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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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“Planning Neurons” in the Monkey Frontal Cortex

In further confirmation of the human clinical evidence about the function of the frontal association cortices, neurons that appear to be specifically involved in planning have been identified in the frontal cortices of rhesus monkeys.

The behavioral test used to study cells in the monkey frontal cortex is called the delayed response task (Figure 26.12A). Variants of this task are used to assess frontal lobe function in a variety of situations, including the clinical evaluation of frontal lobe function in humans (Box C). In the delayed response task, the monkey watches an experimenter place a food morsel in one of two wells; both wells are then covered. Subsequently, a screen is lowered for an interval of a few seconds to several minutes (the delay). When the screen is raised, the monkey gets only one chance to uncover the well containing food and receive the reward. Thus, the animal must decide that he wants the food, remember where it is placed, recall that the cover must be removed to obtain the food, and keep all this information available during the delay so that it can be used to get the reward. The monkey's ability to carry out this task is diminished or abolished if the area anterior to the motor region of the frontal cortex—called the prefrontal cortex—is destroyed bilaterally (in accord with clinical findings in human patients).

Figure 26.12. Activation of neurons near the principal sulcus of the frontal lobe during delayed response task.

Figure 26.12

Activation of neurons near the principal sulcus of the frontal lobe during delayed response task. (A) Illustration of task. The experimenter randomly varies the well in which the food is placed. The monkey watches the morsel being covered, and then the (more...)

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Box C

Neuropsychological Testing. Long before PET scanning and functional MRI were used to evaluate normal and abnormal cognitive function, several “low-tech” methods proved to be reliable means of assessing these abilities in human subjects. (more...)

Some neurons in the prefrontal cortex, particularly those in and around the principal sulcus (Figure 26.12B), generate a response that is correlated with the delayed response task; that is, they are maximally active during the period of the delay, as if their firing represented the information maintained from the presentation part of the trial (i.e., the cognitive information needed to guide behavior when the screen is raised; Figure 26.12C,D). Such neurons return to a low level of activity during the actual motor phase of the behavior, suggesting that they represent working memory and planning rather than the actual movement itself. Delay-specific neurons in the prefrontal cortex are also active in monkeys that have been trained to perform a variant of the delayed response task in which the response is to internally generated memories. Evidently, these neurons are equally capable of using stored information to guide behavior. Thus, if a monkey is trained to associate eye movements to a particular target with a delayed reward, the delay-associated neurons in the prefrontal cortex will fire during the delay, even if the monkey moves his eyes to the appropriate region of the visual field in the absence of the target.

The existence of delay-specific neurons in the frontal cortex of rhesus monkeys, as well as attention-specific cells in the parietal cortex and recognition-specific cells in the temporal cortex, generally supports the functions of these cortical areas inferred from clinical evidence in humans. Nonetheless, functional localization, whether inferred by examining human patients or by recording single neurons in monkeys, is an imprecise business. The observations summarized here are only a rudimentary guide to thinking about how complex cognitive information is represented and processed in the brain, and how the relevant brain areas and their constituent neurons contribute to such important but still ill-defined qualities as personality, intelligence (Box D), or other cognitive functions that define what it means to be a human being.

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Box D

Brain Size and Intelligence. The fact that so much of the brain is occupied by the association cortices raises a fundamental question: does more of it provide individuals with greater cognitive ability? Humans and other animals obviously vary in their (more...)

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK11129

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