The Neuroscience of LanguageFrom Structure to Process
Neuroscience and Language• Aphasia (or dysphasia) – Language disorder of auditory or oral speech, writing (agraphia), or reading (alexia) produced by injury to brain areas specialized for these functions
Blood-Brain Barrier • Blood is toxic to neurons. – Makes evolutionary sense. – Insulates brain from what’s in the blood.
Paul Broca • Broca (1865) described patients who displayed halting, agrammatic speech – Content words were well preservedPaul Broca1824-1880 – Function words (i.e., adjectives, articles) impaired
Broca’s Aphasia• Patient “Tan”• Brain tumor in Left frontal brain region• Broca: Lesion disrupted speech
Broca’s Aphasia• Broca’s Aphasia – Damage to “motor images”• Language comprehension skills relatively preserved• Typically observed in patients with damage to left inferior prefrontal cortex
Broca’s Aphasia• “Yes… ah… Monday… er… Dad and Peter H… (patient’s name), and Dad… er… hospital… and ah… Wednesday… Wednesday, nine o’clock… and oh… Thursday… ten o’clock, ah doctors… two… an’ doctors… and er… teeth…yah Goodglass & Geschwind, 1976
Carl Wernicke • Wernicke (1874) described patients whose speech is fluent, but has no informational value
Wernicke’s Aphasia• Neologisms• Speech appears to have no information content• “fluent nonsense”• Preserved function words, impaired content words• Comprehension impaired• Even simple sentences not well understood• Associated with left temporal lobe damage
Wernicke’s Aphasia• “Well this is… mother is away here working her work out o’here to get her better, but when she’s looking in the other part. One their small tile into her time here. She’s working another time…” Goodglass & Geschwind, 1976
Articulatory Auditory speechspeech “memories” “memories” Geschwind 1979 Scientific American
Articulatory Auditory speechspeech “memories” “memories” Production Comprehension problems problems + empty error-filled production
Broca’s aphasia Wernicke’s aphasia Production Comprehension problems problems + empty error-filled production
Lichtheim• Lichtheim (1885) – cases of patients able to understand and produce speech but unable to repeat words• “Conduction aphasia”• Hypothesis: Broca’s area (speech production) and Wernicke’s area (speech comprehension) intact
Conduction aphasia Error-filled speech, but good comprehensionBroca’s aphasia Wernicke’s aphasia Production Comprehension problems problems + empty error-filled production
Broca’s, Wernicke’s Area, and Connections• Lichtheim’s (1885) and Geschwind’s (1965) model• Auditory input mediated by Wernicke’s area• Motor output mediated byBroca’s area• Regions connected by arcuate fasciculus
Wernicke-Lichtheim “House” Model Concept representations C Widely distributed! M AMotor word images Auditory word images Broca’s area Wernicke’s area
Lichtheim/Geschwind Model Association Cortex ConceptsVentral prefrontal Posterior Temporalcortex Motor word Auditory word Cortex Comprehension Arcuate Comprehension Fasciculus Speech motor output Auditory input
Lichtheim/Geschwind Model Concepts Association Cortex XVentral prefrontal Posterior Temporalcortex Motor word Auditory word Cortex Comprehension Arcuate Comprehension Fasciculus Speech motor output Auditory input Broca’s Aphasia
Lichtheim/Geschwind Model Concepts Association Cortex XVentral prefrontal Posterior Temporalcortex Motor word Auditory word Cortex Comprehension Arcuate Comprehension Fasciculus Speech motor output Auditory input Wernicke’s Aphasia
Lichtheim/Geschwind Model Concepts Association Cortex XVentral prefrontal Posterior Temporalcortex Motor word Auditory word Cortex Comprehension Arcuate Comprehension Fasciculus Speech motor output Auditory input Conduction Aphasia
More aphasias!Aphasia type Production Comp Repetition C1. Broca’s Non-fluent Good Poor 5 4 1 22. Wernicke’s Fluent, Poor Poor paraphasic M 3 A3. Conduction Fluent, Good Poor paraphasic 7 84.Transcortical Fluent, Poor Goodsensory paraphasic5.Transcortical Terse, Good Goodmotor echolalic6. Global Poor Poor Poor7. Aphemia Dysarthric Good Limited only by dysarthria8. Pure word Normal Very poor! As Very poordeafness if deaf!
Problems• Models assumed to map directly onto underlying brain structures• However, patients described as “Broca’s aphasics” had lesions in other areas• Inadequate psychological concepts – e.g., “loss of motor images”
Broca’s Aphasia• Damage to Broca’s area alone is not enough to produce Broca’s aphasia• Usually involves Broca’s area + surrounding areas including M1 & insula.
Wernicke’s Aphasia• Damage to Wernicke’s area alone is not enough to produce Wernicke’s aphasia• Usually involves Wernicke’s area + surrounding areas including MTG & angular gyrus.
Conduction Aphasia Common area?• Damage to the arcuate fasciculus has not been associated with conduction aphasia• Usually two lesion patterns: posterior STG (wernicke’s areas) and/or SMG
Transcortical Sensory Aphasia Common area?• Variable lesion patterns, mostly posterior to Wernicke’s area• Deficit tends to be transient evolving into anomic aphasia
Transcortical Motor Aphasia•Damage often anterior and/or superior to Broca’s area
Global Aphasia• Tend to be large “peri-Sylvian” lesions• But smaller lesions can also cause global aphasia
Pure Word Deafness• Damage to STG bilaterally is the most common pattern, although small left subcortical lesions have also been documented.
Neuropsychology Methods• Early neuropsychology (Broca, Wernicke, Luria, et al.) made inferences based largely on associations – Identify a group of patients with fairly homogeneous functional impairment – fairly homogeneous physical impairment – Identify a “syndrome”
Problems• Absence of rigid quantitative methodology of the day• Patients not always carefully described
Resurgence of Neuropsychology in the 1960s• Renewed interest in using neuropsychological cases to draw inferences about neural substrata of cognition• More powerful inferential methods• Emphasis on – Single case studies – Modern quantitative methods
Neuropsychology Methods• Modern neuropsychology (e.g., Warrington, McCarthy, Farah)• Study single cases or groups with homogeneous physical impariment• Focus on dissociations
Neuropsychology Methods• Single dissociation• Patient with lesion in brain region A• Performs well on task A• Performs poorly on task B• Inference: brain region A mediates performance on task B but not task A
Neuropsychology Methods• Problems with single dissociations• Does not rule out other interpretations• Patient may suffer some global deficit (e.g., general cognitive resource) that Task A does not require (e.g., because it is easier)• Thus, performance differences between Tasks A and B may reflect task difficulty, not the function of Brain Region A
Neuropsychology Methods• Double dissociation• Patient with lesion in brain region A performs well on Task A and poorly on Task B• Patient with lesion in brain region B performs well on Task B but poorly on Task A• Inference: Brain region A mediates Task B Brain Region B mediates Task A
Neuropsychology Methods• Since subjects are “equated” for damage, “global deficit” argument circumvented• Tasks may be assumed to be modular, depending on separate neural processes or resources
Brain Imaging • We now have techniques that allow us to visualize the structure and function of the brain.
PET • Typical brain visualized with Positron Emission Tomography.
Functional Neuroimaging - time (seconds) Deoxy Oxy• Functional magnetic resonance imaging
Functional Neuroimaging Passive word viewing -Activates visual areas -Particularly Left Occipital Cortex Passive word listening -Activates auditory areas -Words (not nonwords) particularly Left Temporal Cortex (Wernicke’s area)• Petersen et al., 1988 PET study
Functional Neuroimaging Repeating words -Bilateral activation of motor and Sensory face areas -No Activation of Broca’s or Wernicke’s Verb Generation -e.g., Cake “Eat” - Left Frontal Lobe Activation (Broca’s Area)• Confirms neuropsychology research
What Have we Learned?• What does localization tell us? – Where processing is occuring. – What is connected to what. – How many parts there are. – What happens when one part gets damaged.• We’d like to know more.
What Would we Like to Know?• How the parts work. – What are the rules and representations that are involved with language processing? – What is the nature of the processing that is taking place. • Digital/discrete/autonomous • Analog/continuous/integrated
Haagort et al. (2003)Het vrouwtje veegde de vloer met een oude bezem gemaakt van twijgen (Thewoman wiped the floor with an old broom made of twigs).*Het vrouwtje veegde de vloer met een oude kliederde gemaakt van twijgen (Thewoman wiped the floor with an old messed made of twigs).
P600 can serve as an index of frequency of a construction• The captain liked the crew was unhappy• The captain heard the crew was unhappy• The captain believed the crew was unhappy• A P600 was strongest in the first sentence, present in the second sentence, and not present in the third sentence.• Larger P600 for syntactically complex sentences (Kaan et al., 2002).
Correct: De houthakker ontweek de schroef op dinsdag (The lumberjack dodged thepropellor on Tuesday).Weak violation: *De houthakker ontweek de schroeft op dinsdag (The lumberjackdodged the propelled on Tuesday).Strong violation: *De houthakker ontweek de omdat op dinsdag (The lumberjackdodged the because on Tuesday).
Summary• The Wernicke-Geschwind model is being replaced by new models.• New models focus on both neural basis and timing of language comprehension, and have moved beyond simplistic divisions of comprehension/production.