3-11 Lateralization Of Function (Part 1)

Front 1

Objective

Explain how the eyes in humans are connected to the two hemispheres of the brain.

Back 1

The hemispheres are connected to the eyes so that each hemisphere gets input from the opposite half of the visual world; that is, the left hemisphere sees the right side of visual space and the right hemispheres sees the left side.

Light from the right half of the visual field—what is visible at any moment—shines onto the left half of both retinas, and light from the left visual field shines onto the right half of both retinas. The left half of each retina connects to the left hemisphere, which therefore sees the right visual field. Similarly, the right half of each retina connects to the right hemisphere, which sees the left visual field. Half of the axons from each eye cross to the opposite side of the brain at the optic chiasm.

Right Visual Field → Left half of each retina → Left Hemisphere

Left Visual Field → Right half of each retina → Right Hemisphere

Front 2

Objective

Indicate the conditions under which the corpus callosum would be cut.

Back 2

Occasionally, surgeons have severed the corpus callosum as a treatment for severe epilepsy, which is characterized by the occurrence of seizures. Antiepileptic drugs block sodium flow across the membrane or enhance the effects of GABA. More than 90% of epileptic patients respond well enough to live a normal life. However, if someone continues having frequent seizures despite medication, physicians consider surgically removing the focus, or point in the brain where the seizures begin.

Front 3

Objective

Describe the characteristics of split-brain patients.

Back 3

People who have undergone surgery to the corpus callosum, referred to as split-brain patients, maintain their intellect and motivation, and they can still walk without difficulty. They also use the two hands together on familiar tasks such as tying shoes. However, if they are asked to pretend they are hitting a golf ball, threading a needle, or attaching a fishhook to a line, they struggle with the less familiar tasks, which have not become automatic for them.

Split-brain patients can use their two hands independently in a way that other people cannot (e.g. drawing circles with both hands simultaneously). They have no trouble planning two actions at once. In one experiment, a split-brain patient stared straight ahead as the experimenter flashed words or pictures on either side of the screen. Info that went to one hemisphere could not cross to the other because of the damage to the corpus callosum. The info stayed on the screen long enough to be visible but not long enough for the person to move his/her eyes. The person could then point with the left hand to what the right hemisphere had seen and could point with the right hand to what the left hemisphere had seen. It was as if each side had only part of the answer. The two halves of the brain had different information, and they could not communicate with each other.

Ex. HAT BAND

When the words hat and band are flashed on a screen, split-brain patients can report only what their left hemisphere saw: “BAND”. However, with their left hand, they can point to a hat, which is what the right hemisphere saw.

A split-brain patient can name an object after viewing it briefly in the right visual field, seeing it with the left hemisphere. But the same person viewing a display in the left visual field (right hemisphere) usually cannot name or describe it. Nevertheless, a patient who cannot name something points to it correctly with the left hand, even while saying “I don’t know what it was.”

Front 4

Objective

Distinguish between the areas of specialization of the left and right hemispheres.

Back 4

The left hemisphere is dominant for speech production in more than 95% of right-handers and nearly 80% of left-handers. It is also better than the right hemisphere at language comprehension for most people, although the right hemisphere does understand speech to some extent.

The right hemisphere is dominant for the emotional inflections of speech and for interpreting other people’s emotions conveyed through either speech or facial expressions. It is instrumental in comprehending spatial relationships and processing visual imagery.

The left and right hemispheres respond about equally to non-language sounds.

Front 5

corpus callosum

Back 5

Bundle of nerve fibers that connect the two hemispheres

Front 6

lateralization

Back 6

Division of labor between the two hemispheres of the brain

Front 7

visual field

Back 7

Area of the world that an individual can see at any time

Front 8

optic chiasm

Back 8

Point at which parts of the optic nerves cross from one side of the brain to the other

Front 9

epilepsy

Back 9

Condition characterized by the repeated occurrence of seizures—episodes of excessive, synchronized neural activity caused by decreases in the release of the inhibitory transmitter GABA

Front 10

focus

Back 10

Point in the brain where someone’s epileptic seizures begin

Front 11

split-brain patients

Back 11

Individuals who have had a surgical procedure that has cut the fibers of the corpus callosum, which would normally connect the two hemispheres of the brain