Notes � L&C VIII Emma, Hemispheric specialisation

About 98% of right handers have left hemisphere lateralisation for language functions.
About 70% of left handers have left hemisphere lateralisation fro language functions.
About 15% of left handers are strongly lateralised for right hemisphere language processing.
About 15% of left handers have bilateral representation of language in the right hemisphere.
(Stuttering is perhaps more prevalent in those left handers with bilateral representation).

Factors which affect the likelihood of language disorder after damage to LH.

Degree of R/L handedness

number/type of tasks for which you use a particular hand

familial L/R handedness

If a R hander has L handers in the family their language is less likely to be fully laterialised in the LH.

`Normal' Right hemisphere language processes

Found in the RH processing of undamaged fully Rhanded people.
1. Processing of prosody - especially emotional tone (Heilman et al. 1977).
2. Logical inferencing (Gardner et al. 1983) RH damaged people often have problems understanding jokes.
3. Only mild language problems (Joanette et al. 1983).
Perhaps the RH used as an 'additional workspace' for language processing - for complex computations.
It may also have a role in focus of attention onto linguistic tasks.

LH and RH differ structurally in terms of:

Size of localised areas ( as measured by the cytoarchitectonic structure - types of cells in each hemisphere)

The temporo-parietal association cortex is up to 7 times larger in the LH than the RH. (Galarburda et al. 1978) Folds of the brain

Planum temporale

folds back more sharply in 57/100 left hemispheres than right hemispheres.
It folds back more sharply in the RH of 18% and evenly in 25%

The Sylvian fissure is longer and more horizontal in the LH (LeMay and Culebras)
There are various handedness asymmetries notable here:

right temporal lobe is wider in 70% of right handers but in only 40% of left handers.

Alternative Non-Lingusitic Account

Left brain may not be specialised for language processing per se. but specialised for

(i) planning of motor sequences or
(ii) analytic processing.

Speech development

Within a few days post-natally the left hemisphere produces larger auditory evoked potentials to speech sounds. By 4-6 months acoustic and optic structures are linked.

Split brain (Zaidel, 1995).

Auditory comprehension in disconnected RH shows evidence of a rich vocabulary (almost full adult range)
though semantic relationships are connotative
rather than the disconnected LH strategy of denotative.
This leads to many semantic errors (thread > sew).

Short sentences of simple constructions (but including passives and negatives) can be understood.
Sentences which make memory load demands or require complex ordering are impaired.

Conclusion

The normal RH has a larger role in processing language than previously thought.
In the RH semantic processing is particularly extensive.
Auditory comprehension is better than reading.
Comprehension is better than speech.
Lexical semantic system is extensive
phonology and syntax are poor.
No grapheme to phoneme conversion
Some profiles of behaviour suggests a complex interhemispheric interaction involving mutual activation and inhibition patterns.

Speaking activates the motor areas of the brain (primary motor cortex & cerebellum) but also activates Wernicke's area(in the left posterior temporal lobe). This shows activity when subjects repeat words that have been spoken aloud to them. (Broca's area is not activated during reading aloud or repetition.

Accessing Meaning.

Subjects were asked to think up verbs which could appropriately be used with the nouns they had just seen. When the activity in 'producing speech' is subtracted from the activity involved in generating then producing speech, we can see the activity involved in accessing meaning. The left frontal cortex, particularly Broca's area , shows much activity. Wernicke's area is also activated during the generation of verbs to auditory stimuli, but not to visually presented stimuli.

Notes � Introduction to cognitive neuropsychology

Broca

Broca�s aphasia - aphasia with severe impairment or loss of speech.

Broca�s area - the region of the frontal cortex of the brain concerned with the production of speech.

Wernicke

Broca�s area vs Wernicke�s area

double dissociation

The structure of the expression of speech originates in Wernicke�s Area.� It is then conveyed through the Arculate Fasciculus to Broca�s Area.� There it elicits a detailed and co-ordinated program of vocalisations.� This program is then conveyed to the motor cortex which activates the appropriate� muscles of the lips, tongue etc.� That is, Broca�s Area mediates the programming of the speech output.

Broca's area is not activated during reading aloud or repetition.

�

Lateralisation stats + split-brain experiments

Diagram-makers + information processing

Globalists

Notes � Sacks, The man who mistook his wife for a hat

aphasia vs agnosia

President�s speech

aphasics who couldn�t understand the words, but can tell from non-linguistic signs whether you�re lying etc.

agnosic who couldn�t tell voice tone/timbre, but could look at the words themselves to see inconsistencies etc.