1. Major Neural Substrates of Expressive and Receptive Language Processes

1. Major neural substrates of expressive and receptive language processes

- language is localized to the left hemisphere in 95% or right-handed people and in most left-handers

- about 15% of left-handed people are right hemisphere dominant for language and 15% have bilateral representation for language

- Broca’s area: opercular/triangular parts of inferior frontal gyrus; planum temporale à superior surface of temporal lobe behind auditory cortex

- Wernicke’s area: posterior superior temporal gyrus; angular/supramarginal gyri

- Broca’s area, Wernicke’s area, and their connection the arcuate fasciculus à perisylvian (located around left sylvian fissure, the lateral sulcus)

- arcuate fasciculus necessary to repeat something heard

- some aspects of language rely on extrasylvian regions of left hemisphere (and right hemisphere)

Compare/contrast areas involved in processing of sound and production of speech

Comprehension of Language

- auditory pathway involves cochlea, CN VIII, brainstem, bilaterally after cochlear nuclei à lateral lemniscus à inferior colliculus à inferior brachium à thalamus, MGN à internal capsule à primary auditory cortex; auditory info from primary auditory areas à projects to Wernicke’s area

- visual pathway for written language involves the retina à optic nerve à optic tract à LGN of thalamus à internal capsule à primary visual cortex in occipital lobe; higher level processing of written info relies on adjacent areas in the temporo-parietal association areas

Production of Language

Speech

- speech production involves moving muscles, bones, and cartilage involved in articulation, in lower airway, larynx, pharynx, and mouth; speech is the motor act that carries the linguistic message

- dysarthria refers to imprecise articulation due to muscle weakness or incoordination resulting from damage to motor areas (motor cortex, basal ganglia, cerebellum, UMN, LMN, NMJ, or muscle)

- Apraxia of speech à motor speech disorder reflecting impaired motor programming for speech that results in difficulty planning and sequencing sound

Language

- representation of conceptual knowledge in left inferior temporal lobe and left inferior frontal lobe

- ability to retrieve words or name things can be disrupted by damage to perisylvian or extrasylvian regions of the left hemisphere

- Expressive language formulation involves Broca’s area; Broca’s area critical for motor planning for speech production (verbal output) as well as syntactic (word ordering) aspects of language

- written language à network including posterior temporal lobe (spelling) and fronto-parietal regions involved in motor planning and control of the hand

2. Aphasia

- acquired language deficit characterized by impaired word selection, language production, and language comprehension

- paraphasia à incorrect word choice and sound substitution

- alexia à reading impairment

- agraphia à writing impairment

- common causes include ischemic stroke of left middle cerebral artery à blood to perisylvian cortical language areas

- extrasylvian aphasias are called transcortical aphasias; lesions here are isolated from regions involved in semantic processing and production of volitional speech

- slow onset from brain tumors or focal cortical atrophy

- transient ischemic attacks (TIA), migraine, seizures à transient aphasia

3. Aphasia classification

- based on fluency, auditory comprehension, repetition, and naming

Fluency

- fluent aphasias à good verbal output consisting of well-articulated utterances of normal length and prosody (variations of pitch, loudness, rhythm)

- fluent aphasias associated with posterior lesions (Wernicke’s) that spare anterior cortical regions for motor control of speech (Broca’s)

- nonfluent aphasias à sparse, effortful utterances of short phrase length and disrupted prosody; struggle with articulation; few nouns, ever fewer verbs; associated with anterior lesions comprising motor and premotor cortical regions involved in speech production (Broca’s)

Auditory comprehension

- usually defective in most aphasias to some degree; anterior lesions result in minor impairments; posterior lesions result in significant impairment of auditory processing

Repetition

- repetition of spoken words requires an intact perisylvian region (Wernicke’s, Broca’s, and arcuate fasciculus); lesions anywhere in perisylvian region disrupt repetition

- extrasylvian region lesions have preserved repetition, but reduction of spontaneous speech, comprehensions disturbance, or both

Naming

- anomia is word finding difficulties; can occur in isolation in anomic aphasia

- perseveration is repetition of word

- neologism is making up a word (and perhaps repeating it)

Classic Aphasia Syndromes

- look at table on next page