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1 Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD 20892, USA; Department of Neurobiology & Physiology, Northwestern University, Evanston, IL 60201, USA; Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
2 Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD 20892, USA; National Institutes of Health Research Scholars Program, Howard Hughes Medical Institute, Bethesda, MD 20892, USA
3 Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD 20892, USA; Mahoney Center for Brain and Behavior and Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY 10032, USA; New York State Psychiatric Institute, New York, NY 10032, USA
* To whom correspondence should be addressed. E-mail: r-hasegawa{at}aist.go.jp.
The self-ordered task is a powerful tool for the analysis of dorsal prefrontal deficits. Each trial consists of a number of steps, and subjects must remember their choices in previous steps. The task becomes more difficult as the number of objects to be remembered increases. We recorded the activity of 156 neurons in the mid-dorsal prefrontal cortex of two rhesus monkeys performing an oculomotor version of the task. Although the task requires working memory, there was no convincing evidence for activity selective for the working memory of the objects that the monkey had to remember. Instead, nearly half of neurons (47%, 74/156) showed activity which was modulated according to the step of the task in any one or more task periods. Although the monkey's reward also increased with step, the neurons exhibited little or no step modulation in a reward control task in which reward increased without a concurrent increase in task difficulty. The activity of some neurons was also selective for the location of saccade target that the monkey voluntarily chose. Neurons showed less step modulation in error trials, and there was no increase between the second and third step responses on trials in which the error was on the third step. These results suggest that the mid-dorsal prefrontal cortex contributes to the self-ordered task not by providing an object working memory signal, but by regulating some general aspect of the performance in the difficult task.
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