Task 2: Symmetry and disconnection syndromes
Learning goals:
1. Are there anatomical differences between left and right?
2. How can lateralization of function be demonstrated?
3. Are there functional differences between left and right hemisphere?
4. Which elements can affect symmetry?
5. How are the left and the right hemispheres connected?
6. What happens after this connection?
PART 1: asymmetry and lateralization___________________________________________________
Kolb, B. & Whishaw, I.Q. (2015). Fundamentals of Human Neuropsychology (7th Edition) H11
11.1: anatomical asymmetries in the human brain
Laterality: the idea that the 2 cerebral hemispheres have separate functions, leads to the notion that
two different minds control our behaviour.
Left hemisphere plays a crucial role in producing and understanding language and in
controlling movement on the right side of the body.
Right hemisphere specializes in perceiving and synthesizing nonverbal information, including
music and facial expression, and controls movement on the left side of the body.
Cerebral asymmetry
MRI scans of living brains confirm eight major anatomical
differences between the 2 hemispheres:
1. The right hemisphere is slightly larger and heavier
than the left, but the left contains more gray matter
relative to white matter.
2. The marked structural asymmetry of the left and right
temporal lobes may provide an anatomical basis for
their observed specialization in language and in music
functions, respectively.
3. The anatomical asymmetry in the temporal lobes’
cortex correlates with an asymmetry in the thalamus.
The left thalamus is dominant for language functions.
4. The slope of the lateral fissure is gentler on the left
hemisphere than on the right. The region of the
temporoparietal cortex lying ventral to the lateral
fissure therefore appears larger on the right.
5. The frontal operculum (Broca’s area) is organized
differently: the area visible on the brain surface is about one-third larger on the right than on
the left, whereas the area of cortex buried in the region’s sulci (ridges) is greater on the left
than on the right. This anatomical asymmetry probably corresponds to the lateralization of
these regions, the left side affecting grammar production and the right side possibly
influencing tone of voice (prosody).
6. The distribution of various neurotransmitters is asymmetrical in both the cortical and the
subcortical regions. The particular asymmetries in the distribution of ACh, GABA, NE, and DA
depend on the structure under consideration.
7. The right hemisphere extends farther anteriorly than does the left, the left hemisphere
extends farther posteriorly than does the right, and the occipital horns of the lateral
ventricles are five times as likely to be longer on the right as on the left.
1
, 8. Analysis of cortical surface area
imaged in 69 brains that were
combined into a single population-
averaged brain reveals an
unexpectedly broad pattern of
asymmetries not visible in individual
brains. The largest asymmetries favor
the left hemisphere in the Sylvian and
medial temporal regions and the right
hemisphere in the posterior parietal
and dorsolateral prefrontal areas
(Figure 11.2A), but an overall pattern
of asymmetry was evident across the
hemispheres (Figure 11.2B).
The largest anatomical asymmetries
enter on the temporoparietal
language areas. These asymmetries
are present before birth supports
the statement that language is innate
in humans. People speculate that asymmetries evolved to subserve (dienen) language.
Neuronal asymmetry
Scheibel and colleagues (1985) compared the
dendritic fields of pyramidal cells in Broca’s area,:
the left frontal operculum (LOP), with
those in the facial area of the motor
cortex in the left precentral cortex (LPC)
and with homologous regions in the right
hemisphere.
Their results show that neurons in each region
have distinct patterns of dendritic branching degree or pattern of branching is important, because
each branch is a potential location for enhancing or suppression graded potentials. Note that the
neurons in Broca’s areas (LOP), have far more abundant branches (overvloedige takken) than in
other areas.
Genetic asymmetry
Tao Sun and colleagues (2006) compared gene expression levels in the perisylvian regions of the left
and right hemispheres of the fetal (foetale) brain. They found genes that are expressed differently
in the two hemispheres predict that epigenetic changes may differentially influence the two
hemispheres. The mechanism is unknown, but an idea is that the asymmetrical expression of genes
may account for functional properties such as handedness, which to date has no clear- cut
(duidelijke) basis.
11.2: asymmetries in neurological patients
Patients with lateralized lesions
The oldest research on hemispheric specialization infers function from behavioural deficits that arise
as a result of strokes or surgery. The study of patients demonstrates that the functions of the two
hemispheres are lateralized, or dissociated. To conclude that the cortical area has a special or
lateralized function, however, it is also necessary to show that lesions in other brain areas do not
produce a similar deficit.
2
, The strongest experimental method for demonstrating lateralization of function is called double
dissociation (Teuber, 1955): 2 neocortical areas are functionally dissociated by 2 behavioural tests.
Performance on each test is affected by a lesion in one zone but not in the other.
Left-hemisphere lesions in right-handed patients consistently produce deficits in language
functions (speech, writing, and reading) that are not produced by right-hemisphere lesions.
Thus, the two hemispheres’ functions are dissociated.
Performing spatial tasks, singing, playing musical instruments and discriminating tonal
patterns are disrupted more by right- hemisphere than left- hemisphere lesions.
Because right-h lesions disturb tasks not disrupted by left- h lesions, and left- h lesions
disturb task not disrupted by right- h lesion, the two hemispheres are doubly dissociated.
A similar logic is used to localize functions within a hemisphere.
o In table 11.2 two hypothetical cortical regions, 102 and 107,
are doubly dissociated on tests of reading and writing: damage
to area 102 disturbs reading, whereas damage to area 107
impairs writing. In principle, this logic can be extended to
dissociate the functions of additional areas concurrently by
triple dissociation, quadruple dissociation, and so on.
Case with 2 patients:
P.G had seizures and neurological investigations revealed a tumor in the anterior part of the
left temporal lobe. Preoperative psychological tests showed P.G. to be of superior
intelligence, with the only significant deficits being on tests of verbal memory. Two weeks
after surgery, psychological testing showed a general decrease in intelligence ratings and a
further decrease in the verbal memory scores. Performance on other tests, including tests of
recall of complex drawings, was at normal.
S.K. had a tumor removed from the right temporal lobe. Preoperative testing of S.K. showed
a low score on recall of complex drawings. Two weeks after surgery, repeat testing showed a
marked decrease in performance IQ rating and a decline in the nonverbal memory score,
both for simple and for complex designs.
Clear example of double
dissociation: subsequent to Begrippen:
removal of the left temporal - Localization of function: theory that different brain regions
lobe, P.G. was impaired only on have different functions
verbal tests, whereas - Laterality: Refers to the side of the brain that controls a
subsequent to removal of the given function. Hence, studies of laterality are undertaken to
determine which side of the brain controls various functions.
right temporal lobe, S.K. was
- Lateralization: process by which function become located
impaired only on nonverbal tests primarily on one side of the brain.
Furthermore, both patients - Double dissociation: Experimental technique by which two
performed at normal on many neocortical areas are functionally dissociated by two
tests, providing evidence for behavioural tests; performance on each test is affected by a
localization, as well as for lesion in one zone but not in the other.
lateralization, of function.
Commissurotomy patients
Epileptic seizures may begin in a restricted region of one hemisphere and spread through the fibers
of the corpus callosum (the commissure) to the homologous location in the opposite hemisphere. To
prevent the spread of a seizure when medication has failed, commissurotomy ( disconnecting the
two hemispheres by cutting the 200 million nerve fibers of the corpus callosum) , was performed first
in the early 1940s. The procedure was medically beneficial, leaving some patients seizure-free
afterward, with minimal effects on their everyday behavior.
3
Learning goals:
1. Are there anatomical differences between left and right?
2. How can lateralization of function be demonstrated?
3. Are there functional differences between left and right hemisphere?
4. Which elements can affect symmetry?
5. How are the left and the right hemispheres connected?
6. What happens after this connection?
PART 1: asymmetry and lateralization___________________________________________________
Kolb, B. & Whishaw, I.Q. (2015). Fundamentals of Human Neuropsychology (7th Edition) H11
11.1: anatomical asymmetries in the human brain
Laterality: the idea that the 2 cerebral hemispheres have separate functions, leads to the notion that
two different minds control our behaviour.
Left hemisphere plays a crucial role in producing and understanding language and in
controlling movement on the right side of the body.
Right hemisphere specializes in perceiving and synthesizing nonverbal information, including
music and facial expression, and controls movement on the left side of the body.
Cerebral asymmetry
MRI scans of living brains confirm eight major anatomical
differences between the 2 hemispheres:
1. The right hemisphere is slightly larger and heavier
than the left, but the left contains more gray matter
relative to white matter.
2. The marked structural asymmetry of the left and right
temporal lobes may provide an anatomical basis for
their observed specialization in language and in music
functions, respectively.
3. The anatomical asymmetry in the temporal lobes’
cortex correlates with an asymmetry in the thalamus.
The left thalamus is dominant for language functions.
4. The slope of the lateral fissure is gentler on the left
hemisphere than on the right. The region of the
temporoparietal cortex lying ventral to the lateral
fissure therefore appears larger on the right.
5. The frontal operculum (Broca’s area) is organized
differently: the area visible on the brain surface is about one-third larger on the right than on
the left, whereas the area of cortex buried in the region’s sulci (ridges) is greater on the left
than on the right. This anatomical asymmetry probably corresponds to the lateralization of
these regions, the left side affecting grammar production and the right side possibly
influencing tone of voice (prosody).
6. The distribution of various neurotransmitters is asymmetrical in both the cortical and the
subcortical regions. The particular asymmetries in the distribution of ACh, GABA, NE, and DA
depend on the structure under consideration.
7. The right hemisphere extends farther anteriorly than does the left, the left hemisphere
extends farther posteriorly than does the right, and the occipital horns of the lateral
ventricles are five times as likely to be longer on the right as on the left.
1
, 8. Analysis of cortical surface area
imaged in 69 brains that were
combined into a single population-
averaged brain reveals an
unexpectedly broad pattern of
asymmetries not visible in individual
brains. The largest asymmetries favor
the left hemisphere in the Sylvian and
medial temporal regions and the right
hemisphere in the posterior parietal
and dorsolateral prefrontal areas
(Figure 11.2A), but an overall pattern
of asymmetry was evident across the
hemispheres (Figure 11.2B).
The largest anatomical asymmetries
enter on the temporoparietal
language areas. These asymmetries
are present before birth supports
the statement that language is innate
in humans. People speculate that asymmetries evolved to subserve (dienen) language.
Neuronal asymmetry
Scheibel and colleagues (1985) compared the
dendritic fields of pyramidal cells in Broca’s area,:
the left frontal operculum (LOP), with
those in the facial area of the motor
cortex in the left precentral cortex (LPC)
and with homologous regions in the right
hemisphere.
Their results show that neurons in each region
have distinct patterns of dendritic branching degree or pattern of branching is important, because
each branch is a potential location for enhancing or suppression graded potentials. Note that the
neurons in Broca’s areas (LOP), have far more abundant branches (overvloedige takken) than in
other areas.
Genetic asymmetry
Tao Sun and colleagues (2006) compared gene expression levels in the perisylvian regions of the left
and right hemispheres of the fetal (foetale) brain. They found genes that are expressed differently
in the two hemispheres predict that epigenetic changes may differentially influence the two
hemispheres. The mechanism is unknown, but an idea is that the asymmetrical expression of genes
may account for functional properties such as handedness, which to date has no clear- cut
(duidelijke) basis.
11.2: asymmetries in neurological patients
Patients with lateralized lesions
The oldest research on hemispheric specialization infers function from behavioural deficits that arise
as a result of strokes or surgery. The study of patients demonstrates that the functions of the two
hemispheres are lateralized, or dissociated. To conclude that the cortical area has a special or
lateralized function, however, it is also necessary to show that lesions in other brain areas do not
produce a similar deficit.
2
, The strongest experimental method for demonstrating lateralization of function is called double
dissociation (Teuber, 1955): 2 neocortical areas are functionally dissociated by 2 behavioural tests.
Performance on each test is affected by a lesion in one zone but not in the other.
Left-hemisphere lesions in right-handed patients consistently produce deficits in language
functions (speech, writing, and reading) that are not produced by right-hemisphere lesions.
Thus, the two hemispheres’ functions are dissociated.
Performing spatial tasks, singing, playing musical instruments and discriminating tonal
patterns are disrupted more by right- hemisphere than left- hemisphere lesions.
Because right-h lesions disturb tasks not disrupted by left- h lesions, and left- h lesions
disturb task not disrupted by right- h lesion, the two hemispheres are doubly dissociated.
A similar logic is used to localize functions within a hemisphere.
o In table 11.2 two hypothetical cortical regions, 102 and 107,
are doubly dissociated on tests of reading and writing: damage
to area 102 disturbs reading, whereas damage to area 107
impairs writing. In principle, this logic can be extended to
dissociate the functions of additional areas concurrently by
triple dissociation, quadruple dissociation, and so on.
Case with 2 patients:
P.G had seizures and neurological investigations revealed a tumor in the anterior part of the
left temporal lobe. Preoperative psychological tests showed P.G. to be of superior
intelligence, with the only significant deficits being on tests of verbal memory. Two weeks
after surgery, psychological testing showed a general decrease in intelligence ratings and a
further decrease in the verbal memory scores. Performance on other tests, including tests of
recall of complex drawings, was at normal.
S.K. had a tumor removed from the right temporal lobe. Preoperative testing of S.K. showed
a low score on recall of complex drawings. Two weeks after surgery, repeat testing showed a
marked decrease in performance IQ rating and a decline in the nonverbal memory score,
both for simple and for complex designs.
Clear example of double
dissociation: subsequent to Begrippen:
removal of the left temporal - Localization of function: theory that different brain regions
lobe, P.G. was impaired only on have different functions
verbal tests, whereas - Laterality: Refers to the side of the brain that controls a
subsequent to removal of the given function. Hence, studies of laterality are undertaken to
determine which side of the brain controls various functions.
right temporal lobe, S.K. was
- Lateralization: process by which function become located
impaired only on nonverbal tests primarily on one side of the brain.
Furthermore, both patients - Double dissociation: Experimental technique by which two
performed at normal on many neocortical areas are functionally dissociated by two
tests, providing evidence for behavioural tests; performance on each test is affected by a
localization, as well as for lesion in one zone but not in the other.
lateralization, of function.
Commissurotomy patients
Epileptic seizures may begin in a restricted region of one hemisphere and spread through the fibers
of the corpus callosum (the commissure) to the homologous location in the opposite hemisphere. To
prevent the spread of a seizure when medication has failed, commissurotomy ( disconnecting the
two hemispheres by cutting the 200 million nerve fibers of the corpus callosum) , was performed first
in the early 1940s. The procedure was medically beneficial, leaving some patients seizure-free
afterward, with minimal effects on their everyday behavior.
3
