The Neuroscience of Language

From 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

Nature’s Experiments

Nature’s Experiments

• Stroke – Status post 5 hours

Nature’s Experiments

• Status post 5 days.

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 preserved– Function words (i.e.,

adjectives, articles) impaired

Paul Broca1824-1880

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

Auditory speech “memories”

Articulatory speech “memories”

Geschwind 1979Scientific American

Comprehension problems

Production problems

+empty error-filled

production

Auditory speech “memories”

Articulatory speech “memories”

Wernicke’s aphasiaBroca’s aphasia

Comprehension problems

Production 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

Comprehension problems

Production problems

+empty error-filled

production

Conduction aphasia

Wernicke’s aphasiaBroca’s aphasia

Error-filled speech, but good comprehension

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

C

AM

Wernicke-Lichtheim “House” Model

Auditory word imagesWernicke’s area

Motor word imagesBroca’s area

Concept representationsWidely distributed!

Lichtheim/Geschwind Model

Concepts

Motor wordComprehension

Auditory wordComprehension

Speech motor outputAuditory input

Association Cortex

Ventral prefrontalcortex

Posterior Temporal Cortex

ArcuateFasciculus

Lichtheim/Geschwind Model

Broca’s Aphasia

Concepts

Motor wordComprehension

Auditory wordComprehension

Speech motor outputAuditory input

Association Cortex

Ventral prefrontalcortex X Arcuate

Fasciculus

Posterior Temporal Cortex

Lichtheim/Geschwind Model

Wernicke’s Aphasia

Concepts

Motor wordComprehension

Auditory wordComprehension

Speech motor outputAuditory input

Association Cortex

Ventral prefrontalcortex XArcuate

Fasciculus

Posterior Temporal Cortex

Lichtheim/Geschwind Model

Conduction Aphasia

Concepts

Motor wordComprehension

Auditory wordComprehension

Speech motor outputAuditory input

Association Cortex

Ventral prefrontalcortex XArcuate

Fasciculus

Posterior Temporal Cortex

More aphasias!

C

AM

Aphasia type Production Comp Repetition

1. Broca’s Non-fluent Good Poor

2. Wernicke’s Fluent, paraphasic

Poor Poor

3. Conduction Fluent, paraphasic

Good Poor

4.Transcortical sensory

Fluent, paraphasic

Poor Good

5.Transcortical motor

Terse, echolalic

Good Good

6. Global Poor Poor Poor

7. Aphemia Dysarthric Good Limited only by dysarthria

8. Pure word deafness

Normal Very poor! As if deaf!

Very poor

1 2

3

45

7 8

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

• Damage to the arcuate fasciculus has not been associated with conduction aphasia

• Usually two lesion patterns: posterior STG (wernicke’s areas) and/or SMG

Common area?

Transcortical Sensory Aphasia

• Variable lesion patterns, mostly posterior to Wernicke’s area

• Deficit tends to be transient evolving into anomic aphasia

Common area?

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

Aphemia

• Damage to lower motor cortex

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.

MRI

• MRI of a typical brain.

PET

• Typical brain visualized with Positron Emission Tomography.

Functional Neuroimaging

• Functional magnetic resonance imaging

DeoxyOxy

-time (seconds)

Functional Neuroimaging

• Petersen et al., 1988 PET study

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)

Functional Neuroimaging

• Confirms neuropsychology research

Repeating words-Bilateral activation of motor andSensory face areas-No Activation of Broca’s orWernicke’s

Verb Generation-e.g., Cake “Eat”- Left Frontal Lobe Activation (Broca’s Area)

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

ERP studies

• Semantics– N400

• Syntax– LAN– P600/SPS

N400 (2)

P600

Haagort et al. (2003)

Het vrouwtje veegde de vloer met een oude bezem gemaakt van twijgen (The woman wiped the floor with an old broom made of twigs).*Het vrouwtje veegde de vloer met een oude kliederde gemaakt van twijgen (The woman 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 the propellor on Tuesday).Weak violation: *De houthakker ontweek de schroeft op dinsdag (The lumberjack dodged the propelled on Tuesday).Strong violation: *De houthakker ontweek de omdat op dinsdag (The lumberjack dodged the because on Tuesday).

Combining methods

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.