Left Brain | Right Brain Introduction & Historical Overview - - PowerPoint PPT Presentation

Left Brain | Right Brain Introduction & Historical Overview

Janet Hsiao Cog Sci 200

Outline of Historical Overview

! Asymmetry in the damaged brain ! Splitting the brain ! Asymmetry in the normal brain ! In what way do the hemispheres differ? ! Future research

Human Brain

! Symmetrical appearance = symmetrical

functions?

Asymmetry in the damaged brain

! Correlation between speech disturbances

and paralysis of right half of the body.

! Two sides (18xx)

– Doctrine of cerebral localization

(Franz Gall): Phrenology: shape of skull reflects underlying brain tissue and mental characteristics.

– Holistic processes.

Asymmetry in the damaged brain

! The findings of Paul Broca (1861, a meeting of the

Society of Anthropology in Paris)

– Postmortem examination

• f a patient’s brain

(loss of speech): left frontal lobe.

Asymmetry in the damaged brain

! A turning point:

– “I have been struck with the fact that in my first aphemics

(aphasics) the lesion always lay not only in the same part of the brain but always the same side – the left. Since then, from many postmortems, the lesion is always left sided. One has also seen many aphemics alive, most of them hemiplegic on the right side. Furthermore, one has seen at autopsy lesions on the right side in patients who had shown no aphemia. It seems from all this that the faculty of articulate language is localized in the left hemisphere, or at least that it depends chiefly upon that hemisphere” – Broca, 1864)

– How about Mark Dax?

Asymmetry in the damaged brain

! Broca’s rule:

– Both speech and manual dexterity are attributable

to the inborn superiority of the LH in right-handers.

– How about left-handers?

Asymmetry in the damaged brain

! Cerebral dominance – LH is the “leading”

brain?

– Jackson: LH is the so-called “will” and RH is the

automatic side.

– Different language problems resulted from

damage to different areas within the LH. e.g. Wernicke area.

– Apraxia: inability to perform purposeful

movements – LH injury.

Asymmetry in the damaged brain

! Discovery of RH abilities:

– Jackson: Right posterior lobe for visual ideation – Psychological tests:

! LH damage:

poor in verbal ability

! RH damage:

poor in visual spatial abilities (geometric figures, form, space, distance relationships …) e.g.

Asymmetry in the damaged brain

! Problems of inferring brain function from

brain damage:

– Logic of associations and dissociations. – Individual variations in performance. – Complex cerebral interactions. – The brain compensates the injury. – What should we do? Combine knowledge in

different ways …

Splitting the brain

! The first split brain operations on humans:

– Wagenen, neurosurgeon from Rochester, New

York, performed on patients with epilepsy in 1940s, attempting to stop spread of epileptic discharge between hemispheres " didn’t really work, unfortunately ....

Splitting the brain

! Sperry & Myers’ discoveries in cats

– If completely severed,

Visual tasks trained with one eye did not transfer to the

• ther.

Splitting the brain

! Complete commissurotomy for intractable

epilepsy (Vogel & Bogen)

– Successfully reduced seizure activity while left

patients unchanged in personality, intelligence, and behavior in general.

– BUT! Not that simple … (Sperry, Nobel prize laureate)

Splitting the brain

! Testing with a split brain

Splitting the brain

! Everyday behavior after split-brain surgery

– Acute disconnection syndrome: mute, difficult to

control left side of the body, “splitting headache”.

– In some rare cases, their left struggles against

right " commissure transmission is inhibitory in nature.

– They have difficulty:

! Associating names with faces. ! Solving geometry problems. ! "coordination between the two hemispheres.

Splitting the brain

! Language processing in the RH

– Moves from a printed word to its meaning without

phonological decoding.

– May have auditory lexicon but not visual lexicon. – May be underestimated "Speech following LH

injury may be a result of RH competencies and the loss of complex interaction between LH and

• RH. (Zaidel)

Splitting the brain

! Visuospatial functions in the RH and LH

– Right hand (LH) performs much worse than left

hand (RH) for tasks involving complex visual and spatial processes.

Splitting the brain

! Different information processing: analytic (LH)

• vs. holistic (RH) (Levy)

– LH and RH differ in kinds of information they pick

up from visual stimuli.

Splitting the brain

! Question: how can the two separate

hemispheres act as a unit during the everyday activities of these patients?

Splitting the brain

! Subcortical pathways can mediate the

direction of visual attention (superior colliculus), although additional information is very limited.

– Cueing effects between

the two hemispheres.

Splitting the brain

! Insights from the study of split-brain patients:

– Hemispheric specialization is not an all-or-none

phenomenon but represents a continuum:

! LH and RH differ in both approach and efficiency.

– Epilepsy may have produced changes in the

patient’s brain; there may also be compensation after surgery " need to combine different approaches.

Asymmetry in the normal brain

! Techniques to study the normal brain

– NeuroImaging: building the link between

psychology and physiology

! fMRI, EEG, ERP, MEG ! There is little evidence to support the notion that either

• ne or the other hemisphere turns on to perform a

specific task all by itself.

– Behavioral studies:

! Divided visual-field study ! Dichotic listening

Asymmetry in the normal brain

! Divided visual-field study

– Assumption: Performance will be superior when a

stimulus is initially presented to the hemisphere specialized in the processing.

! RVF/LH advantage for visual word recognition in English.

pink

Asymmetry in the normal brain

! Dichotic listening (Kimura, 1967)

– Ipsilateral pathways are weaker, less numerous,

and slower than contralateral fibers " right ear advantage.

Asymmetry in the normal brain

! Why does lateralized presentation result in

asymmetric performance?

– Direction access model: information will be

processed by the hemisphere that first receives it.

– Callosal relay model: information will always be

processed by the hemisphere best equipped to deal with it.

– Attentional advantage model: explains the role of

attentional bias.

In what way do the hemispheres differ?

! The nature of information: verbal vs. non-

verbal

– LH: language related or verbal – RH: faces, spatial arrays of dots, sound & music .. – It seems to be over-simplified.

In what way do the hemispheres differ?

! The information processing approach:

analytical vs. holistic processing

– Divided field study of Kana & Kanji in Japanese

writing system.

In what way do the hemispheres differ?

! Representation of information: high vs.

low spatial frequency (Sergent, 1986)

– LH is more sensitive to high-spatial frequency whereas RH

is more sensitive to low-spatial frequency. They are equivalent in processing spatial frequency information at a sensory level; differences emerge at a later stage.

– Dynamic view of asymmetry (Gabowska & Nowicka, 1996)

! Early stage: RH is superior for whole frequency range. ! Later stage: RH and LH bias information to low and high

frequency range.

Summary

! Asymmetry in the damaged brain ! Splitting the brain ! Asymmetry in the normal brain ! In what way do the hemispheres differ?

Future research

! How do the two hemispheres interact with each

• ther?

– Direct access or callosal relay? – How does the brain combine the information in the two

visual hemifields?

! Must there be a single dichotomy?

– The underlying organizing principles are still unclear …

! Why does the asymmetry exist? ! Individual differences?

– Handedness, sex, etc. … Is degree of asymmetry related to

learning performance?

References

! Springer & Deutsch. (1997). Left Brain, Right Brain: Perspective

from Cognitive Neuroscience.

! Ivry & Robertson (1999). The two sides of perception.