Cerebral asymmetry and the specific functions of the

General overview of the cerebral asymmetries and the functions of the corpus callosum
Specific functions of the right hemisphere, and consequences for neuropsychological assessment
Cerebral asymmetry and
the specific functions of the right hemishere

Cerebral asymmetry
Left
hemisphere
Right
hemipshere
Corpus callosum
RH
LH

Cerebral asymmetry
“ Brain asymmetry has been observed in animals and humans structurally, functionally, and behaviorally. This lateralization is thought to originate from evolutionary, hereditary, developmental, experiential and pathological factors”
Toga & Thompson, Nature Reviews Neuroscience, 2004

Human brain asymmetries
Structural brain asymmetries
Functional brain asymmetries
Perceptual asymmetries
Motor asymmetries
Neurochemical differences
Research methods

Human brain asymmetries
Patients with unilateral lesions
Split-brain patient
Normal subjects (age, sex, hand preference)
Bilateral lesions (e.g., Alzheimerpatiënten)
Developmental disorders
Psychiatric patients
Clinical relevance

Structural brain asymmetries
Macroscopical
posterior
anterior
LH
RH
Counterclockwise torque
Sylvian fissure
LH
RH
W
B
Gebied
van Broca
Gebied
van Wernicke

Structural brain asymmetries
Microscopical
Left hemisphere: more detailed processing and expression of information
Right hemisphere: more focused on connectivity between different brain regions

Left hemisphere: dopamine (motor activation)
Right hemisphere: norepinephrine (alertness, orientation to new stimuli)
Structural brain asymmetries
Neurochemical

Functional brain asymmetry
Not a simple dichotomy
Not a modern phrenology
Is not absolute, but relative
Modular brain circuits (neuronal networks)
- subprocesses
- patterns of activations (intra- and inter-hemispheric connectivities)

Unilateral lesions of the left hemisphere
- primary perceptual and motor disturbances at the right side of the body (contralaterally)
- aphasia
- language-related disorders and verbal disorders
(alexia, agraphia, acalculia, verbal memory disorders)
- apraxia
- disturbances in temporal order / sequencing
‘conscious
awareness’
LH

Unilateral lesions of the right hemisphere
- primary perceptual and motor disturbances at the left side of the body (contralaterally)
hemi-inattention (neglect); anosognosie and denial
nonverbal information processing and nonverbal memory
(faces, visuospatial processes, environmental sounds, music (melodies, but not rhythm; LH involved by experienced musicians)
Semantic, paralinguistic and affective aspects of language (‘context’)
- emotions (‘somatic markers’); m.n. negative emotions
RH

LH patients draw the overall global form of the figure, not on local parts of the figure
Example LH-pt RH-pt
RH patients draw the local parts of the figure, but the overall global form is incorrect
Task: drawing
Constrctive apraxia

Actual social interaction
Ekman’s faces varies from anxiety to depression
Right hemisphere:
Examples of emotional and social behavior
Anxiety in animals

Functional brain asymmetries
Left hemisphere
high spatial frequency
local processing
analytic, detailed processing
Right hemisphere
low spatial frequency
global processing
holististic processing
Right hemisphere: stronger ‘connectivity’ between different brain regions than the left hemisphere

Left and Right
Left side of the body
Left hand
Left ear
Left Visual Field (LVF)
Right side of the body
Right hand
Right ear
Right visual field (RVF)
LH
RH
R-ear
RVF
L-ear
LVF

Brain asymmetries and the role of the corpus callosum
Left
hemisphere
Right
hemisphere
corpus callosum
Transfer of information from the LH to the RH, and vice versa
Inhibition of equivalent regions in the other hemisphere, and at the same time increased activation of the surrounding neurons for encoding related or contextual informaton
Language writing
reading
spatial emotions
Inter-hemispheric interaction across the corpus callosum

Split-brain patiënten

Corpus callosum
Corpus callosum
LH
RH
Visual system
Motor system
Somatosensory system
Auditory system

Split-brain patiënt
Left hand Right hand
Pre-
operative
Post-
operative
Postoperatively, the right hand performed poorly due to the disconnection of the LH (right hand) and the RH (spatial functions)

Visual
system
Optische radiatie
Colliculus superior
Lateral genuculate nucleus
Pulvinar
Primaire visuele cortex
Optisch chiasma
nasaal
temporaal
temporaal
13
13
LVV
LVV
bycicle
bycicle
Primary visual cortex
Optic radiation
Colliculus superior
Lateral geniculate nucleus
Pulvinar
Optic chiasm
nasal
temporal
temporal
LH

Visual
system
Optische radiatie
Colliculus superior
Lateral genuculate nucleus
Pulvinar
Primaire visuele cortex
Optisch chiasma
nasaal
temporaal
temporaal
13
13
Fiets
LVV
bycicle
bycicle
Primary visual cortex
Optic radiation
Colliculus superior
Lateral geniculate nucleus
Pulvinar
Optic chiasm
nasal
temporal
temporal
RH

Split-brain patient

Disconnection syndromes in patients with
lesions of the corpus callosum
(split-brain patients, partial disconnection)
‘alien’ hand syndroom (Lh)
unilateraal L-hand anomie
hemi-alexie (LV)

Perceptual asymmetries
Visual half-field method
Dichotic listening task

Lear
Dichotic listening and hemispheric asymmetry
Kimura’s structural model
LH
RH
Results: R-ear > L-ear
Presentation time:
500 msec pear stimulus pair
Rear
Bouma, 1998
7
4
1
3
2
5
LH
RH
325
741

Effects of attention on dichotic listening
(focused attention task)
Left ear
Right ear
Attended R-ear condition
Attended L-ear condition
L1
L2
R2
R1
Attended Rear condition: Recall first Rear digits and then Lear digits
Attended Lear condition: Recall first Lear digits and then Rear digits
LH
RH
R-ear
L- ear
CC

Dichotic listening: language perception and laterality
Attented R-ear condition
automatic processiing
= ‘bottom up’ process
Attented L-ear condition
Inhibitory control / EF
= ‘top down’ process
7
4
1
3
2
5
LH
RH

Dichotic listening is based on a
distributed network of
different hemispheric regions
intra-hemispheric (temporal, frontal, parietal)
inter-hemispheric (corpus callosum)
Lear
LH
RH
Rear
CC
Attended L-ear condition:
= particularly sensitive to disconnectivy

Age effects in dichotic listening
40%
70%
80%
60%
50%
90%
100%
60-70 yrs
70-80 yrs
80-90 yrs
Att R-ear
Unatt R-ear
Att L-ear
Unatt L-ear
% correct responses
corpus callosum dysfunction
RH-dysfunction (‘hemi-aging’)
attentional deficits (EF: inhibitory control dysfunctions)
Bouma, 1998

Dichotic listening in Alzheimer’s Disease
20%
50%
60%
40%
30%
70%
80%
L-ear
% correct responses
Effects enhanced in Alzheimer patients
corpus callosum dysfunction
RH-dysfunction (‘hemi-aging’)
attentional deficits (EF: inhibitory control dysfunctions)
R-ear
L-ear
R-ear
90%
10%
ATT-LE
ATT-RE
Bouma, 1998
Alzheimer
Alzheimer
Controls
Controls

Methods of cerebral asymmetry
Methods of cerebral asymmatry play an important role in measuring:
inter-hemispheric connectivity (corpus callosum)
intra-hemispheric connectiviy
Corpus callosum
LH
RH
RH > LH

Clinical applications in neuropsychological assessment
patients with unilaterale lesions
aging and dementia (e.g., Alzheimer patiënten)
patients with (partial) lesions of the corpus callosum
patients with multiple sclerose
psychiatric patients
developmental disorders

Left-right asymmetries
Handedness
Footedness
Head turning
Eye preference
Ear preference
Facial expression
Cradling

Handedness questionnaire (Van Strien & Bouma)
Writing hand (social pressure?)
1. drawing left / right / both
2, tooth-brush left / right / both
3. bottle opener left / right / both
4. throwing ball left / right / both
5. hammer left / right / both
6. (tennis)racket left / right / both
7. cutting robe with knife left / right / both
8. stirring with spoon left / right / both
9 rubbing out left / right / both
10. Striking a match left / right / both
Scoring -1 / +1 / 0 (range –10, + 10)

Handedness and language dominance
Broca’s law of contralateral dominance:
(until mid-20th century):
- In right-handers, the left hemisphere is dominant for language and the use of the preferred hand
- In left-handers the right hemisphere is dominant
Broca’s law is not supported by the data!

Handedness and language dominance
extreme left-handedness : 25%
ambidexter : 15%
- extreme right-hander : 5%
Likelihood of right-hemisphere dominance (%)
Formula: 15% - handedness score (-10 to + 10)

Left-handedness - prevalence
In the Netherlands, but also in other countries
females 9.6%
males : 11.8%
CBS (1985)

Left-handedness
a) genetic models (‘nature’)
b) developmental factors (‘nurture’)
- prenatal environment (hormonal factors: testosterone; stress)
- developmental disorders (e.g., neural tube defects, stuttering, dyslexia, schizophrenia, autism)
- birth stress (e.g., premature birth, respiratory problems)
- low birth weight
c) cultural influence (e.g., cultural pressure; costs of left-handedness)
d) evolutionary factors (benefits of left-handedness)

Sex differences in cerebral asymmetries
Male advantage:
visuospatial skills
mental rotation
perceptual closure
embedded figures
mathematical reasoning
target directed motor skills
Females advantage:
verbal skills
verbal fluency
speed of articulation
grammar
earlier language acquisition
perceptual speed
fine motor skills and rapid sequential movements

Sex differences in cerebral asymmetry
Cerebral asymmetries tend to be smaller in females than in males, presumably related to stronger inter-hemispheric connectivitiy
Higher level of testosterone during development of the brain:
leads to
reduced inter-hemispheric connectivity and
increased cerebral asymmetries