The propositional nature of human associative learning

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The propositional nature of human associative learning

  1. 1. BEHAVIORAL AND BRAIN SCIENCES (2009) 32, 183 –246Printed in the United States of Americadoi:10.1017/S0140525X09000855The propositional nature of humanassociative learning Chris J. Mitchell School of Psychology, University of New South Wales, Kensington 2052, Australia chris.mitchell@unsw.edu.au http://www.psy.unsw.edu.au/profiles/cmitchell.html Jan De Houwer Department of Psychology, Ghent University, Henri Dunantlaan 2, B-9000 Ghent, Belgium jan.dehouwer@ugent.be http://users.ugent.be/~jdhouwer/ Peter F. Lovibond School of Psychology, University of New South Wales, Kensington 2052, Australia p.lovibond@unsw.edu.au http://www.psy.unsw.edu.au/profiles/plovibond.htmlAbstract: The past 50 years have seen an accumulation of evidence suggesting that associative learning depends on high-level cognitiveprocesses that give rise to propositional knowledge. Yet, many learning theorists maintain a belief in a learning mechanism in whichlinks between mental representations are formed automatically. We characterize and highlight the differences between thepropositional and link approaches, and review the relevant empirical evidence. We conclude that learning is the consequence ofpropositional reasoning processes that cooperate with the unconscious processes involved in memory retrieval and perception. Weargue that this new conceptual framework allows many of the important recent advances in associative learning research to beretained, but recast in a model that provides a firmer foundation for both immediate application and future research.Keywords: association; associative link; automatic; awareness; conditioning; controlled; dual-system; human associative learning;propositional1. Introduction of food. The biologically neutral bell is usually referred to as a conditioned stimulus (CS), and the biologically rel-The idea that behavior is determined by two independent evant food (to a hungry dog) is referred to as an uncondi-and potentially competing systems has been used repeat- tioned stimulus (US). Most contemporary animal learningedly in psychology (see Evans [2008] for a recent review theorists now consider that the dogs salivated on hearingof some of these ideas). The diversity of research areas the bell because a link formed between the mental rep-in which this idea has been reproduced is striking. It resentations of the bell (CS) and food (US). This link ¨includes, for example, fear learning (e.g., Ohman & allowed the presentation of the bell to activate theMineka 2001), memory (e.g., Schacter 1987), reasoning mental representation of food (see Fig. 1) and, therefore,(e.g., Evans 2003), decision making (e.g., Kahneman & produce salivation in much the same way as would actualFrederick 2002), and the activation of attitudes (e.g., presentation of the US itself.Wilson et al. 2000). In each case, one system is generally It is clear from this description of Pavlov’s (1927) hugelycharacterized as conscious, cold, and calculating; the influential work, that the term associative learning has twoother, as unconscious, affective, and intuitive. In this meanings. These meanings are often confused. The firsttarget article, we reconsider (and reject) one of the refers to a phenomenon – the capacity possessed by aoldest and most deeply entrenched dual-system theories broad range of organisms to learn that two or morein the behavioral sciences, namely the traditional view of events in the world are related to one another. That is,associative learning as an unconscious, automatic process one event may refer to, signal, or cause the other. Thisthat is divorced from higher-order cognition. meaning of associative learning is silent as to the psycho- The classic empirical demonstration of associative learn- logical mechanism responsible for learning. The seconding comes from Pavlov (1927). He presented his dogs with meaning of associative learning does specify a psychologi-a ringing bell followed by food delivery. As a consequence, cal mechanism. This mechanism is the formation of linksthe dogs would salivate on hearing the sound of the bell, between mental representations of physical stimuli aseven in the absence of food. This shows that Pavlov’s illustrated in Figure 1. The links are said to be formed pas-dogs learned to associate the bell with the presentation sively and automatically as a direct consequence of# 2009 Cambridge University Press 0140-525X/09 $40.00 183
  2. 2. Mitchell et al.: The propositional nature of human associative learning links are formed. In order to distinguish the two main approaches to theorizing about mechanisms of associative learning, we refer descriptively to the automatic link-for- mation mechanism and its alternative, the propositional approach.Figure 1. Elipses indicate mental representations (of the bell A core difference between the two approaches (prop-and the food). The arrow between the two elipses indicates the ositional and link-based) is related to the way in whichmental link formed as a consequence of bell-food pairings. The knowledge is assumed to be represented. As Shanksbell ringing produces salivation because it activates the mental (2007, p. 294) points out, propositional representations:representation of food, which, in turn, produces salivation. have internal semantic or propositional structure in the same way that language does. The English sentences “John chased Mary” and “Mary chased John” have the same elements butcontiguous (with some restrictions) pairings of those phys- do not mean the same thing as they are internally structuredical stimuli. These mental links then allow the presentation in different ways. The alternative to such propositional or cog-of one stimulus to activate the representation of – that is, nitive representations is an association that simply connectsbring to mind – the other stimulus. Many researchers the mental images of a pair of events in such a way that acti-assume that learning about the relationships between vation of one image causes activation (or inhibition) of theevents in the environment (the phenomenon) takes place other.via the formation of links between mental representations Dickinson (1980, p. 85) similarly describes “an excit-of those events (the mechanism). Our target article argues atory link which has no other property than that of trans-against this position and aims to show that associative mitting excitation from one event representation tolearning results, not from the automatic formation of another.”links, but from the operation of controlled reasoning pro- These quotes reveal that a proposition differs from acesses. These processes result in beliefs about the world in link in that it specifies the way in which events arethe form of propositions, rather than simply links that related. For instance, a proposition can specify that theallow one representation to activate another. Hence, in bell signals food. In contrast, a link between represen-the context of the present argument, the term “associative tations only allows activation to pass between those rep-learning” refers to the ability to learn about relationships resentations. The link itself has no representationalbetween events, not to a mechanism by which mental content; there is nothing stored to indicate the nature of the relationship between the stimuli (Fodor 2003). CHRIS MITCHELL is senior lecturer at the University of This means that a proposition has a truth value (see New South Wales. From 1991– 1997 he investigated Strack & Deutsch 2004), but a link does not. That is, a associative learning in rats at University College proposition can be shown to be true or false. In the London. He then worked at Unilever Research, Port case above, it can be demonstrated that the bell does or Sunlight, as a consumer psychologist, before returning does not signal food. A link cannot be shown to be true to academia in 2000 to pursue his interests in attention, or false because it does not represent any particular memory, and associative learning. He has published in relationship between the bell and food. the Journal of Experimental Psychology: Animal Beha- Proponents of the automatic link mechanism do not vior Processes, the Journal of Experimental Psychology: deny that propositional reasoning processes can generate Learning, Memory, and Cognition, and the Journal of knowledge of relationships between events in the world. Experimental Psychology: General. He is an Associate Editor of the Quarterly Journal of Experimental However, they argue that the link-formation mechanism Psychology. is able to produce learning independently and in an auto- matic manner. This point has already been made by JAN DE HOUWER is Professor of Psychology at Ghent Shanks (2007). As he says, University. He is the author of over 100 publications It is important to realise that when arguing for a contribution in the field of experimental psychology, including pub- of associative processes, supporters of this approach have lications in Psychological Bulletin and the Journal of never denied that rational causal thinking takes place . . . Experimental Psychology: General. His main research Rather, the question is whether all causal thought is of this interest is the manner in which spontaneous (auto- form, or whether instead there might be a separate type of matic) preferences are learned and can be measured. thinking (associative) when people make intuitive judgments He is Editor of Cognition and Emotion and was Laure- under conditions of less reflection. (Shanks 2007, p. 297) ate of the Royal Flemish Academy of Belgium for Science and the Arts. Likewise, McLaren et al. (1994) “agree there exist two qualitatively different types of learning,” (p. 315) “an PETER LOVIBOND is Professor of Psychology at the associative system which cumulates information about University of New South Wales. He has qualifications contingencies between events and a cognitive system in experimental and clinical psychology, and has with beliefs and reasons for those beliefs” (p. 327). “By carried out research on animal learning and motivation, associative learning, we mean learning that can be charac- human learning, cognition, psychophysiology, and terised in terms of the establishment of links between rep- anxiety and depression. He is a Consulting Editor for resentations” (p. 316). They assume that the formation of Learning and Behavior, the Journal of Experimental Psychology: Animal Behavior Processes, and the links occurs “automatically, regardless of the subject’s Journal of Abnormal Psychology. He is a Fellow of plans or intentions” (p. 321). Thus, the alternative to the the Australian Psychological Society and the Associ- propositional approach is a dual-system approach; beha- ation for Psychological Science. vior is determined by both the propositional reasoning system and the automatic link-formation mechanism.184 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  3. 3. Mitchell et al.: The propositional nature of human associative learningA critical issue then is whether there is evidence for the events (e.g., Mackintosh 1975; Pearce 1987; Pearce &second component of the dual-system approach, the auto- Hall 1980; Rescorla & Wagner 1972; Wagner 1981). Formatic link-formation mechanism. example, it is generally accepted that links will be It is important to be clear that our aim is not to evaluate formed only if the CS is attended (e.g., Mackintoshindividual models of learning or propositional reasoning, 1975; Pearce & Hall 1980). Similarly, Rescorla andof which there are many. Our aim is simply to compare Wagner (1972) proposed that contiguous pairings of athe broad class of dual-system models with the broad CS and US will not produce an associative link betweenclass of propositional models. It is for this reason that we the two if the representation of the US is already activateduse the terms propositional approach and dual-system (or is unsurprising), for instance because a second pre-approach. These two approaches differ in fundamental trained CS is present on that trial. This is the phenomenonand testable ways. To summarize, the propositional of blocking (Kamin 1969) – the pre-trained CS will blockapproach suggests that controlled reasoning processes the formation of a link between the target CS and theare necessary for learning to take place, and learning US – which is an example of competition between cuesresults in beliefs about the relationship between events. to gain “associative strength.” Blocking is a very importantThis can be contrasted with the idea that learning is some- phenomenon in the study of learning, precisely because ittimes the consequence of the automatic formation of excit- shows that contiguous stimulus pairings do not alwaysatory and inhibitory links between stimulus nodes or produce associative learning.representations. The link-formation mechanism is thought to be respon- In this target article, we present a brief and selective sible not only for blocking, but also for many other con-survey of the literature on associative learning (for more ditioning phenomena (e.g., conditioned inhibition,complete reviews of some specific aspects of the literature, overexpectation effects, etc.) and is thought to applysee De Houwer 2009; De Houwer et al. 2005; Lovibond & equally to all stimuli across different modalities and in aShanks 2002). In this survey, we find clear support for the wide range of species. The generality of the phenomenarole of propositional processes in learning. In stark con- (perhaps most importantly, blocking) across these differ-trast, little unambiguous support is found for an automatic ent situations and species is often argued to demonstratelink-formation mechanism. We conclude that there is very that all species possess a common learning mechanismlittle to be lost, and much to be gained, by the rejection of (e.g., Dickinson et al. 1984). The mechanism must, it isthe dual-system approach that incorporates an automatic sometimes further argued, be very simple and automaticlink-formation mechanism. This is true for our under- because surely species such as the humble rat could notstanding of the basic processes of associative learning possess the complex hypothesis testing abilities of humans.(at both the psychological and physiological level) and inthe application of learning theory to pathological behaviors 2.2. Performancein the clinic. The link model provides a ready explanation for con- ditioned responses (CRs) such as salivation to a CS that2. The dual-system approach to learning has been paired with food. Once the link is formed, acti- vation can be transmitted from one representation toThe dual-system approach incorporates all of the reason- another just as a piece of copper wire conducts electricity.ing processes of the propositional approach plus an Thus, when a CS such as a bell is presented on test, it acti-additional automatic link-formation mechanism. There- vates the mental representation of that bell. This activationfore, it is this link formation mechanism that is the focus is then transmitted along the link, and so the US represen-of section 2. tation also becomes activated (see Fig. 1). Salivation (the CR) is observed because activation of the US represen- tation is functionally equivalent to actual physical presen-2.1. Learning tation of food. Thus, the link mechanism provides a veryAs outlined in section 1, the usual view is that links simple and intuitive account of why, when a CS is pre-between representations can be formed automatically in sented in the absence of the US on test, behaviors consist-the sense that they are independent of the goals, proces- ent with actual US presentation, such as salivation, aresing resources, and causal beliefs of the individual (see often observed.Moors & De Houwer [2006] for an analysis of the Of course, this characterization of operation of the linkconcept “automatic”). Thus, as Le Pelley et al. (2005a, model is overly simplistic and easily discredited (seep. 65) have argued, imposing a cognitive load will Wagner & Brandon 1989). Within this model, activation“hamper participants’ use of cognitive strategies in contin- of the US representation by the CS (via the link) is equiv-gency learning, instead forcing them to rely on ‘automatic’ alent to activation of the US representation by presen-associative processes.” This implies that these (link-based) tation of the US itself. Associative learning theorists areassociative processes are automatic in the sense that they well aware that presentation of the CS and US do notare efficient (see also, Dickinson 2001, p. 23). have exactly the same consequences; the CS is not a sub- Although the link mechanism is often thought to be effi- stitute for the US. Wagner’s (1981) influential Sometimescient and to operate independently of the subject’s goals, Opponent Processes (SOP) model of associative learninglink formation is not assumed to be completely uncondi- addresses this issue. Wagner distinguishes between ational. A number of different learning rules have been pro- primary and a secondary state of activation, termed A1posed that can be seen as setting restrictions on the and A2, respectively. It is only when a US is physicallyconditions under which the pairing of events leads to the present that its representation (or some part thereof) willformation of a link between the representations of those be activated into the (primary) A1 state. Following BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 185
  4. 4. Mitchell et al.: The propositional nature of human associative learningearlier CS-US pairings (conditioning), presentation of the ˜ pattern of responding seen on test (Pineno et al. 2005;CS will associatively activate the US representation into Vadillo & Matute 2007; see also Waldmann 2000, for athe (secondary) A2 state. Thus, Wagner’s model postulates similar argument in the context of causal and diagnostictwo different states of activation to distinguish between learning). The simple link mechanism, because it cannotperception of the US when it is physically present (the capture the precise nature of the associative relationshipA1 state) and anticipation of that US (the A2 state). between cue and outcome, cannot explain these effects There are also other ways in which a US representation and so cannot explain many aspects of human associativecan be activated into the A2 state. When a US is presented learning. Of course, as was pointed out in section 1, the(and its representation is activated into A1), removal of automatic link-formation mechanism has been argued tothat US will allow the representation to decay into A2. be only one system in a dual-system approach to learning.In this case, A2 activation of the US representation It is open to proponents of this approach to argue that thewould seem to equate to memory of the US. One thing differences observed between causal and predictive cuesthat is striking about this model is that it does not dis- are a consequence of the second, propositional, process,tinguish between memory for a US in the recent past not the automatic links (e.g., Vadillo & Matute 2007).and anticipation of a US in the future (which have very We shall return to this issue further on.different behavioral consequences; see Bolles & Fanselow In summary, the dual-system approach suggests that, in1980). That is, both US memory and US anticipation are addition to the reasoning processes that produce consciousrepresented by A2 activation of the US representation. propositional knowledge, there exists an automatic, hard-Further refinement would be needed to accommodate wired mechanism that produces links between CSs andthis important distinction. However, what is important USs (or cues and outcomes). In Pavlovian conditioning,is that if one postulates different states of activation, then these links allow the presentation of the CS to activatethe idea of simple activation can come to mean different the US representation, and this produces a CR. Thethings, and the link model becomes much more flexible. link-formation mechanism is also thought (under certain Anticipatory CRs such as salivation or fear are not the circumstances) to be responsible for the learning ofonly responses said to be produced by the link mechanism. other types of relations, including predictive, causal, andLearning theorists have also applied this same approach to referential relations, and is assumed to operate in allthe analysis of human contingency learning. An example of species, including humans.a contingency learning task is the allergist task (e.g., Larkinet al. 1998). Participants play the role of an allergist who isasked to determine which food cues produce an allergic 3. The propositional approach to learningreaction outcome in a fictitious Mr. X. In the case ofsimple conditioning, Mr. X eats a food such as carrots on According to the propositional approach, associative learn-each trial and always suffers an allergic reaction. Partici- ing depends on effortful, attention-demanding reasoningpants learn that carrots are associated with the allergic processes. The process of reasoning about the relationshipreaction. The automatic link-formation mechanism is between events produces conscious, declarative, prop-thought to operate in this scenario just as it does in Pavlo- ositional knowledge about those events.vian conditioning; a carrot-allergic reaction (cue-outcome)link is formed, such that presentation of the cue is able to 3.1. Learningactivate the representation of the outcome. When a foodthat has been followed by the allergic reaction during When we learn that Mr. X has an allergy to carrots, or thattraining is judged to be allergenic on test, it is argued a bell will be followed by food, we use the same processesthat this judgment is the consequence of the cue- of memory and reasoning that we use to plan our groceryoutcome link that has formed. shopping, to play chess, or to behave appropriately at a In fact, Pearce and Bouton (2001) suggest that the link black-tie function. When presented with a bell, we maybetween cue and outcome can serve to represent a whole recall that the last time the same bell rang, we receivedrange of different associative relationships. This further food. Given a number of assumptions (e.g., that relationsimplies that a causal relationship between the cue and are stable over time and that the bell is a potential signaloutcome (e.g., drinking alcohol causes a headache) is rep- for food), this might lead us to hypothesize that when weresented in exactly the same way as a predictive relation- hear that bell, we are about to receive food again. Weship (e.g., hearing the platform announcement predicts, may also recall having previously hypothesized that thebut does not cause, the arrival of a train). It also implies bell signals food. When we do indeed receive food, thatthat causal and predictive relationships are represented experience constitutes a test (a confirmation) of ourin the same way as purely referential relationships, in hypothesis. Thus the strength of our belief in the bell-which the cue merely refers to the outcome without an food relationship will increase. The encoding of thisexpectation that the outcome will actually occur (e.g., hypothesis in memory, and the degree to which we havethe word “sun” uttered at night refers to the sun but confidence in it, constitutes learning. There is no mentaldoes not produce an expectation that the sun will appear link between the bell and food, but a proposition of thein the immediate future), or to the relationship between form, “When I hear a bell, I will receive food.”a category (e.g., animals) and an exemplar of that category Propositions can be regarded as qualified mental links,(e.g., a cat). that is, links that specify how two events are related. However, these relationships are not equal. It is known, This approach is also adopted in the Bayesian networkfor example, that whether the cues and outcomes in an approach to the analysis of belief acquisition and revisionassociative learning experiment are presented in a causal (see Lagnado et al. 2007, for a very useful overview).or a predictive scenario has a profound effect on the In Bayes nets, events are joined by, for example, a causal186 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  5. 5. Mitchell et al.: The propositional nature of human associative learninglink – an arrow that has a particular strength and direc- processes that are responsible for learning. Other kinds oftion. Thus, an arrow that points from “bacterial infection” input will include, for example, the knowledge that thereto “peptic ulcer” might indicate that bacterial infection was no other signal for food present when bell-food pair-causes peptic ulcers. Because the links in Bayes nets rep- ings were experienced, and the belief that bells are, inresent propositions about relationships, they, like all prop- general, potential signals for food delivery.ositions, have truth value (e.g., it is either true or not true It is important to make clear that allowing automaticthat bacterial infection causes peptic ulcers). Therefore, processes of memory (and, indeed, perception) to play athe arrows do not simply indicate that activation can role in learning, does not imply that the propositionalspread from one mental representation to another in approach is simply another dual-system approach. Thethat direction. Despite these similarities, the Bayes net fra- way in which automatic and controlled processes interactmework and the propositional approach are not identical. to produce learning in the propositional approach isMost importantly, the Bayesian approach is silent as to quite unlike that of the dual-system approach. In thewhether belief acquisition involves controlled or automatic dual-system approach, two incompatible CS-US (e.g.,processes. The propositional approach presented here bell-food) relationships might simultaneously be learnedmakes the strong claim that associative learning is never by the two systems (although, it should be noted, it isautomatic and always requires controlled processes. seldom explained how these systems might interact Associative learning theorists are often concerned not under such circumstances). For example, a strong bell-simply with whether or not a CR is produced, but with food link may form in the absence of any belief that pres-the strength of the CR, which is thought to be a measure of entation of the bell signals food delivery. In contrast, in the“associative strength.” Within the propositional approach, propositional approach this is not possible because theassociative strength relates to two things. The first is the automatic processes of perception and memory servebelief about the strength of the CS-US relationship. only as an input to the non-automatic processes of prop-Thus, a belief may be held that a CS is followed by a US ositional reasoning. These two types of process areon 50% of occasions. This will, of course produce a simply different parts of the same learning system.weaker CR than a belief that the CS is followed by the Lastly, it is important to be clear on the way in which theUS on 100% of occasions. The second is the strength of propositional approach deals with the role of conscious-the belief, which will typically be low at the start of training ness in learning. We do not claim that people are necess-and high after many CS presentations. Therefore, associat- arily aware of all of the processes that lead to the formationive strength will be jointly determined by how strong the of propositions about relationships between events,CS-US relationship is believed to be (the content of the including the reasoning processes. What we do claim isbelief) and the strength of that belief (the degree of confi- that the propositional beliefs themselves are available todence with which it is held). consciousness. Thus, it is not possible to have learned The description of learning presented above leaves about a relationship between two events in the environ-some important issues unspecified. First, we do not ment without being, or having been, aware of that relation-specify the nature of the controlled processes, beyond ship (see De Houwer 2009).characterizing them as propositional reasoning. That is,we do not propose a new model of propositional reasoning. 3.2. PerformanceThere are many ways to model reasoning processes (e.g.,Braine 1978; Chater et al. 2006; Evans & Over 1996; The consequence of entertaining a belief that the bell CSJohnson-Laird 1983), some of which are specifically signals the food US (or, in other cases, that the CS causesdesigned to account for the learning of causal relationships the US) is that, “When I next hear the bell, I shall (allbetween events (e.g., Cheng 1997; Kruschke 2006). We things being equal) anticipate, or expect, the food to bewould not argue for the virtues of any particular model presented.” Early cognitive psychologists also viewed con-of reasoning, only that associative learning requires ditioned responses to be the consequence of US expect-reasoning, in whichever way this is achieved. ancy. They assumed that the strength of the CR (e.g., Second, even though we postulate that associative learn- skin conductance elevation) in conditioning with a shocking is influenced by memory for prior events, we do not US would be a product of the strength of the expectancypropose a new model of memory. Probably the simplest of shock and the value (intensity or aversiveness) of thatmemory model that would be consistent with our view is shock (e.g., MacCorquodale & Meehl 1954). However,an instance model of memory (e.g., Hintzman 1986). expectancy was thought of in terms of a link that allowedAccording to this model, separate experiences are stored the CS to activate the US representation. The prop-as separate memory traces that can be retrieved on the ositional approach departs from these early theories inbasis of similarity with the current stimulus input. Thus, that the knowledge of the associative relationshipa bell can retrieve memories of past occasions on which between CS and US is a belief represented in prop-the bell was presented, and therefore, of past bell-food ositional form. Thus, the expectancy of the US when thepairings. CS is presented is a consequence of the belief that the Third, we do not rule out a role for automatic processes CS causes or signals the US.in learning. Memory retrieval has many features of auto- One problem that is often raised in the context ofmaticity, and so some of the processes that result in learn- expectancy-based models of emotional and physiologicaling must also be automatic. However, this does not imply conditioned responses is how an expectancy can give risethat learning itself is automatic. According to the prop- to such responses. We do not have a solution to thisositional approach, recollections of past bell-food pairings long-standing problem. However, we already know thatalone cannot produce learning. These recollections only instructions can produce physiological and emotionalserve as one kind of input into the propositional reasoning responses in the absence of any CS-US link. For instance, BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 187
  6. 6. Mitchell et al.: The propositional nature of human associative learningthe mere instruction that an electric shock is forthcoming automatic (in the sense that it is efficient) should be lessleads to an increase in fear and skin conductance (Cook & dependent on cognitive resources. Third, hypothesesHarris 1937). Hence, if it is assumed that instructions about how events are related to each other can be acquiredproduce CRs by generating an expectancy of the US, by verbal instruction and will be influenced by abstractthen there must be a process by which US expectancy rules and deductive reasoning processes. Therefore, thecan generate physiological CRs, even though this process propositional approach predicts that learning will similarlyis not yet well understood. be affected by these factors. The automatic link-formation A related issue is that skin conductance and heart rate mechanism is non-propositional. It cannot, therefore, beCRs seem uncontrollable and, in this sense, therefore, affected directly by verbal instruction, rules, or deduction.automatic. This seems to imply that an automatic learning In section 4, we present the findings that lend support tosystem is in operation. However, the idea that conditioned the propositional approach. In section 5, we outline theresponses can arise automatically can be accounted for evidence that has been argued to provide strong supportwithin the propositional approach in two ways. First, we for the dual-system approach. It will be suggested at thedo not argue that subjects have control over their end of section 5 that the balance of evidence stronglyresponses to expected USs, but rather that learning to favors the propositional approach.expect those USs is a non-automatic process. Once thereis an expectancy that the US will occur (the subject haslearned that the CS that has been presented predicts the 4. Evidence for the propositional approachUS), this can automatically lead to emotional and physio- 4.1. The role of awareness in associative learninglogical responses; if one believes that a painful shock isimminent, it is difficult not to experience fear. Second, Because learning is assumed to involve the strategiconce a proposition has been formed that the CS causes testing of hypotheses and to result in conscious prop-or signals the US, it will be stored in memory and may ositional knowledge about relations between events inbe activated automatically. Hence, the presentation of a the world, a propositional approach predicts that learningCS can automatically lead to the expectation of the US should be found only when participants have conscious(and thus to conditioned responding) if the previously awareness of the relevant relations. If evidence forformed CS-US proposition is retrieved automatically unaware conditioning were uncovered, this would, there-from memory. Whether the CS-US proposition is fore, strongly support the existence of multiple learningretrieved automatically from memory will depend on a mechanisms (Lovibond & Shanks 2002; see also Boakesnumber of factors, including the number of times that 1989; Brewer 1974; Dawson & Schell 1985; Shanks &the CS-US proposition has been consciously entertained. St. John 1994).In summary, although learning results from controlled In Pavlovian conditioning of human autonomicprocesses, performance may be automatic. responses, for example, a CS (e.g., a light) is paired with With regard to performance in causal or contingency an aversive US such as an electric shock. On test, learninglearning, the propositional approach applies in a very is evidenced by the ability of the CS to increase the partici-straightforward way. Take the example of the food- pant’s skin conductance, a measure of fear. The resultsallergy paradigm. Participants are assumed to form prop- consistently show evidence for skin conductance CRsositions about the relation between foods and allergies only in participants who are aware of the CS-US contin-(e.g., carrots cause an allergic reaction). When asked to gency (for reviews, see Dawson & Schell 1985; Lovibondrate the contingencies between different foods and aller- & Shanks 2002). Moreover, CRs occur only after the par-gies, participants simply need to express their prop- ticipants become aware of the CS-US contingency. Suchositional knowledge. That is, the report of contingency results have led to the conclusion that awareness of theknowledge is merely the verbal expression of a belief. CS-US contingency is a necessary condition for Pavlovian conditioning to occur (Dawson & Shell 1985). Other studies of conditioning with shock USs suggest3.3. Predictions of the propositional and dual-system that the close link between learning and awareness is approaches due to the fact that consciously available hypotheses deter-The propositional and dual-system approaches make a mine how the participant will respond. For instance, inter-number of different predictions about the conditions individual differences in human autonomic conditioningunder which learning will occur, and about the pattern are closely related to interindividual differences in theof responding that might be observed when different con- extent to which the US is expected at a particulartingencies are in place. First, whether learning can take moment in time (e.g., Epstein & Roupenian 1970).place in the absence of awareness of the CS-US (or cue- When participants have incorrect beliefs about the associ-outcome) contingencies is relevant to this debate. The ation between events or between a response and an event,propositional approach assumes that learning involves their conditioned behavior is most often in line with thetesting hypotheses and that it results in conscious prop- incorrect beliefs rather than with the objective contingen-ositional beliefs. One would, therefore, expect participants cies (e.g., Parton & DeNike 1966).who successfully learn the CS-US contingencies to be Lovibond and Shanks (2002) concluded that the avail-aware of, and be able to report, those contingencies. By able evidence, from a whole range of conditioning pro-contrast, if learning is automatic, it may take place in the cedures, is consistent with the idea that conditioning isabsence of such awareness. Second, the propositional accompanied by awareness. Although there are manyapproach suggests that all learning is effortful and so papers arguing for unaware conditioning, close inspectionshould depend on the availability of sufficient cognitive reveals, in almost all cases, that the measure of condition-resources. The link-formation mechanism, because it is ing was most likely more sensitive than that of awareness.188 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  7. 7. Mitchell et al.: The propositional nature of human associative learningThis may have been because, for example, a recall rather to interfere with the formation or deployment of prop-than a recognition test of contingency awareness was ositional knowledge about the CS-US relation shouldused, or because contingency awareness was only tested also reduce CRs to that CS. One way in which processesafter an extinction phase (see Dawson & Schell [1985; can be automatic is that they require only limited cognitive1987] for excellent reviews of these issues). These flaws resources. Hence, if reduced attention to the targethave the potential to lead to an apparent dissociation relationship leads to a reduction in learning of thatbetween conditioning and awareness when, in fact, none relationship, this would seem to suggest that learning isexists. Only two possible exceptions were identified by cognitively demanding and, in this sense, not automatic.Lovibond and Shanks, evaluative conditioning (e.g., The most thorough investigation of the effect of atten-Baeyens et al. 1990a) and the Perruchet effect (e.g., Perru- tional manipulations on conditioning was conducted bychet 1985). We shall return to these in section 5. Dawson and colleagues in the 1970s (e.g., Dawson 1970; Before we accept that the absence of evidence for Dawson & Biferno 1973). They embedded a differentialunaware conditioning constitutes evidence against the autonomic conditioning design within an “auditory per-automatic link mechanism, we should consider the ception” masking task that required participants toalternatives. For example, perhaps the observed concor- answer several questions at the end of each trial concern-dance between awareness and CRs does not result from ing the pitch of a series of six tones. In fact, one tone wasthe US expectancy causing the CR (as we have suggested), paired with shock (CSþ) and another tone was neverbut rather from the CR causing the US expectancy. Thus, paired with shock (CS2). Propositional knowledge offollowing CS-shock training, presentation of the CS will the differential contingency was assessed by online expect-elicit CRs such as increased anxiety, heart rate, and ancy ratings and by a post-experimental interview. Thearousal. When participants experience these physiological results were clear-cut. The addition of the masking taskCRs, they may then draw the conclusion that the shock is substantially reduced both contingency knowledge andabout to be presented, and so they become aware of the differential electrodermal CRs. Participants who were ¨CS-US contingency (Katkin et al. 2001; Ohman & Soares classified as unaware of the differential contingency1993; 1994; 1998). Alternatively, it may be argued that, failed to show any differential CRs. Furthermore, thealthough the link-formation mechanism is automatic in expectancy ratings and electrodermal CRs were closelysome respects (e.g., it is efficient and independent of the related. When the data for “aware” participants werelearner’s goals), it is not automatic in the sense that it is aligned around the trial on which they first showed expect-unconscious. This would be a second way in which the ancy discrimination, the electrodermal measure similarlyabsence of unaware conditioning might be argued not to showed differentiation after, but not before, that point.be inconsistent with the dual-system approach. Dawson’s results are not unusual; the same pattern has To summarize, a demonstration of unaware conditioning been observed repeatedly across different conditioningwould be highly damaging to the propositional approach, preparations, and there is no convincing example of aand would provide strong evidence for a second (automatic) differential impact of reduced attention on verbalizablelearning mechanism. However, a large body of literature knowledge and CRs (see Lovibond & Shanks 2002).shows a clear concordance between conditioning and The finding that learning processes are disrupted by theawareness, and provides, therefore, no unique support for addition of a masking task suggests that learning requiresan automatic learning mechanism. So what can be con- cognitive resources and is, in this sense, not automatic.cluded from these data? The observed concordance It is, therefore, evidence against an automatic link-between conditioning and awareness is strongly predicted formation mechanism. However, it might be argued that noby the propositional approach. And, although the absence psychological mechanism or process places zero require-of unaware conditioning cannot be taken as decisive evi- ments on cognitive resources; there are no automatic pro-dence in the present debate (an absence of evidence cesses in this very strict sense. There are degrees ofrarely is decisive), it is only consistent with the existence automaticity (Moors & De Houwer 2006). Thus, theof the link-formation mechanism if certain additional link-formation mechanism, although cognitively demand-assumptions are made. Thus, if anything, the data support ing, may be less demanding than other tasks such asthe propositional approach. Finally, it should be noted reasoning and problem solving. Alternatively, perhaps cog-that if we acknowledge that learning depends on awareness, nitive load does not prevent the automatic link-formationthen we remove one of the reasons for postulating a dual- mechanism itself from operating, but rather, it reducessystem approach in the first place. If all learning is aware, the degree to which the stimulus input (the CS and US)there is less to be gained from postulating an automatic is processed. If the participant fails to notice the stimuli,link-formation mechanism in addition to a propositional there will be no input to the automatic learning system,reasoning mechanism. and nothing will be learned. Either of these interpret- ations of the effect of cognitive load would, of course, con- stitute quite a large concession. If all learning depends on4.2. Cognitive load and secondary tasks cognitive resources, then one of the reasons for postulatingAccording to the propositional approach, learning the existence of an automatic link-formation mechanismdepends on the involvement of propositional reasoning has been removed (as was the case for the role of aware-processes that require attentional/cognitive resources. ness in conditioning; see section 4.1 above). Moreover,Therefore, secondary tasks that consume cognitive such a concession weakens the testability of Dickinsonresources, or instructions that divert attention away from (2001) and Le Pelley et al.’s (2005a) claim that when thethe target association, are predicted to impair learning. cognitive system is overloaded, the operation of the linkA small decrease in attention may not be sufficient mechanism will be revealed. If the link-formation mechan-to reduce learning, but any manipulation that is sufficient ism depends on cognitive resources, then imposing a BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 189
  8. 8. Mitchell et al.: The propositional nature of human associative learningmental load during a learning task cannot, as has been acquired by direct experience are represented in aclaimed, reveal the operation of that mechanism in the similar way. Thus, the implication is that the knowledgeabsence of propositional reasoning. acquired by experience is propositional in nature. Furthermore, one recent study seems to suggest that It is very difficult to explain effects such as instructedthe introduction of a secondary task does not simply conditioning in terms of an automatic link mechanism.reduce stimulus processing. This time the evidence Perhaps the mention of the bell activates the represen-comes from studies of blocking in human contingency tation of the bell, and the mention of the shock activateslearning. In blocking, as described previously, pairing of a representation of shock. This contiguous activationone cue, A, with the outcome (Aþ) in a first phase prevents might foster the formation of a link between these two rep-learning about the target cue T on subsequent ATþ trials. resentations (mediated learning; Holland 1990). OfDe Houwer and Beckers (2003) found that blocking in course, this theory is easily refuted; verbal instructionshuman contingency learning was less pronounced when that “on none of the following trials will the bell be fol-participants performed a demanding secondary task lowed by shock” activate the bell and shock represen-during the learning and test phases, than when they per- tations in the same way, but these instructions will notformed an easy secondary task. In other words, increasing produce an anticipatory response.the demands of the secondary task increased the degree to Perhaps knowledge in propositional form creates CS-which participants learned a T2outcome relationship. US links in some way that we have not yet considered.Waldmann and Walker (2005) obtained a similar result, However, even if this translation process were possible,attesting to the reliability of this finding. This is the there is a deeper problem with this general idea. Propo-precise opposite of the outcome predicted by the nents of the dual-system approach would like to argueaccount outlined above, according to which cognitive for a distinction between the acquisition of consciousload has an effect on learning by reducing the degree of propositional knowledge, on the one hand, and automaticstimulus processing. By that account, the secondary task learning, on the other. Allowing that a single verbalshould have reduced learning about T on ATþ trials. instruction might produce a link between two represen-The result is, however, in line with the hypothesis that tations of the same kind as does the experience of multipleblocking depends on effortful controlled processes, as pre- training trials, seems to blur this distinction. Rememberdicted by the propositional approach; participants were that, in their analysis of causal learning, the dual-systemprevented from reasoning that, because A is a cause of theorists also argue that the links formed by the automaticthe outcome, T is, therefore, redundant. system can generate propositional knowledge. Taken together, these two ideas suggest that all propositional knowledge is immediately translated into links, and all4.3. Verbal instructions knowledge in the form of links can be translated into prop-Many studies have shown that informing participants verb- ositional form. One of the two systems is, therefore, redun-ally about a relationship between stimuli is sufficient to dant. The only coherent solution to this problem is toproduce evidence of learning. In an example presented assume that there is a single system, and the evidence pre-earlier (see sect. 3.2), if one informs a participant that a sented here suggests that this system is propositional intone will always be followed by a shock, the tone will nature. The experiments presented in the followingproduce an increase in skin conductance, even though section, concerning the effects of abstract rules and deduc-the tone and shock have never actually been presented tive reasoning in conditioning, lend further support to thistogether (Cook & Harris 1937). Likewise, if one first pre- conclusion.sents tone-shock trials and then verbally instructs the par-ticipants that the tone will no longer be followed by the 4.4. Abstract rules and deductive reasoningshock (instructed extinction), the skin conductance CRwill be dramatically reduced (e.g., Colgan 1970). Thus, Shanks and Darby (1998) reported a striking demon-verbal instructions can lead to the same effects as the stration of the use of rules in associative learning. Theyactual experience of a contingency, and can interact with presented Aþ, Bþ, AB2, C2, D2, and CDþ trialsknowledge derived from actual experience. together with Iþ, Jþ, M2, and N2 trials. During a test Recent studies have shown that these conclusions also phase, participants judged that the outcome was morehold for more complex learning phenomena. Lovibond likely to occur after the (previously unseen) compound(2003), using an autonomic conditioning procedure, MN than after the (also previously unseen) IJ compound.trained a compound of A and T with shock (ATþ) and In terms of links between representations, this is thethen presented CS (A) without the US (A2). The A2 reverse of the prediction based on the elements thattraining in the second phase increased the CR observed made up the compounds. Participants appeared to haveto T on test, a phenomenon known as release from over- learned a rule from observing trials on which cues A –Dshadowing. Release from overshadowing could result were presented, that the outcome of compounds of twofrom reasoning that (a) at least one of the cues A or T stimuli (i.e., AB2, CDþ) is the reverse of the outcomemust signal the shock on ATþ trials and (b) because A of the individual elements that make up that compoundwas subsequently found to be safe, T must be the signal. (i.e., Aþ, Bþ, C2, D2). They then applied this reversalImportantly, Lovibond (2003) also found release from rule to cues I –N.overshadowing when the ATþ and A2 trials were Other evidence for the role of propositional reasoning indescribed verbally (Experiment 2) and when the ATþ human associative learning comes mainly from studies ontrials were actually presented, but the subsequent A2 cue competition, in particular, blocking (see De Houwercontingency was described verbally (Experiment 3). This et al. 2005, for review). For example, De Houwer et al.shows that the knowledge acquired verbally and that (2002) observed blocking only when it was possible to190 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  9. 9. Mitchell et al.: The propositional nature of human associative learninginfer deductively that cue T in the Aþ/ATþ design was both the link-formation and propositional reasoningnot associated with the outcome. Because T does not systems. However, what is important is that, within theadd anything to the effect of A alone (i.e., the outcome dual-system account of the data outlined above, the linkwas as probable and as intense on Aþ trials as on ATþ mechanism itself is redundant. We now turn to the evi-trials), it can be inferred that T is not a cause of the dence that has been argued to provide unique supportoutcome. However, De Houwer et al. (2002) argued that for the link-formation mechanism.this inference is valid only if it is assumed that the effectof two causes is additive (that when two causes are pre-sented in compound, a larger than normal effect will be 5. Evidence for the automatic formation of linksproduced). De Houwer et al. (2002) provided one groupof participants with an alternative explanation for why T Dual-system theorists point to a number of sources of evi-did not add anything to the effect of A. They told these dence that they believe provide unique support for link-participants that A alone already caused the outcome to formation models. First, although associative learning isa maximal extent. That is, the outcome was at ceiling on generally accompanied by awareness of the CS-US contin-Aþ trials. In this case, participants can reason that no gency, there are two learning procedures that do seemincrease in the effect was seen on ATþ trials, not to provide some evidence of unaware conditioning (seebecause T was non-causal, but because an increase in Lovibond & Shanks 2002). These are evaluative condition-the size of the effect was impossible. In line with the ing and Perruchet’s (e.g., 1985) findings relating to theidea that blocking is based on propositional reasoning, effects of trial sequence in partial reinforcement sche-no blocking effect was found in this condition (causal dules. Second, some experiments have demonstratedratings of T were not reduced as a consequence of prior learning that is not always rational (or normative). TheAþ trials). absence of rationality has been argued to support the Many other studies have confirmed this result. Beckers idea that learning can result from an automatic link mech-et al. (2005; see also Lovibond et al. 2003) raised doubts in anism. Lastly, it has been suggested that some neuroscien-their participants’ minds about the inference underlying tific data indicate the existence of a multiple learningblocking by giving pretraining in which the effect of two system. We address these lines of evidence in turn.cues was shown to be subadditive (i.e., Gþ, Hþ, GHþ,and Iþþ, where þ stands for a US of low intensity and 5.1. Unaware associative learningþþ for a US of high intensity). Blocking was significantlysmaller after this type of pretraining than after pretraining In evaluative conditioning research (see De Houwer et al.that confirmed the additivity of causes (i.e., Gþ, Hþ, 2001; De Houwer 2007, for reviews), neutral stimuliGHþþ, Iþ). Mitchell and Lovibond (2002), using a (across a range of modalities) have been shown to increasesimilar approach, showed blocking of skin conductance or decrease in rated pleasantness as a consequence of pair-CRs only when blocking was a valid inference. Finally, ings with strongly liked or disliked stimuli. Some research-Vandorpe et al. (2007a) obtained the same result in a ers have provided evidence for evaluative conditioning incausal judgment study that involved a very complex the absence of awareness (Baeyens et al. 1990a; Dickinsondesign. This is important because dual-system theorists & Brown 2007; Fulcher & Hammerl 2001; Walther &often argue that the link-formation mechanism will be Nagengast 2006; and see Stevenson et al. 1998, for arevealed in very complex tasks such as that used by Van- related finding). However, insensitivity of testing pro-dorpe et al. (see the discussion above in section 4.2 con- cedures and aggregating awareness scores across both par-cerning cognitive load), and so the propositional system ticipants and items may have hidden some contingencyis unable to operate or is off-line (e.g., Dickinson 2001; awareness in these studies (see Lovibond & ShanksLe Pelley et al. 2005a). Vandorpe et al.’s (2007a) results [2002] for a review). An example of this second issue canshowed, however, that propositional reasoning processes be seen in Dickinson and Brown (2007). They foundcan operate even in these complex tasks. that their participants, when analyzed as a single group, did not demonstrate reliable contingency awareness but did show evaluative conditioning. However, Wardle et al4.5. Conclusions (2007) reanalyzed these data and found that when partici-Many experiments, using a wide range of procedures, have pants were divided into two groups, aware and unaware, itshown a concordance between associative learning and was only the aware group that produced a reliable con-contingency awareness. Furthermore, results of exper- ditioning effect. Other researchers have suggested animents in which a secondary task was imposed are consist- even more fine-grained analysis. They have argued that,ent with the operation of a cognitively demanding although participants might show very little contingencyreasoning process, especially in the case of blocking. awareness when the cues are aggregated, they are, never-Thus, manipulations that prevent reasoning also prevent theless, aware of the outcomes with which a subset of cuesthe learning mechanism from operating. Many more were paired. It is possible that it is this subset of cues thatexperiments have demonstrated the impact of verbal are responsible for the evaluative conditioning observed ininstructions, rules, and deductive reasoning processes on earlier studies (Pleyers et al. 2007).the acquisition of associative knowledge. These data It is very difficult to provide a satisfactory demonstrationmake a very strong case for the idea that associative learn- of unaware conditioning simply by showing conditioninging is based on reasoning processes that yield conscious in the absence of awareness. This is because it is verypropositional knowledge. difficult to be sure that the awareness measure and the Of course, the dual-system approach cannot be said to conditioning measure are equally sensitive. Lovibondbe inconsistent with these findings, because it incorporates and Shanks (2002) identified Baeyens et al.’s (1990a) BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 191
  10. 10. Mitchell et al.: The propositional nature of human associative learningfinding as being the most convincing evidence of unaware evidence for dissociation between a CR and the consciousevaluative conditioning, because flavor-flavor conditioning expectancy of a US.was seen in the absence of any contingency awareness, butcolor-flavor conditioning was not seen despite awareness 5.2. Rationalityof the color-flavor contingency. The latter findingappears to confirm that the awareness measure used was It is often assumed that rationality is a hallmark of thesensitive (albeit to contingencies involving different propositional system. If behavior is rational, then a prop-stimuli). Thus, participants in the flavor-flavor condition ositional mechanism was in operation; if it is not rational,appear to have been unaware of the contingencies they an automatic mechanism was in operation (Shanks 2007;were exposed to. Given the uniqueness of this finding, it Shanks & Dickinson 1990). Therefore, if it can be shownis important that Baeyens et al’s design is replicated, that associative learning is non-rational, it must be basedperhaps with the awareness measure used by Dickinson on the automatic formation of links. The example ofand Brown (2007), and that the awareness-learning irrational behavior that most readily comes to mind isrelationship is analyzed at the item level. Even more con- phobia. For example, arachnophobes can be fearful ofvincing than Baeyens et al’s (1990a) dissociation would be spiders despite claiming to know that spiders are nota demonstration of conditioning in participants unaware of harmful. This would appear to undermine the idea thatthe flavor-flavor contingencies, but not in participants learning is a propositional process – how could such aaware of those same contingencies (rather than system produce behavior that contradicts the verballycolor-flavor contingencies). This is exactly the reverse reported belief?association (see Dunn & Kirsner 1988) sought by Pierre There are three ways that the irrational behavior of ara-Perruchet in his analysis of eyeblink conditioning and chnophobes can be explained which are consistent withcued reaction time learning. It is to this work that we the propositional approach to learning: (1) The verballynow turn. reported belief that spiders are not harmful may simply Perruchet (1985) exposed participants to a pseudo- be a consequence of social demands; the patient mayrandom series of tone-air puff and tone-alone trials and believe the spider to be harmful but not wish to contradictmeasured both eyeblink CRs and expectancy that an air the clinician’s view that the spider is harmless. (2) Thispuff would be delivered on the following trial (tones phenomenon may relate to performance, not to learning.appeared on every trial). Participants’ self-reported The patient may have a long-standing and strong beliefexpectancy of an air puff followed the gambler’s fallacy. that spiders will do him or her harm. He or she may alsoHence, after a run of three tone-air puff trials, participants have acquired more recently a perhaps more fragiletended to predict that the tone would not be followed by appreciation that certain spiders are not harmful. On pres-an air puff on the next trial. Conversely, after a run of entation of a harmless spider, the old belief that spidersthree tone-alone trials, an air puff was strongly predicted are harmful may be retrieved automatically fromto follow the tone on the next trial. The eyeblink CR, memory and thus lead to fear (see sect. 3.2). Becausehowever, followed the opposite pattern; eyeblinks to the the retrieval of the old belief occurs automatically, theCS were most likely to be observed on trials following a resulting fear might seem irrational and difficult torun of tone-air puff trials and least likely following a run control. According to the propositional model, bothof tone-alone trials. Thus, recent CS-US pairings appeared beliefs (that the spider is harmful and that it is notto strengthen the CS-US link and increase the probability harmful) will have been acquired through a process ofof the CR, despite a reduction in US expectancy. Perru- propositional reasoning. (3) There is, in fact, little evidencechet has more recently observed the same dissociation that specific phobias of this kind result from learning at all,using a simple cued reaction time task (Perruchet et al. and therefore they may have a genetic etiology (see2006). Menzies & Clarke 1995, for review). If fear of spiders Perruchet’s dissociations between US expectancy and has a large genetic component that affects behavior inde-the occurrence of the CR in eyeblink conditioning (and pendently of learning, the fact that fear remains eventhe equivalent effect in the cued reaction time task) are when it is known that spiders are not harmful does not rep-certainly intriguing. However, the findings are somewhat resent a challenge to the propositional approach to associ-peculiar and are open to alternative interpretation. They ative learning.are peculiar in the sense that the dissociation is not Nevertheless, there are examples of what appears to bereally between contingency awareness and the observation irrational associative learning. Karazinov and Boakesof the response (CR or reaction time). Participants know (2007) trained participants on a causal learning task withthe contingency from the start of the experiment and the a conditioned inhibition design (Xþ/XT2). Thus, X wastraining trials confirm this; the tone will be followed by followed by the outcome when presented alone (Xþ)the US on 50% of trials. The effect observed seems to be but not when it was presented in compound with themuch more a performance effect. Furthermore, the target cue (XT2). This training can give rise to inhibition;recency of CS-US pairings is perfectly confounded with presentation of T has the ability to reduce the causal attri-recency of US presentations in this experiment. The bution to another exciter, Y, on test. This seems to be aobserved fluctuation in the CR may, therefore, be due to rational inference because T prevented the outcome pro-sensitization produced by US recency alone, and not an duced by X in training, and so might prevent theassociative phenomenon at all. Perruchet’s own exper- outcome that would otherwise have been produced by Yiments (see also Weidemann et al., in press) go some on test. Karazinov and Boakes (2007) found the reverseway to ruling out this alternative explanation, but further effect, however, when participants were given little timework remains to be done. Despite these issues, Perruchet’s to think during training. Thus, participants did not learngambler’s fallacy effect remains the strongest available that T prevented the outcome, but they appeared to192 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  11. 11. Mitchell et al.: The propositional nature of human associative learninglearn that it caused the outcome. Karazinov and Boakes between A-O1O2 trials and AT-O1O3 trials, but theyconcluded that participants did not have time to reason could not remember exactly what had changed. As a con-about the relationship between T and the outcome, and sequence, they may have concluded that it was safest toso their behavior was the result of the automatic formation assume that T caused O1 and O3 equally.of a (second-order) link between T and the outcome (or Finally, in the studies Shanks (2007) refers to, partici-between T and the response of giving a high causal rating). pants may merely have been confused about the There are other related findings in the literature. For meaning of the term “probability” in the test instructions.example, Le Pelley et al. (2005a) paired cue A with two It is not at all obvious that participants would readily dis-outcomes (A-O1O2) in a first phase of training and tinguish between probability and contingency in the wayfound blocking following a second phase in which cue T that the experimenters did. Alternatively, participants inwas added (AT-O1O2); pretraining with A reduced the the non-contingent condition probably assumed thatdegree to which an association between T and the two out- there existed some other cause of the outcome. Then, oncomes was learned. This blocking was disrupted, however, test, they may have thought that the experimenter waswhen one of the outcomes changed in the second phase asking about the probability of the outcome following(AT-O1O3). Not only did participants learn to associate the cue, but in the absence of any other potential causes.T and O3 (they failed to show blocking with respect to That is, an assumption may have been made that the cuethe outcome not predicted by A), but also T and the was presented in a different context on test.unchanged outcome, O1. Le Pelley et al. (2005a) argued These alternative explanations might be argued to bethat, because learning an association between T and O1 somewhat far-fetched. However, they are presented onlyis not rational (O1 is predicted by A), and was not observed to demonstrate that irrational behavior is not inconsistentin a much simpler version of the task, the learning of T-O1 with the operation of an imperfect propositional reasoningassociation must be a result of a non-rational, automatic system cooperating with an imperfect memory system. Itmechanism. might also be argued that this position leaves the prop- Shanks (2007) presented the following phenomenon as ositional approach untestable. This is not so.the most compelling evidence of an irrational link-for- First, one can test propositional explanations ofmation mechanism in the context of contingency learning. irrational behavior empirically. For instance, if Le PelleyIn one condition, the probability of the outcome in the et al.’s (2005a) finding is due to confusion as to whichpresence of the cue (P(O/C)) was 0.75, and the outcome outcome changed between the two phases of training,did not occur in the absence of the cue (P(O/C) ¼ 0). increasing the distinctiveness of the two outcomesIn the other condition, the probability of the outcome should reduce the unblocking effect with respect to O1.both in the presence and in the absence of the cue was If the impact of contingency on probability judgments fea-0.75. Thus, although the probability of the outcome fol- tured by Shanks (2007) depends on confusion about thelowing the cue was equivalent in both cases (0.75), the instructions given on test, then the effect should beoutcome was contingent on the cue in the first condition, reduced in magnitude if these instructions leave lessbut not in the second. It has been found that judgments of room for misunderstanding. Also, presenting the test ques-the probability that the outcome will follow the cue are tion in terms of frequency (“You will see ten further trialsgreater in the former case than in the latter. Thus, the on which the cue will be present, on how many will thecue-outcome contingency appears to have an impact on outcome occur?”), rather than probability, should reducethe judgment of outcome probability, despite the fact the size of the effect (see Gigerenzer Hoffrage [1995]that this probability is identical in both cases (see De for an example of frequency formats reducing base rateHouwer et al. 2007; Lagnado Shanks 2002; Lopez ´ neglect). If, on the other hand, the participants assumedet al. 1998a; Price Yates 1993). It is irrational to give a that the test context was different from the traininghigher rating of probability when the contingency is context, then making it explicit that the cue was presentedincreased but the probability of the outcome stays the in the same context on test should eliminate the effect.same. Shanks (2007) attributed these higher probability Second, and more importantly, evidence that participantsratings to the formation of links between cues and out- are not always rational when they learn does not under-comes that have a contingent relationship. mine the main predictions of the propositional approach; We agree that these are very interesting findings, and that learning will occur only when participants are awareeach suggests that our reasoning abilities are sometimes of the cue-outcome (or CS-US) contingencies, will be dis-not optimal. However, we do not think that these findings rupted by secondary tasks, and will be affected by verbalprovide evidence for an automatic link-formation mechan- instructions, rules, and deductive reasoning processes.ism. The irrational behavior observed can equally beattributed to sub-optimal operation of the reasoning 5.3. Dissociable systems within the brainsystem.1 In each case, an explanation for the behaviorcan be given that is consistent with the propositional One could argue that a dual-system approach is supportedapproach. For example, when given little time to ponder by neurological data showing that different brain regionsover the implications of seeing Xþ and XT2 trials, are involved in different types of learning. These differentperhaps Karazinov and Boakes’ (2007) participants mista- brain regions could be seen as the neurological basis ofkenly thought that T might somehow signal the presence different learning systems. For example, there is nowof X, which itself caused the outcome. Such an inference abundant evidence that the amygdala plays an importantwould lead to the conclusion that T itself might be associ- role in, for instance, fear learning (e.g., Le Doux 2000;ated with the outcome to a greater extent than the control ¨ Ohman Mineka 2001). A quite different area of thecue. Perhaps Le Pelley et al.’s (2005a) participants knew brain, the cerebellum, has been shown to be important inthat something about the outcomes had changed conditioning of the nictitating membrane (Thompson BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 193
  12. 12. Mitchell et al.: The propositional nature of human associative learning2005). Therefore, based on such neuroscientific dissociation (1) the learning models developed within this traditionaldata, it might be argued that the amygdala is part of a fear approach (e.g., Rescorla Wagner 1972) seem parsimo-learning system that is quite separate from the system nious; (2) mental links, and the way they increase andresponsible for nictitating membrane conditioning. decrease in strength, provide a very intuitive analogy for This conclusion, however, is not necessarily correct (see neural plasticity; and (3) researchers are resistant to theHenson [2006] for a detailed discussion of the validity idea that nonhuman animals engage in propositionalof theoretical inferences based on neuroscientific dis- reasoning. We will evaluate the relative strengths andsociation data). One alternative interpretation is that weaknesses of the propositional and link-basedneither the amygdala nor the cerebellum is able to approaches with regard to these conceptual issues.produce learned behavior alone, but that they operate asindividual components in a coordinated learning system. 6.1. Simple models of learningFor instance, these brain regions might be important inprocessing specific kinds of stimuli or generating specific The first and perhaps strongest reason for learning theor-kinds of responses rather than being responsible for the ists’ adherence to the idea of a link-formation mechanismlearning process as such. Thus, the learning may take is that a range of very tightly specified theories have beenplace neither in the amygdala nor cerebellum but in developed within this approach. Theories such as thoseanother part of the brain entirely, or, indeed, in many proposed by Mackintosh (1975), Pearce and Hall (1980),parts of the brain simultaneously. A related argument Rescorla and Wagner (1972), and Wagner (1981) are for-can also be applied to the idea that the striatum and its malized, can be simulated on a computer, and can, there-dopaminergic afferents are responsible for habitual beha- fore, make precise and testable predictions. The power ofvior (Jog et al. 1999), but prefrontal areas are responsible these models comes from the fact that they often make fewfor higher-level cognition. Again, these dissociations seem assumptions but apply to a wide range of phenomena. Forto imply separate learning systems. However, they may this reason, it could be argued that these models are pre-simply reflect a single learning system solving problems ferable to the propositional approach to learning.of differing complexity or concreteness (see Chater, in The first thing that needs to be pointed out is that thepress). precision of the predictions of associative models from Although there can be no doubt that recent advances in the link-formation tradition is somewhat overstated. A lotthe neurosciences have provided a wealth of knowledge depends on the particular parameter values and the par-about the brain mechanisms necessary for learning, ticular model variant from which the predictions arethese findings are not inconsistent with the single-system derived. In fact, from experience we have learned that itview of learning. Furthermore, the available behavioral is difficult to produce a pattern of data that cannot beevidence concerning human associative learning does explained by one or the other variant of these associativenot support the view that there are multiple learning models. For example, one can explain blocking (Kaminsystems. The behavioral evidence, therefore, presents a 1969) and the opposite phenomenon, augmentationchallenge to neuroscientists to discover how a single, inte- (Batsell et al. 2001). One can also explain overshadowinggrated, propositional learning system with multiple sub- (Pavlov 1927) and the opposite phenomenon, potentiationcomponents might be implemented in the brain. (Garcia et al. 1989). For each case of competition between cues, the opposite pattern of results can be explained by postulating links (“within-compound associations”)5.4. Conclusions between the stimuli that might otherwise be in compe-To summarize the data presented in the present section, it tition (e.g., Durlach Rescorla 1980).would appear that two or three studies provide support for The notion of within-compound associations is only onethe link-formation mechanism. These are demonstrations way in which freedom is gained to explain results that areof the Perruchet effect (Perruchet 1985; Perruchet et al. not predicted by the formal versions of the models.2006) and perhaps one example of flavor-flavor evaluative Another way is to postulate different levels of generaliz-conditioning (Baeyens et al. 1990a). It is important, there- ation between cues. Schmajuk and Larrauri (2008), forfore, that these findings are subject to the closest empirical instance, added such assumptions to a variant of theand conceptual scrutiny in the future. Findings that Rescorla-Wagner model in order to explain the findingprovide evidence for irrational learning should also be that additivity pretraining can influence blockingstudied further, but they do not provide direct evidence (Beckers et al. 2005; see section 4.4). To recap, blockingagainst the propositional approach. Lastly, it is not at all is the finding that little is learned about T in a design inclear that evidence from studies of the brain can inform which Aþ trials precede ATþ trials. According to theus as to the existence of distinct learning systems. Rescorla-Wagner model, blocking occurs because, onOverall, therefore, we see no reason to postulate the exist- ATþ trials, the outcome is already predicted by A. Schma-ence of a link-formation system in addition to a prop- juk and Larrauri (2008) argued that more blocking is seenositional reasoning system. following additivity pretraining (Gþ, Hþ, GHþþ, Iþ) than subadditivity pretraining (Gþ, Hþ, GHþ, Iþþ) because learning about GH during pretraining generalizes6. Conceptual arguments to later ATþ trials. In Beckers et al.’s (2005) experiment, the AT compound can be expected to acquire more gener-There are a variety of reasons why the link mechanism has alized associative strength from GH following GHþþ pre-been so popular as an explanation for associative learning, training (the additive group) than following GHþeven in the absence of strong supporting data. In pretraining (the subadditive group). This is because thethe present section, we discuss three of these reasons: associative strength of GH is higher in the additive194 BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2
  13. 13. Mitchell et al.: The propositional nature of human associative learninggroup. In other words, participants expect the outcome to that proposes only one of those systems, no matter howa larger extent at the start of ATþ trials in the additive than parsimonious the second system might be.in the subadditive group. It follows from the Rescorla- Nevertheless, the apparent precision and parsimony ofWagner model, therefore, that less can be learned about traditional learning models might be an important reasonthe T2outcome relation (more blocking will be observed) why many researchers are not ready to give up thesein the additive group. models. It is important to realize, therefore, that adopting There are two problems with this alternative expla- a propositional approach does not imply that one must givenation. Firstly, Schmajuk and Larrauri (2008) focus on up traditional models of learning. The propositionalgeneralization between compounds (e.g., GH and AT). approach is not an alternative to specific learning modelsHowever, generalization between elements is ignored, as such as the Rescorla-Wagner model (or any of its rela-is generalization from compounds (e.g., GH) to elements tives); but it is an alternative to the dual-system approachof those compounds (e.g., G). Hence, Schmajuk and Lar- that postulates an automatic link-formation mechanism.rauri (2008) can explain the results of Beckers et al. (2005) We can clarify this argument using Marr’s (1982) distinc-only by choosing very specific and selective parameters of tion between functional and algorithmic levels of expla-generalization. It is not clear whether the model would still nation. Both functional and algorithmic models makebe able to explain the findings of Beckers et al. when more predictions about which pattern of input (e.g., learningrealistic assumptions are made about generalization trials) leads to which pattern of output (e.g., CRs orbetween different kinds of cue. causal ratings). Only algorithmic models, however, incor- Secondly, as Schmajuk and Larrauri (2008) admit, the porate assumptions about the processes and represen-explanatory power of this model is limited. There are, tations that translate the input into the output. That is,for example, other experiments presented by Beckers models at the algorithmic level make assumptions aboutet al. (2005) that the model is unable to account for, how the stimulus input is processed to produce thesuch as the effects of additivity on backward blocking, in output. The propositional approach and the automaticwhich ABþ training is given before Aþ training. To link-formation mechanism are thus clearly explanationsexplain these data, further assumptions would be required. at the algorithmic level, because they do incorporateElsewhere in the literature there are other similar effects (different) assumptions about how the input is processedthat this model cannot explain. For example, in a similar to produce the output (i.e., controlled reasoning vs.experiment to that of Beckers et al. (2005), Mitchell automatic link-formation and activation) and about theet al. (2005) showed that Gþ, Hþ, and GH2pretraining nature of the representations over which these processes(subtractivity) can also produce a strong blocking effect. operate (i.e., propositions vs. links between stimulusIn this case, the compound of two causal cues in pretrain- representations).ing (Gþ and Hþ) was non-causal (GH2). The variant of Many individual models of associative learning,the Rescorla-Wagner model proposed by Schmajuk and however, can be regarded as functional models. Take theLarrauri (2008) cannot account for blocking in this case; example of the Rescorla-Wagner model. In essence, thisit predicts very little blocking here, because the GH com- is a mathematical formula that allows one to predictpound acquires no associative strength in pretraining. In whether a CR will be observed given information as tocontrast, the propositional approach provides a straightfor- the nature of the learning trials experienced. Hence, it isward explanation for the strong blocking seen in both a functional model. It is not an algorithmic modelMitchell et al.’s (2005) subtractivity condition and because Rescorla and Wagner (1972) do not commit to aBeckers et al.’s (2005) additivity condition. Participants particular type of underlying process. Their model wasin both of these conditions can reason that T was non- developed to account for what is learned under certaincausal because the AT compound did not produce a differ- conditions. This can be contrasted with models at the algo-ent outcome (either smaller or larger) from that observed rithmic level that give an account of how this learningwhen the A cue was presented alone. takes place. In fact, Rescorla and Wagner (1972) are expli- The conclusion from the examples above seems clear. citly agnostic about algorithmic level explanations (that is,While individual models such as the Rescorla-Wagner how organisms learn and therefore why they behavemodel are quite parsimonious, the entire class of theories according to the Rescorla-Wagner model). They offerthat are assumed to describe the way in which links are two quite different algorithmic level explanations, one informed is not. Although extending models in a post hoc the language of links and another in terms of the con-manner is not, in principle, problematic, the evaluation structs of expectancy and surprise. Hence, when theof the extended model against only a single data set (for Rescorla-Wagner model is tested against other modelswhich that extension was specifically designed) is danger- such as the Pearce-Hall model, it is the fit of the math-ous. The generalizability of the new model to other data ematical formulae to the behavior that is being testedsets must be demonstrated; otherwise there is a risk that (i.e., predictions at the functional level), not the naturea different link-based model will be generated post hoc of the underlying processes or representations (e.g., auto-to account for each observed experimental result. matic formation of links or propositional reasoning). From There is also another issue related to parsimony. In this perspective, a functional model such as the Rescorla-order to account for our manifest ability to, for example, Wagner model is not incompatible with the propositionalsolve problems and play chess, traditional learning theor- approach because the two can be seen as focusing differ-ists must supplement the link-formation system with a ent levels of explanation.system that forms propositions on the basis of reasoning. In fact, from this point of view, the Rescorla-WagnerAs we argued above, these theorists are calling for a model can even be thought of as a simple mathematicaldual-system approach. No approach that needs two model of propositional reasoning, not, as is usuallysystems can be more parsimonious than an approach assumed, a model of link formation. At the functional BEHAVIORAL AND BRAIN SCIENCES (2009) 32:2 195

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