1.1.1 Localization

Wilder Penfield

(Essential understanding: attempts to establish strict localization of function have culminated in the creation of cortical maps) used the method of neural stimulation - he stimulated various parts of the cortex with electrodes while the patient was awake and observed the effects this stimulation had on sensations and behavior. Little by little he covered the whole cortex and created the *cortical homunculus*, a map that shows the relative representation of various parts of the body in the cortex

Sperry (1968) and Gazzaniga (1967) AMPRCE

*Aim*: to investigate how the two hemispheres function independently when the connection between them is severed *Methods*: In-depth case study of four patients who underwent novel treatment for epilepsy that involved surgically cutting the corpus callosum *procedure*: A technique was used that allowed researchers to project stimuli to either the left or right eye of the participant. They used a table with a board on it. Participants sat in front of the board, fixing their eyes on the dot in the center. stimuli were then flashed on the far right or the far left of the board for one tenth of a second. The idea was that since optic nerves from the right eye are connected in our brain to the left hemisphere and vice versa, the researchers could present stimuli to one of the hemispheres only. For some trials the table also had a curtain with some objects behind it. Participants could reach behind the curtain and feel the objects with their hands. *results*: - When a picture of a spoon was shown to the left visual field (connected to the right hemisphere) and participants were asked to describe the object, they said nothing. However, when asked to pick an object behind the curtain, they could feel around and pick a spoon but only with their left hand (because it's connected to the right hemisphere). Participants could not explain why they picked the spoon. This result supports the idea that language is localized in the left hemisphere - the right hemisphere saw the object and was able to tell the left hand what to do, but since language is localized in the left hemisphere, participants were not able to explain what they did and why. - When a simple word (such as "pencil") was flashed to the right hemisphere, participants were able to reach behind the curtain and pick a pencil. This goes contrary to the previous finding and shows that the right hemisphere was able to process some simple speech. Perhaps while language production may be confined to the left hemisphere, language comprehension may be a function of both. This shows weak localization of language comprehension. - When researchers placed four plastic letters in a pile behind the curtain and asked participants to "spell a word", one participant was able to spell "love" with just his left hand. He was not able to name the word he just spelled. This shows that even language production, in some rudimentary form, may be present in the right hemisphere in some but not all people. *conclusion*: These results support the idea that localization of language is not strict. Both language production and language comprehension are mostly localized in the left hemisphere, but the right hemisphere can also perform some simple tasks.

Sperry (1968) and Gazzaniga (1967) essential understading

- research with split brain patients - As demonstrated by split brain research, both production and comprehension are weakly localized (lateralized) in the left hemisphere

Karl Lashley (1929): Essential understanding

Experiments with induced brain damage in rats showed that memory is widely distributed rater than localized anywhere in the cortex. This research used carefully controlled induced brain damage in the cortex of rats that were trained to run through a maze. *some psychological functions are distributed rather than localized*

Carl Wernicke (1874)

In a case study discovered the brain area responsible for comprehension of written and spoken language. Wernicke's area is located in the temporal lobe of the dominant hemisphere (usually the left). People with Wernicke's aphasia have an impairment of speech comprehension, but their articulate speech is intact.

Karl Lashley (1929) AMPRCE

Procedure: In a typical study, he trained a rat to go through a maze without mistakes in search of food, then removed a part of its cortex and observed what effect this would have on its memory of the maze. He removed 10-50% of the cortex on different trials. Results: Results of these experiments did not support Lashley's original hypotheses and led him to formulate the following ideas - *the principle of mass action*: Memory of the maze depended on the percentage of cortex destroyed, but not on the location of the lesion. - *equipotentiality*: the idea that one part of the cortex can take over the functions of another part of the cortex when necessary Conclusion: Based on these observations Lashley concluded that memory is not localized; it is widely distributed across the cortex as a whole. Even if one part of the cortex is lost, other parts may take over the functions of the missing part

Relative localization of function

The accumulated body of evidence suggests that localization of function in the human brain is relative. This idea of relative localization includes the following aspects: - Some functions are indeed strictly localized - Broca's aphasia, Wernicke's aphasia - Some functions are widely distributed - Karl Lashely and memory - Some functions are weakly localized rather than strictly (several brain areas are responsible for the same function, with one that is dominant) - see split brain research - some components of a function may be localized while other components of the same function may be distributed. For example, speech production seems to be more localized than speech comprehension. - Localization is not static: brain areas can respecialize due to neuroplasiticy

Broca's aphasia and Wernicke's aphasia

The first case studies used post mortem examination of the brains of patients with unique speech disorders. Results of these studies inspired the idea of strict localization. It appeared that the production of articulate speech is localized in Broca's area, and speech comprehension in Wernicke's area

Corticol Homunculus

a map that shows the relative representation of various parts of the body in the cortex

Paul Broca (1861)

documented the case study of "Tan", a patient who lost the ability to speak or write but retained all other functions such as intelligence and speech comprehension. He understood everything said to him and tried to communicate back, but the only sound he could make was the syllable "tan". This condition, the loss of articulated speech, became known as *Broca's aphasia*. When Tan died, an autopsy was performed and it was discovered that his brain had a very specific lesion in the left hemisphere, in the posterior inferior frontal gyrus. this brain region is now known as Broca's area.

Equipotentiality

established by Karl Lashley: the idea that one part of the cortex can take over the functions of another part of the cortex when necessary

The principle of mass action

established by Karl Lashley: the idea that one part of the cortex can take over the functions of another part of the cortex when necessary

weak localization

the idea that one brain area may be responsible for a function, but not exclusively, and other areas may also take over the function (several brain areas may be responsible for the same function, but only one of these areas is dominant)

strict localization

the idea that there is a clear correspondence between psychological functions and brain areas, and that all functions can be clearly mapped onto the brain

Localization of function

the theory that certain areas of the brain are responsible for certain psychological functions