Chapter 4
Hemispheric specialization
Anatomy of the hemispheres
● Wada test: Used to identify which hemisphere is responsible for language before
brain surgery is performed → each hemisphere is temporarily anesthetized.
Anatomical correlates of hemispheric specialization:
● Macroscopic anatomical asymmetries:
○ Sylvian fissure: Has a more prominent upward curl in the right hemisphere
than it does in the left hemisphere, where it is relatively flat → the cortical
area at the center of Wernicke’s area (understanding of language) was larger
in the left hemisphere.
○ Ninety-six percent of humans, regardless of which hand is dominant, have a
left-hemisphere specialization for language.
○ Differences in neural connectivity and organization may underlie many of the
gross asymmetries between the hemispheres.
● Microscopic anatomical asymmetries: Pyramidal cell bodies are larger on the left
than on the right in Heschl’s gyrus.
● The anatomy of communication:
○ Corpus callosum: Connects the two halves of the cerebral cortex → largest
fiber system in the brain → includes more than 250 million axons.
■ Homotopic connections: Connect the corresponding regions of each
hemisphere → V1 on the right to V1 on the left.
■ Heterotopic connections: Connect different areas of each
hemisphere → V1 on the right to V2 on the left.
○ The commissures: Two smaller bands of fibers, the anterior and posterior
commissures, also connect the two hemispheres.
Function of the corpus callosum:
● Interhemispheric connectivity: Synchrony between neurons that respond to the
same stimulus that activates cells with receptive fields in both hemifields.
● Differences in neural connectivity and organization may underlie many of the gross
asymmetries between the hemispheres.
Splitting the brain; cortical disconnection
The surgery: William P. Van Wagenen performed the first partial callosotomies in 10
patients suffering from severe epileps.
● Results were mixed, 80% of patients found 60-70% relief → probably due to partial
cut of corpus callosum.
Methodological considerations in studying split-brain patients:
● Patients were not neurologically normal before the operation.
● Important to consider whether the transcortical connections were
completely sectioned, or whether some fibers remained intact.
● Patient reports through speaking hemisphere only the items flashed to the
right half of the screen → denies seeing left-field stimuli or recognizing
, objects presented to the left hand → the left hand correctly retrieves
objects presented in the left visual field.
Functional consequences of the split-brain procedure:
● Split-brain evidence for callosal function specificity:
○ When the posterior half of the callosum is sectioned in humans,
transfer of visual, tactile, and auditory sensory information is
severely disrupted.
○ The anterior part of the callosum is involved in the higher order,
can still transfer semantic information.
Hemispheric specialization
Evidence from split-brain patients:
● Language and speech: The left hemisphere is dominant for language, speech, and
major problem solving.
○ There may be two lexicons, one in each hemisphere → right hemisphere’s
lexicon seems organized differently from the left hemisphere’s lexicon →
lexicons are accessed in different ways.
● Visuospatial processing: The right hemisphere appears specialized for
visuospatial tasks → drawing cubes and other three-dimensional patterns.
○ The left hemisphere outperforms the right when faces are dissimilar,
and the right hemisphere outperforms the left when faces are similar.
● The interactions of attention and perception: Although touching any part
of the body is noted by either hemisphere, patterned somatosensory
information is lateralized → a split-brain patient who is holding an object in
the left hand is unable to find an identical object with the right hand.
○ Attentional resources are shared: In control patients, search times
increase in with set size. In split brain patients, search times
increase half with set sizes, compared to control patients, when
items presented to two hemispheres → two half brains working
separately can do the job in half the time that one whole brain can.
○ Global and local processing: The global shape is composed of the
small letters and the local shape are the small letters.
■ Global detection: Is done faster by the right hemisphere.
■ Local detection: Is done faster by the left hemisphere.
Theory of mind: Research suggests that if it is lateralized, it’s the right hemisphere
→ if true, information about beliefs of others isn’t transferred to the speaking, left
hemisphere → patients expected to suffer disruption in social reasoning, but don’t.
The interpreter: The left hemisphere is trying to explain, interpret, the actions
performed by the right hemisphere → post hoc rationalizations.
● When asked why these items were chosen, left brain answers ‘chicken goes
with chicken claw, and you need a shovel to clean out the chicken shed’.
● Evidence from patients with unilateral cortical lesions: Comparing the
performance of patients with right-hemisphere lesions against those with left-
hemisphere lesions → no need to lateralize the stimuli to one side or the other.
, Chapter 7
Attention
Attention: Ability to focus awareness on a stimulus, thought, or action while ignoring others.
● Influences how we process sensory inputs, store that information in memory, process
it semantically, and act on it.
Bálint’s syndrome: Severe disturbance of visual attention and awareness, caused by
bilateral damage to regions of the posterior parietal and occipital cortex.
● Only one or a small subset of available objects are perceived at any one time and are
mislocalized in space.
Selective attention: Describes what we attend and ignore within any specific level (high vs.
low) of arousal → global physiological and psychological brain state.
The anatomy of attention
The neuropsychology of attention
Neglect: Result from damage to the (right) parietal, temporal, and/or frontal cortices, as well
as subcortical structures → damage typically occurs from a stroke.
● Right-hemisphere lesion would bias attention toward the right,
resulting in a neglect of what is going on in the left visual field.
● Neglect affects external personal hemispace and objects as well as
internal memory for objects arrayed in space.
Neuropsychological tests of neglect:
● Lines: Asked to bisect lines precisely in the middle with a vertical
line → tendency to bisect the lines to the right of the midline of each page and/or
each line, owing to neglect for contralesional space and the contralesional side of
individual objects.
Hemispheric specialization
Anatomy of the hemispheres
● Wada test: Used to identify which hemisphere is responsible for language before
brain surgery is performed → each hemisphere is temporarily anesthetized.
Anatomical correlates of hemispheric specialization:
● Macroscopic anatomical asymmetries:
○ Sylvian fissure: Has a more prominent upward curl in the right hemisphere
than it does in the left hemisphere, where it is relatively flat → the cortical
area at the center of Wernicke’s area (understanding of language) was larger
in the left hemisphere.
○ Ninety-six percent of humans, regardless of which hand is dominant, have a
left-hemisphere specialization for language.
○ Differences in neural connectivity and organization may underlie many of the
gross asymmetries between the hemispheres.
● Microscopic anatomical asymmetries: Pyramidal cell bodies are larger on the left
than on the right in Heschl’s gyrus.
● The anatomy of communication:
○ Corpus callosum: Connects the two halves of the cerebral cortex → largest
fiber system in the brain → includes more than 250 million axons.
■ Homotopic connections: Connect the corresponding regions of each
hemisphere → V1 on the right to V1 on the left.
■ Heterotopic connections: Connect different areas of each
hemisphere → V1 on the right to V2 on the left.
○ The commissures: Two smaller bands of fibers, the anterior and posterior
commissures, also connect the two hemispheres.
Function of the corpus callosum:
● Interhemispheric connectivity: Synchrony between neurons that respond to the
same stimulus that activates cells with receptive fields in both hemifields.
● Differences in neural connectivity and organization may underlie many of the gross
asymmetries between the hemispheres.
Splitting the brain; cortical disconnection
The surgery: William P. Van Wagenen performed the first partial callosotomies in 10
patients suffering from severe epileps.
● Results were mixed, 80% of patients found 60-70% relief → probably due to partial
cut of corpus callosum.
Methodological considerations in studying split-brain patients:
● Patients were not neurologically normal before the operation.
● Important to consider whether the transcortical connections were
completely sectioned, or whether some fibers remained intact.
● Patient reports through speaking hemisphere only the items flashed to the
right half of the screen → denies seeing left-field stimuli or recognizing
, objects presented to the left hand → the left hand correctly retrieves
objects presented in the left visual field.
Functional consequences of the split-brain procedure:
● Split-brain evidence for callosal function specificity:
○ When the posterior half of the callosum is sectioned in humans,
transfer of visual, tactile, and auditory sensory information is
severely disrupted.
○ The anterior part of the callosum is involved in the higher order,
can still transfer semantic information.
Hemispheric specialization
Evidence from split-brain patients:
● Language and speech: The left hemisphere is dominant for language, speech, and
major problem solving.
○ There may be two lexicons, one in each hemisphere → right hemisphere’s
lexicon seems organized differently from the left hemisphere’s lexicon →
lexicons are accessed in different ways.
● Visuospatial processing: The right hemisphere appears specialized for
visuospatial tasks → drawing cubes and other three-dimensional patterns.
○ The left hemisphere outperforms the right when faces are dissimilar,
and the right hemisphere outperforms the left when faces are similar.
● The interactions of attention and perception: Although touching any part
of the body is noted by either hemisphere, patterned somatosensory
information is lateralized → a split-brain patient who is holding an object in
the left hand is unable to find an identical object with the right hand.
○ Attentional resources are shared: In control patients, search times
increase in with set size. In split brain patients, search times
increase half with set sizes, compared to control patients, when
items presented to two hemispheres → two half brains working
separately can do the job in half the time that one whole brain can.
○ Global and local processing: The global shape is composed of the
small letters and the local shape are the small letters.
■ Global detection: Is done faster by the right hemisphere.
■ Local detection: Is done faster by the left hemisphere.
Theory of mind: Research suggests that if it is lateralized, it’s the right hemisphere
→ if true, information about beliefs of others isn’t transferred to the speaking, left
hemisphere → patients expected to suffer disruption in social reasoning, but don’t.
The interpreter: The left hemisphere is trying to explain, interpret, the actions
performed by the right hemisphere → post hoc rationalizations.
● When asked why these items were chosen, left brain answers ‘chicken goes
with chicken claw, and you need a shovel to clean out the chicken shed’.
● Evidence from patients with unilateral cortical lesions: Comparing the
performance of patients with right-hemisphere lesions against those with left-
hemisphere lesions → no need to lateralize the stimuli to one side or the other.
, Chapter 7
Attention
Attention: Ability to focus awareness on a stimulus, thought, or action while ignoring others.
● Influences how we process sensory inputs, store that information in memory, process
it semantically, and act on it.
Bálint’s syndrome: Severe disturbance of visual attention and awareness, caused by
bilateral damage to regions of the posterior parietal and occipital cortex.
● Only one or a small subset of available objects are perceived at any one time and are
mislocalized in space.
Selective attention: Describes what we attend and ignore within any specific level (high vs.
low) of arousal → global physiological and psychological brain state.
The anatomy of attention
The neuropsychology of attention
Neglect: Result from damage to the (right) parietal, temporal, and/or frontal cortices, as well
as subcortical structures → damage typically occurs from a stroke.
● Right-hemisphere lesion would bias attention toward the right,
resulting in a neglect of what is going on in the left visual field.
● Neglect affects external personal hemispace and objects as well as
internal memory for objects arrayed in space.
Neuropsychological tests of neglect:
● Lines: Asked to bisect lines precisely in the middle with a vertical
line → tendency to bisect the lines to the right of the midline of each page and/or
each line, owing to neglect for contralesional space and the contralesional side of
individual objects.
