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Posner task results

Posner task results



We tried to expand the Posner paradigm, a framework which links attentive effects to early events of cognitive processing. ...

We tried to expand the Posner paradigm, a framework which links attentive effects to early events of cognitive processing.
In a spatial cueing task, the influence of attentive effects on the P1 component onset was confirmed. The discussion includes speculation about the underlying neural mechanisms.



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    Posner task results Posner task results Document Transcript

    • Attention modulates P1 component onset Research Report Cognitieve Neurowetenschappen K. Bangel, B. Hilhorst, K. Jagersberg, D.Portain, A. Siebold, 4/8/2009
    • ABSTRACT Posner et al proposed a three-factor model of attention addressing different cognitive and neural correlates. Following a study from Luck et al (1998), we confirmed the link between attention and P1 amplitude for P1 onset latency as well. In the current study, ERP recordings of six participants executing a variant of the Posner paradigm are investigated. P1 latency was analyzed for segments containing trials with attention paid to the left versus attention paid to right visual field. For each of both sides, slow versus the fast responses were compared, as obtained from individual reaction times. The results yield that P1 component is strongest on the side contralateral to the attended visual field compared to the ipsilateral side, suggesting that attention has a mediating effect on the P1 component. Attention might act as a mechanism that increases neural sensitivity, leading to an increased activity during stimulus detection. Additionally, attention might optimize neural pathways to fasten the response upon stimulus detection. When comparing fast RT opposed to slow RT, small differences in P1 onset latency suggest that P1 serves as an indicator of RT which can be observed before the actual response. Key Terms: Attentional modulation, Spatial Cueing Task, ERP, P1. 2
    • INTRODUCTION The shifting and focus of attention by humans is a topic that has been examined in depth within the cognitive science. Different mechanisms have been identified, such as the Simon- effect (Simon, 1969) or change blindness (Henderson and Hollingworth., 1999). Posner and Peterson's 3-factor model of attention suggests three general attentional functions incorporated by distinct regions of the brain: Orienting to sensory events (primarily processed in posterior parietal cortical areas and subcortical areas), detection of target stimuli (located in an anterior attention system) and alerting (frontal attentional systems on the right hemisphere) (Posner & Peterson, 1990). Attentional processes can be examined by means of event-related potentials (ERPs). Research on attention commonly utilizes ERPs based on recorded EEG data. This technique offers the possibility to observe ERP waveforms that are associated with attentional processes. One of the earliest components of the evoked response to a visual stimulus is the P1 component and is visible in the ERP waveform. The P1 component typically occurs 80 to 140 ms after stimulus onset and is strongest in the Parieto-Occipital areas PO7 and PO8. Luck, Heinze, Mangun and Hillyard (1989) lined out that the P1 component can be understood as reflecting a facilitation of sensory processing of items at an attended location and can therefore serve as a measure for the presence and absence of attention. It their experiment, differences in amplitude were observed in the P1 component for attended stimuli and unattended stimuli when subjects performed a stimulus detection task. It is possible that not only the amplitude of the P1 component can be facilitated by attention but its latency as well. In that case, visual attention directed at one side of the visual field may 3
    • result in an earlier onset of the P1 component for attended stimuli compared to unattended stimuli. The task used in the study by Luck et al. (1989) required the subject to perform a visual search for a target stimulus from a group of stimuli. As we are interested in latency differences of the P1 component we use a task that omits this search and only requires stimulus detection, the Posner task. In this experiment subjects perform an altered version of the Posner spatial cueing task paradigm. Subjects are asked to attend to either the left of the right side of a computer screen as indicated by a visual cue. After a short interval two stimuli appear, one on each side of the screen. The subjects have to respond to the orientation of the stimulus (which is either horizontal or vertical) on the attended side with a corresponding key press. Since we are interested in attention influencing the P1 component, the task does not involve invalid cueing, unlike the original Posner task. Stimuli will be presented bilaterally in each trial. As the stimuli will be presented in different visual fields, each stimulus will be processed in the corresponding hemisphere, hence each trail two different P1 components will be measures (unattended and attended). Both reaction times and brain activity are measured by a computer and EEG measurement requipment. ERPs will be constructed using the EEG data and will be examined on their amplitude and latency. We expect to observe amplitude differences in the P1 component of attended and unattended stimuli in conformity with the study by Luck et al. (1989). We also expect to see a latency difference of the P1 component onset as well for attended or unattended stimuli. Finally we also observe behavioral data (response times) in order to examine if the Posner task may be subject to a Simon effect as the mapping of the response keys in relation to the target stimulus can be either congruent or incongruent. If this effect is present, it may 4
    • influence the P1 component since it is attention-related. We expect the Simon effect to be present, possibly to a lesser degree as attention is directed by the cue. 5
    • METHODS P ARTICIPANTS EEG recordings were made from two female and four male students aged between 21 and 28 (mean 23.8), following a practical EEG recording course at the University of Twente. All participants were right handed, not under actual medical treatment and had no history of psychiatric or neurological disorders. All participants had normal or corrected to normal vision, and accurate hearing abilities. One student did not have intact color vision and two participants were excluded from analysis due to procedural errors. All participants gave informed consent and the experiment was approved by the local ethics committee. Figure 1. Example of the stimuli and their temporal order as employed in the experiment. 6
    • STIMULI , APPARATUS AND RECORDING PROCEDURE All stimuli were presented on a black 17’’ computer screen. During each trial a small white fixation circle was continuously presented in the centre of the screen, attended by two white circles located at 12.2° to the left and right of the fixation dot. After 700ms showing the default display the fixation dot changed into a bigger dot for 400ms, again the default display appeared for 600ms after which the cue (two opposing red and green triangles, each pointing to one of the circles) replaced the fixation point for 400ms. Next, the default display was presented again for another 600ms followed by the target, presented in one of the circles (see Figure 1). Targets consisted either of a horizontal or a vertical black line appearing for 200ms within one of the white circles. The default display was shown again for 1800 ms after target onset. Participants were seated in front of a screen at a distance of about 70 cm in a silenced and partly darkened chamber. The software Presentation (version 11.0) was used for stimulus presentation. D ATA ACQUISITION EEG data were recorded using 12 Ag/AgCl ring electrodes placed on a standard 10/10 cap. The channels were recorded at the specific locations: (F3 Fz F4 C3 Cz C4 P3 Pz P4 PO7 Oz PO8). Electrode impedance was held below 5 kΩ. Button triggers, EEG en EOG data were amplified by a Quick-Amp (BrainProducts GmbH) and recorded at a sample rate of 1000 Hz with BrainVisionRecorder (Version 1.4). EOG were measured above and below the left eye, and horizontally on the outer canthi of both eyes to determine the vEOG and the hEOG. 7
    • T ASK AND PROCEDURE Participants were instructed to keep their eyes on the fixation dot while performing 320 trials, divided into four blocks. At the beginning of the first two blocks, participants were instructed to pay attention to the direction of the green arrow, pressing the left CTRL key whenever the indicated circle was filled with a vertical line and pressing the right CTRL button when the indicated circle was filled with a horizontal line. Following 20 practice trails, they completed two blocks of each 80 randomized experimental trials varying in color (green vs. red) and direction (left vs. right) of arrow cue and type (horizontal vs. vertical) and location of target cues. Responses with the corresponding hand were regarded as correct response, whereas responses with the non-corresponding hand were regarded as false response. Prior to the start of the third and forth block instructions were chanced. Participants were then asked to pay attention to the direction indicated by the red arrow. D ATA ANALYSIS Behavioral data was analyzed by one-way-ANOVA. RT were splitted into two conditions: “congruent” and “incongruent”. To account for the Simon effect, the congruent condition was specified as directing attention to the same spatial side as the desirable button response. Accordingly, trials including button responses after attending to contrary side were specified as incongruent conditions. EEG data were digitally filtered (TC = 0 s, high-cutoff filter of 200 Hz, notch filter of 50 Hz) by Brainvision Recorder. Using the median of all RT for each participant, our data covered by the time window of interest between -100 and 250ms from stimulus onset were distinguished into 8
    • a slow and a fast condition. We determined the baseline from -100 to 0 ms before the stimulus was presented. Reactions which occurred 100ms after target presentation were regarded premature and rejected. Also rejected were segments showing false responses. Segments were then removed from EOG artifacts by rejecting horizontal EOG amplitudes greater than 60µV and vertical EOG amplitudes greater than 120µV. Subsequently, EEG artifacts were removed by rejecting segments with amplitudes greater than 100µV. To determine which side of the screen was attended, we distinguished the remaining EEG data into two halves representing attentive processes on either the left or the right target. After creating a grand average over all subjects, ERP data was filtered through a lowpass of 16 Hz to further enhance the signal-to-noise-ratio. After selecting the most promising electrodes (PO7 and PO8) for further analysis, we compared the segments containing trials with attention paid to the left versus attention paid to the right screen side. For each of both lateral sides, we compared the slow versus the fast condition. 9
    • RESULTS If participants showed more than 5% incorrect trials on basis of their behavioral data, they were excluded from further analysis. This criterion applied for one participant, leaving 4 participants with usable data for further analysis. EEG rejection criteria ultimately lead to the exclusion of 14.4% from all trials. BEHAVIORAL DATA The analysis of reaction times did not yield any significant results. In particular there were no significant differences between trials in congruent and incongruent conditions (F<1; p>0.05). EEG DATA LATERAL COMPARISON Regarding both sides of the scalp, we observed a difference in P1 latency between conditions in which attention was paid to the left side versus attention was paid to the right side. In both cases, activation of the contralateral electrode showed a significant shorter latency (by about 8 ms) than the activation of ipsilateral electrode, as can be seen in Figure 2. The difference in amplitudes regarding P1 components was insignificant. 10
    • Figure 2. Comparison of P1 components in the “attend left” and “attend right” condition for the electrodes P7 (top) and P8 (bottom) RESPONSE SPEED COMPARISON Only results regarding the right visual field yielded a significant shorter onset delay for the P1 component in the fast condition. The difference in amplitudes indicated a weaker response for the slow condition, compared to the fast condition (Figure 3 and Figure 4). These effects were significant for both hemispheres. 11
    • Figure 3. Comparison of P1 component for fast (straight line) vs. slow responses (dotted line) in the “attend left” condition for the electrodes P7 (top) and P8 (bottom) Figure 4. Comparison of P1 component for fast (straight line) vs. slow responses (dotted line) in the “attend right” condition for the electrodes P7 (top) and P8 (bottom) 12
    • DISCUSSION The results obtained in this experiment suggest that attention has a mediating effect on the P1 component when subjects perform the Posner task. This mediating effect manifests in two ways: when comparing the P1 component on locations PO7 and PO8, we can see that the amplitude of the P1 component is strongest on the side contralateral to the visual field that the subject is attending to. It may be possible that attention acts as a mechanism that increases sensitivity for a stimulus for a specific region of the brain, in this case either PO7 or PO8. This increased sensitivity may lead to an increased neural response when a stimulus is detected, with more neurons firing after stimulus detection resulting in increased amplitude of the P1 component. Second, the faster onset of the P1 component for attended stimuli versus unattended stimuli may suggest that attention even serves as a mechanism that optimizes neural pathways in order to fasten the response upon stimulus detection. When comparing the P1 components of fast reaction times opposed to slow reaction times for either visual field, there are some small differences in the onset of the P1 component. This suggests that before the actual response the P1 component serves as an indicator of the reaction time of the response. Only did this effect occur for the condition that subjects were attending to the right visual field. This can be explained from research on hemispatial neglect patients, indicating an attentional bias for the left visual field by default (eg. Kinsbourne, 1987), resulting in less noticeable differences between the P1 components of fast or slow response times. Analysis of the response times did not provide evidence that responses were faster when a subject was presented a congruent stimulus compared to incongruent stimuli. As such there is no Simon-effect (Simon & Rudell, 1967) present in the Posner task, most likely due to the 13
    • presence of a cue that directs attention to the relevant visual field before the stimulus is shown. Some considerations have to be made when drawing conclusions from the results of this experiment. First, the experiment only featured the data of four test subjects out of a total of six. Certain effects may be more apparent when there are more subjects participating in the experiment. One of those effects is the LRP, which was not clearly visible for both the stimulus-locked and response-locked waveforms. The same is true for the analysis of the P1 component for the fast and slow responses when subjects are attending the left visual field. Second, for analyzing the P1 components for fast and slow responses, all responses were divided into either “fast” responses (all responses faster then the median response time) or “slow” responses (all responses slower then the median response time). A different technique features the division of the response times into three groups rather than two and using the fastest group as “fast” responses and the slowest group as “slow” responses. Such a division may have yielded more pronounced effects. 14
    • FUTURE RESEARCH Following from this study there are some recommendations regarding possible future research possibilities. As has been stated before, there are indicators that the P1 component can be influenced by the attentional state of the subject. First, a replication study featuring can be conducted featuring more subjects. With more subjects it is possible to test the effects for significance and draw solid conclusions regarding the indicators of this study. Another topic for future research is to explore to which extent the preparation interval can influence the P1 component. It seems likely that some preparation is required on the neural level when a subject directs his or her attention to a particular visual field and that this preparation takes some time. In this study the time delay between the cue and the stimulus was constant so the preparation time was the same for every trial. It is, however, possible that the length of the preparation interval determines the degree of neural preparation and that adjusting the length of this interval may influence the onset and amplitude of the P1 component. 15
    • REFERENCES Henderson, John M.; Hollingworth, Andrew (1999), The role of fixation position in detecting scene changes across saccades, Psychological Science 10, 438-443. Kinsbourne, M. (1987). Mechanisms of unilateral neglect. In M. Jeannerod (Ed.), Neurophysiological and neuropsychological aspects of spatial neglect. Amsterdam: Elsevier. Luck S. J., Heinze H. J., Mangun G. R., Hillyard S. A. (1989). Visual event related potentials index focused attention within bilateral stimulus arrays. 2. Functional dissociation of P1 and N1 components. Electroencephalography and Clinical Neurophysiology, 75, 528–542. Posner M. I., Peterson S. E. (1990). The attention system of the human brain. Annual Review of Neuroscience, 13, 25-42. Simon, J. R. (1969). Reaction toward the source of stimulation. Journal of Experimental Psychology, 81, 174- 176. Simon, J. R. & Rudell, A. P. (1967). Auditory S-R compatibility: the effect of an irrelevant cue on information processing. Journal of applied psychology, 51, 300-304. 16