NEURONAL ACTIVITY OF THE PREFRONTAL CORTEX IN RELATION TO TIME CODING.

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The purpose of the present study was to examine how neuronal activity in the prefrontal cortex is involved in the time cording. For the first step to answer this question, we recorded neuronal activity in the prefrontal cortex during the performance of time period creation task.The monkey (Macaca fuscata) was trained to create the three different time periods (2, 4 or 8 sec). Each trial was started by pressing the hold key attached at the end of the forelimb brace. One to two seconds after the key press, one of the three lights was turned on for 2 sec, each of which indicating the minimum waiting period between the light onset to the hold key release. Yellow, red and green lights means 2, 4 and 8 sec of minimum waiting period, respectively. In addition, we also applied reversed rule, where the meaning of green and red lights was reversed. That is, green light indicated 8 sec of minimum waiting period and red light, 4 sec. This task was performed in a block manner. Each block consisted of 5 trials. Reversed rule was indicated by the lights flashed randomly for 2 sec, before the start of new block. The correct movement was rewarded with a drop of fruit juice (0.1 ml).In prefrontal cortex, about 70% of task-related neurons were active during the interval between the light onset and the initiation of movement Many prefrontal cortex neurons showed marked increased activity just after the light onset or relatively constant activity throughout the waiting period. A substantial number of prefrontal cortex neurons showed different activity between 2, 4, and 8 waiting periods. In some prefrontal cortex neurons, the magnitude of activity in 4 or 8 sec waiting period was different in the same waiting period in reversed rule. These observations indicate that the prefrontal cortex plays an important role in the execution of this task requiring to estimate and measure the different time periods.