Presentation � L&C VIII Emma, Hemispheric specialisation

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.

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.

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Diagram-makers + information processing

Globalists

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

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.

Stats for lateralisation

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.
more easily recover

`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.
processing metaphorical speech (Portini, 1994) � PET � straight vs metaphorical sentences, lexical decision tasks � use strucutres in the RH much more � RH maybe used in deciphering complex meanings
metaphors, jokes etc. require more analysis than a normal phrase
propositionality � Code�s conclusion, based on Hughlings Jackson � propositional vs non-propositional � non-propositional = automatic phrases treated as single lexical entities, e.g. �good morning�, �do you have a light� � these are dealt with by the RH to free up the LH � hemispherectomy patients could still use these stock phrases
i.e. the LH is for processing areas that require more analytical/sequential thinking (+ for things you�re experienced with???)
aphasics produce expletives more often � may be because LH damage disinhibits them because of � RH connectivity to the limbic system
semantic priming by Beemen and Chiarello � pairs/sequences of words, see if lexical decision is imaffected by preceding words. there�s a right ear advantage with direct connections, but ambiguous and less obvious connections prime a left ear advantage. cross-lateral connections to the ear. the RH is more involved in ambiguous/distributed connections/representations.
classic study with dichotic listening (commisurotomy) � Kimmura � one with music � tried to see if left-handed people had RH dominance and so a left ear advantage � repeat digits presented to one ear or the other � found a difference between the ears based on handedness and hemispheric dominance � experienced musicians were LH dominant at recognising music, but non-musician was RH dominant. a non-musician follows the prosody etc with the RH.

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) � this is true of the foetus, so it doesn�t seem to be learned
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.

Misc

PET scans of normal brain activity (Posner & Raichle, 1994)

left vs right brain � there is something in the logical/creative divide, but probably not as strongly as the pop psych