3-12 Lateralization Of Function (Part 2)

Front 1

Objective

Describe an anatomical difference between the two hemispheres.

Back 1

Research has shown that one section of the temporal cortex, called the planum temporale, is larger in the left hemisphere for 65% of people. The difference in its size between the left and right hemispheres is slightly greater, on the average, for people who are strongly right-handed. Smaller but still significant differences are found between the left and right hemispheres of chimpanzees and gorillas, suggesting that this difference is part of our ancient genetic heritage.

Front 2

Objective

Cite evidence showing that the corpus callosum is not fully developed in young children.

Back 2

The corpus callosum matures gradually over the first five to 10 years of human life. The developmental process is not a matter of growing new axons but of selecting certain axons and discarding others. At an early stage, the brain generates far more axons in the corpus callosum than it will have at maturity.

Because the connections take years to develop their mature adult pattern, the behavior of young children in some situations resembles that of split-brain patients. An infant who has one arm restrained will not reach across the midline of the body to pick up a toy on the other side before the age of 17 weeks. Evidently, in younger children, each hemisphere has too little access to information from the opposite hemisphere. In one study, three- and five-year-old children were asked to feel two fabrics, either with one hand at two different times or with two hands at the same time, and say whether the materials felt the same or different. The five- year-olds did equally well with one hand or with two. The three-year-olds made 90% more errors with two hands than with one. The likely interpretation is that the corpus callosum matures sufficiently between ages 3 and 5 to facilitate the comparison of stimuli between the two hands.

Front 3

Objective

Explain why people born without a corpus callosum can perform some tasks that split-brain patients fail.

Back 3

People born without a corpus callosum are unlike people who have it cut later in life. First, whatever prevented formation of the corpus callosum undoubtedly affects brain development in other ways. Second, the absence or near absence of the corpus callosum induces the remaining brain areas to develop abnormally. Because of this, people born without a corpus callosum can perform some tasks that split-brain patients fail. They can verbally describe what they feel with either hand and what they see in either visual field; they can also feel one object with the left hand and another with the right and say whether they are the same or different.

How do they do so? They do not use their right hemisphere for speech. Rather, each hemisphere develops pathways connecting it to both sides of the body, enabling the left (speaking) hemisphere to feel both the left and right hands.

Also, the brain’s other commissures become larger than usual. In addition to the corpus callosum, people have the anterior commissure, which connects the anterior parts of the cerebral cortex, the hippocampal commisssure, which connects the left and right hippocampus, and the smaller posterior commissure. The extra development of these other commissures partly compensates for the lack of a corpus callosum.

Front 4

Objective

Indicate specific ways in which the brains of left-handers and right-handers differ.

Back 4

For more than 95% of right-handed people, the left hemisphere is strongly dominant for speech. Left-handers are more variable. Most left-handers have left-hemisphere dominance for speech, but some have right-hemisphere dominance or a mixture of left and right. The same is true for people who were left-handed in early childhood but forced to switch to writing right-handed. Many left-handers who have partial right-hemisphere control of speech are also partly reversed for spatial perception, showing more than the usual amount of left hemisphere contribution. A few left-handers have right hemisphere dominance for both language and spatial perception. Hand preference relates to some other asymmetries in brain and behavior. In one study, people wore a device on their belt that counted the number of times they turned left or right over three days. On the average, right-handers turned mostly to the left, and left-handers turned mostly to the right.

Front 5

planum temporale

Back 5

Area of the temporal cortex that for most people is larger in the left hemisphere than in the right hemisphere

Front 6

anterior commissure

Back 6

Set of axons connecting the 2 cerebral hemispheres; smaller than the corpus callosum

Front 7

hippocampal commissure

Back 7

Set of axons that connects the left and right hippocampus

Front 8

Rasmussen’s encephalopathy

Back 8

Rare condition in which an autoimmune disorder attacks first the glia and then the neurons of one or the other hemisphere of the brain