hemispheric specialisation ref. banich, ch 4, pp. 113-130

Hemispheric Specialisation

ref. Banich, Ch 4, pp. 113-130

Examples of "popular" view•“Right-brained people This means that the right side of your brain is your dominant side. Usually this is the case with most left-handed people... Right-brained people are usually very good problem solvers and much more creative… Often they are also very visual, learning better by visual images rather than auditory instruction…”

•“Left-brained people This means that the left side of your brain is dominant. Many times a left-brained person is right-handed... Although left-brained people are not quite as creative, but are much more logical or analytical than their right-brained counterparts. Many times, these individuals are better at science and math…”

•“...as high as 65% of students are now right-brained, as opposed to back in the 40’s and 50’s when that percentage was left-brained.”

• from http://kyky.essortment.com/amirightbrain_opr.htm

Examples of "popular" view

• “Biologists figured out years ago, that the left hemisphere of the brain is the seat of most logical thought, and the right half of the brain is where most creativity occurs...”

• “…In most people, the two halves of the brain have difficulty passing information back and forth. Scientists discovered this by studying head-trauma patients. In most people, the left half of the brain is jealously dominant. This, also, has been shown by studying head-trauma patients. These two principles coordinate to insure most people in our society are quite logical, and not very creative.”

• from http://www.mitra.net.id/business/course/171098.htm

Examples of "popular" view

• “… Betty Edwards, director of the center and author of the best selling book "Drawing on the Right Side of the Brain," insists anyone can learn to draw provided they use their right brain functions, as opposed to the left.... Edwards realized that those who could draw were using images formed in the right side of the brain and those who could not were attempting to draw from the logical left side.”

• “… Edwards’ first step in teaching students to draw from the right side of the brain begins by having students recreate a picture while the picture is upside down..”

• from http://www.acs.csulb.edu/~d49er/Issue28/28nbrain.html

Hemispheric Specialisation: Methods of Studying

1. Individuals with unilateral lesions

2. WADA Technique

3. "Split Brain" patients

4. Lateralised presentation (e.g. visual half-field technique)

1. Individuals with Unilateral Lesions

• Compare effects of damage to RH and LH

Does damage result in different types of Does damage result in different types of impairments in each case?impairments in each case?

Left hemisphere damage

•Bill Reiger had been a rising star in high school - academically talented and a top athlete. But then his mother died unexpectedly. Confused by her death, he turned down a scholarship to college and joined the army. During a combat mission in Vietnam, he was hit by shrapnel that damaged the left hemisphere of his brain. When asked to tell his story, he said:

•"My mother died...uh... me... uh fi'tenn. Uh, oh, I guess six month... my mother pass away. An'uh... an'en,,, un... ah... seventeen... seventeen... go... uh High School. An uh... Christmas... well, uh, I uh... Pitt'burgh."

• Goodglass 1976, p.239

Right hemisphere damage

•Thirty years ago, Lincoln Holmes was in a car accident that rendered him completely "face blind". "In those moments when I am suddenly alone, and I don't know where anybody that I am with is, there can be a surge of fear, and it is lonely in that sense" When shown a series of slides of inanimate objects, he is able to identify them correctly - but finds it completely impossible to recognise a picture of Marilyn Monroe. Even when shown a picture of himself, he has to be prompted before he realises he is staring at his own image. "For me it is a face, it is not my face, and there is some sense of incompleteness there. So be it." "When I am asked by people, 'do faces all look the same?', the answer to that question is 'no' - they don't all look the same, but none of them look like anyone."

• Adapted from the BBC TV programme "Brain Story"

Right hemisphere damage

Poor on visuospatial tasks e.g. Block designPoor on visuospatial tasks e.g. Block design

Other findings

• More often affected after LH damage:

More often affected after RH damage:

Language (90%) Face processing

Reading & writing Visuospatial tasks

ArithmeticArithmetic

Complex movements Complex movements e.g. opening e.g. opening cancan

Melody (“amusica”)Melody (“amusica”)

IntonationIntonation

Limitations

• - Can’t compare R/L hemispheres in

same person

• - Can’t get data on small sub-samples

(e.g. left-handers)



2. WADA Technique



2. The WADA Technique

• Sodium Amytal

injected into carotid

artery

• Anaesthetises one

hemisphere

• Creates temporary

unilateral "lesion"

The WADA Technique (cont)

• Example: Language localisation in r. and l. handers

LH Bilateral RH

Right handers 96% 0% 4%

Left handers 70% 15% 15%

Total 84% 7% 9%

can compare hemispheres in same personcan compare hemispheres in same person

good for studying "unusual" subgroupsgood for studying "unusual" subgroups



2. WADA Technique



3. Split Brain Patients

• Corpus callosum is cut

• LH/RH intact, but don't communicate

The Split Brain Syndrome

•Patients who have undergone this procedure recover to perform at a normal intellectual and social level. In fact, they may be totally unaware of having a deficit. One patient, WJ, was described as "living happily in Downey, California, with no sense of the enormity of the findings or for that matter any awareness that he had changed."

•Nevertheless, these patients have some unusual traits…. They sometimes give evidence of having two differing minds. For example, one patient found his left hand struggling against his right hand when trying to pull up his pants in the morning. While the right hand tried to pull them up, the left was trying to pull them down. On another occasion, he was angry with his wife and attacked her with his left hand while simultaneously trying to protect her with his right!

• From Gazzaniga et al (1998) Cognitive Neuroscience

Split Brain Studies• Present stimulus to one hemisphere only:

Stimulus processed by Stimulus processed by ______ hemisphere______ hemisphereleftleft

Which hand could he use? Which hand could he use? Left right or both?Left right or both?

NoNo

YesYes

Left onlyLeft only

If ball presented on If ball presented on leftleft side of screen:side of screen:

Could P name the item?Could P name the item?

Could he pick the Could he pick the right object? right object?

Split Brain Studies (cont.)

•

E:E: “What was it?”“What was it?”““What goes on it?" What goes on it?"

P:P: “I don't know."“I don't know."

E: E: “Can you draw it?”“Can you draw it?”

Split Brain Studies (cont.)

• Can demonstrate asymmetries in other domains:Can demonstrate asymmetries in other domains:

Other findings

• LH can produce and understand all words, even the most complex sentences

• RH can’t produce speech, but understands some concrete words, simple sentences

• BUT Beware: split-brain P's may have different brains premorbidly

Language Tasks:Language Tasks:



2. WADA Technique



4. Lateralised Presentation in Normals

Hemispheres communicateHemispheres communicate

BUT first hemisphere to process has BUT first hemisphere to process has advantage (e.g. accuracy, RT)advantage (e.g. accuracy, RT)

Example: Visual half-field techniqueExample: Visual half-field technique

+ CAT

Presentation is brief (< 200msec). Why?Presentation is brief (< 200msec). Why?

Lateralised Presentation (cont.)

Applications:Applications:

word recognition tasks word recognition tasks e.g. lexical decisione.g. lexical decision

face processing face processing e.g. same-different matchinge.g. same-different matching

Can use in other modalitiesCan use in other modalities

e.g. dichotic listeninge.g. dichotic listening

RH advantage for musicRH advantage for music

LH advantage for wordsLH advantage for words

Lateralised Presentation (cont.)

normals, no pre-existing conditionsnormals, no pre-existing conditions

can be performed anywhere, anytimecan be performed anywhere, anytime

Advantages:Advantages:

Drawbacks:Drawbacks:

what do the RT differences what do the RT differences actually meanactually mean??

Theories of Hemispheric Specialisation

A. Type of Material: verbal vs. visual

B. Nature of Processing: analytical vs. holistic

Analytical vs. Holistic theory: Evidence

• 1. Split-brain studies: visual matching tasks

Match by visual similarity

= RH advantage

Match by function

= LH advantage

Target pictureTarget picture

• 2. Normals:


Discrimination of grossly diff. faces = RHDiscrimination of grossly diff. faces = RH

Discrim. of faces differing by one feature = LHDiscrim. of faces differing by one feature = LH

• 3. Patients with unilateral brain damage


LH damage (relies on RH)LH damage (relies on RH) RH damage (relies on LH)RH damage (relies on LH)

"Hierarchical" stimuli:"Hierarchical" stimuli:

Alternative models

• Kosslyn: language-driven specialisation:

• * Language specialisation is cause of asymmetries

• * Other asymmetries reflect categorical nature of language

Dangers of "dichotomies" -> circularityDangers of "dichotomies" -> circularity

hemispheric specialisation ref. banich, ch 4, pp. 113-130

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