Single-cell activity was recorded from three different motor areas in the cerebral cortex: the primary motor cortex (MI), supplementary motor area (SMA), and premotor cortex (PM). Three monkeys (Macaca fuscata) were trained to perform a sequential motor task in two different conditions. In one condition (visually triggered task, VT), they reached to and touched three pads placed in a front panel by following lights illuminated individually from behind the pads. In the other condition (internally guided task, IT), they had to remember a predetermined sequence and press the three pads without visual guidance. In a transitional phase between the two conditions, the animals learned to memorize the correct sequence. Auditory instruction signals (tones of different frequencies) told the animal which mode it was in. After the instruction signals, the animals waited for a visual signal that triggered the first movement. Neuronal activity was analyzed during three defined periods: delay period, premovement period, and movement period. Statistical comparisons were made to detect differences between the two behavioral modes with respect to the activity in each period. Most, if not all, of MI neurons exhibited similar activity during the delay, premovement, and movement periods, regardless of whether the sequential motor task was visually guided or internally determined. More than one-half of the SMA neurons were preferentially or exclusively active in relation to IT during both the premovement (55%) and movement (65%) periods. In contrast, PM neurons were more active (55% and 64% during the premovement and movement periods) in VT. During the instructed-delay period, a majority of SMA neurons exhibited preferential or exclusive relation to IT whereas the activity in PM neurons was observed equally in different modes. Two types of neurons exhibiting properties of special interest were observed. Sequence-specific neurons (active in a particular sequence only) were more common in SMA, whereas transition-specific neurons (active only at the transitional phase) were more common in PM. Although a strict functional dichotomy is not acceptable, these observations support a hypothesis that the SMA is more related to IT, whereas PM is more involved in VT. Some indications pointing to a functional subdivision of PM are obtained.
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