PSYB65H3 Chapter Notes - Chapter 11: Supplementary Motor Area, Neocortex, Microstructure

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21 Jul 2016
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Chapter 11: Cerebral Asymmetry
Anatomical Asymmetry in the Human Brain:
Perhaps no idea about human brain organization has so fascinated neuroscientists as has laterality, the idea
that the two cerebral hemispheres have separate functions.
Laterality leads to the notion that two different minds control our behavior.
The left hemisphere plays a special role in producing and understanding language and in controlling
movement on the right side of the body, whereas the right hemisphere specializes in perceiving and
synthesizing nonverbal information, including music and facial expression.
And the right hemisphere controls movement on the left side of the body.
Four variables complicate the research on laterality:
1. Laterality is relative, not absolute. Both hemispheres play a role in
nearly every behavior; thus, although the left hemisphere is especially
important for the production of language, the right hemisphere also has
some language capabilities.
2. Cerebral site is at least as important in understanding brain function
as cerebral side. The frontal lobes are asymmetrical, but their functions
are more similar to each other than they are to those of the posterior
cortex on the same side. In fact, in the absence of neurological data, it is
often very difficult to localize lesions in neurological patients to one
hemisphere even though the site (frontal rather than temporal or parietal)
may be immediately obvious. Perhaps it is best to think of many of the
functions of the cerebral cortex as being localized and of hemispheric side
as being only one feature of the localization.
3. Laterality is affected by environmental and genetic factors.
the cerebral organization of some left-handers and females appears less asymmetrical than that of right-handers and
males.
4. Laterality is exhibited by a range of animals. A functionally asymmetrical brain was once believed to be a
uniquely human characteristic and related to language, but certain songbirds, rats, cats, monkeys, and apes have
functionally and anatomically as asymmetrical brains as well.
Cerebral Asymmetry
According to John Hughlings-Jackson, Pierre Gratiolet first observed in the 1860s that the cortical
convolutions (gyri and sulci) on the left hemisphere mature more rapidly than those on the right.
Anatomical asymmetry was described again later in the nineteenth century by a number of researchers, but
these observations were largely ignored until the 1960s, when Norman Geschwind and Walter Levitsky
described a significant anatomical asymmetry of the planum temporale in the temporal lobes.
Also called Wernicke’s area, the planum temporale lies just posterior to the primary auditory cortex
(Heschl’s gyrus) within the Sylvian, or lateral, fissure.
On average, in 65 of the 100 brains studied by Geschwind and Levitsky, the planum temporale in the left
hemisphere was nearly 1 cm longer than that in the right hemisphere.
Geschwind and Levitsky’s finding has been replicated by numerous investigators, with the percentage of
cases having a larger planum temporale in the left hemisphere varying from 65% to 90% in different
samples.
-In contrast, the neighboring primary auditory cortex of Heschl’s longer than that in the right hemisphere.
Geschwind and Levitsky’s finding has been replicated by numerous investigators, with the percentage of
cases having a larger planum temporale in the left hemisphere varying from 65% to 90% in different
samples.
-In contrast, the neighboring primary auditory cortex of Heschl’symmetrical brains asgyrus is larger in the
right hemisphere because there are usually two Heschl’s gyri in the right hemisphere and only one in the
left.
MRI scans of living brains confirm eight major anatomical differences between the two hemispheres:
1. The right hemisphere is slightly larger and heavier than the left, but the
left contains more gray matter relative to white matter.
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2. The temporal lobes display a marked structural asymmetry that may
provide an anatomical basis for the observed specialization of the left and
right temporal lobes in language and in music functions, respectively. (See
Geschwind and Levitsky).
3. The asymmetry in the cortex of the temporal lobes is correlated with a
corresponding asymmetry in the thalamus. This anatomical asymmetry
complements an apparent functional asymmetry in the thalamus, the left
thalamus being dominant for language functions. (See Eidelberg and
Galaburda.)
4. The slope of the lateral fissure is gentler on the left hemisphere than on
the right (see Figure 11.1, top). The region of the temporoparietal cortex
lying ventral to the lateral fissure therefore appears larger on the right.
(See Toga and Thompson.)
5. The frontal operculum (Broca’s area) is organized differently on the left and
right. The area visible on the surface of the brain is about one-third larger
on the right than on the left, whereas the area of cortex buried in the sulci
(ridges) of the region is greater on the left than on the right. This anatomical asymmetry probably corresponds to the
lateralization of the
regions, the left side affecting the production of grammar in language and
the right side possibly influencing tone of voice.
6. The distribution of various neurotransmitters is asymmetrical, in both
the cortical and the subcortical regions. The particular asymmetries in
the distribution of acetylcholine, gamma-aminobutyric acid (GABA),
norepinephrine, and dopamine 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. These asymmetries presumably correspond to
some gross difference in cerebral organization that has yet to be identified.
8. The details of anatomical asymmetry are affected by both sex and handedness.
Overall, anatomical asymmetries center on the language areas, with most of the frontal and parietal lobes showing
little gross asymmetry. It is thus tempting to speculate that the asymmetries evolved to subserve language.
Moreover, these asymmetries are present in preterm infants, which seems to support the proposition that language is
innate in humans.
In fact, the brains of australopithecines had many anatomical asymmetries in common with modern
humans, but the hominids had no vocal apparatus that allowed language as we conceive of it.
In addition, some asymmetries, such as a heavier and larger right hemisphere and a longer lateral fissure,
can also be seen in many nonhuman primate species.
Neuronal Asymmetry
The identification of structural differences in the neurons in any two areas of the brain is a formidable task
in view of the sheer number of neurons.
Nonetheless, Arnold Scheibel and his colleagues 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 the neurons in each of these regions have distinct patterns of dendritic branching.
-The degree or pattern of branching is important, because each branch is a potential location for the
enhancement or suppression of the graded potentials in the dendritic tree.
-Thus, more branch points allow more degrees of freedom with respect to the final activity of the cell.
*the abundant branches in neurons in Broca’s area (LOP), far more than in the other areas.
We must approach Scheibel’s data on neural asymmetry with caution, because the sample of brains was
small (n _6).
However, five of the six brains were similar to the pattern shown in Figure 11.2. These five brains came
from right-handers; the atypical brain came from a left-handed person.
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Genetic Asymmetry
Asymmetry in Neurological Patients
Cerebral asymmetry was first established by studying patients with neurological disease, such as epilepsy,
that is lateralized to one hemisphere.
Improved neurosurgical treatment for such disorders has provided researchers with a large source of
subsequently healthy subjects who are usually very willing to participate in neuropsychological studies.
Patients with Lateralized Lesions
The oldest research on hemispheric specialization infers function from behavioral deficits that arise as a
result of strokes or surgery.
Such circumscribed, unilateral lesions in the left hemisphere of right-handed patients can produce aphasias
that do not develop from lesions in the right hemisphere.
The study of such 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 areas of the brain do not produce a similar deficit.
In the strongest experimental method for demonstrating the lateralization of function, called double
dissociation by Hans-Leukas Teuber, two areas of the neocortex are functionally dissociated by two
behavioral tests.
Each test is affected by a lesion in one zone but not in the other.
Lesions in the left hemisphere of right-handed patients consistently produce deficits in language functions
(speech, writing, and reading) that are not produced by lesions in the right hemisphere.
Thus, the functions of the two hemispheres are dissociated.
-However, performing spatial tasks, singing, playing musical instruments, and discriminating tonal patterns
are more disrupted by right-hemisphere than by left-hemisphere lesions.
Because right-hemisphere lesions disturb tasks not disrupted by left-hemisphere lesions and vice versa, the
two hemispheres are doubly dissociated.
Behavioral tests that are especially sensitive to damage to a specific locus but not to others can be used.
In principle, this logic can be extended to dissociate the functions of additional areas concurrently by triple
dissociation, quadruple dissociation, and so on.
Patients with Commissurotomy
Epileptic seizures may begin in a restricted region of one hemisphere and then 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 to impose control, commissurotomy, the
surgical procedure of disconnecting the two hemispheres by cutting the 200 million nerve fibers of the
corpus callosum, was performed first in the early 1940s by William Van Wagnen, an American
neurosurgeon.
The therapeutic outcome of the procedure initially appeared too variable and was subsequently abandoned
until the 1960s, when research with monkeys and cats by Ron Myers and by Roger Sperry led neurologists
to reconsider it.
At the time, two California surgeons, Joseph Bogen and Philip Vogel, performed complete sections of the
corpus callosum and of the smaller anterior commissure in a new series of about two dozen patients
suffering from intractable
epilepsy. The procedure was medically beneficial, leaving some patients
virtually seizure free afterward, with minimal effects on their everyday
behavior. More extensive psychological testing by Sperry and his colleagues
soon demonstrated, however, a unique behavioral syndrome that has been a source of new insights into the nature of
cerebral asymmetry. After sectioning, the two hemispheres are independent: each receives sensory input from all
sensory systems, and each can control the muscles of the body, but the two hemispheres can no longer communicate.
Because the functions in these separate cortexes, or split brains, are thus isolated, sensory information can be
presented to one hemisphere, and its function can be studied, without the other hemisphere having access to the
information.
how information seen in a particular part of the visual world by both eyes is sent to only one hemisphere.
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