Unit activity of motor cortex during acoustically signalled reaching in rats.

Changes of unit activity, accompanying a discriminated instrumental reaction were studied in the motor cortex of rats (n = 15) trained to reach into a narrow tubular feeder for food pellets. A shutter opened the feeder for 1280 ms while the presence of food was signalled by 600 Hz (CS+) tone and its absence by a 3000 Hz (CS-) tone. The electrophysiological experiments were performed after the rats had developed a preference for reaching with the left or right forepaw and when their discriminations were 75% correct. Unit activity was recorded in the motor cortex contralateral to the preferred forepaw using capillary microelectrodes and a head-mounted microdrive. Post-stimulus histograms (PSHs) to CS+ (n = 71) showed an increase of firing rate in the first 120 ms after CS onset (40% units) and a tonic excitation (55% units) starting some 200 ms later. The second component was related to movement as revealed by simultaneously plotted peri-reach histograms. The first component was related to preparatory activity and was also present in the PSHs obtained during correct reactions to CS- (n = 29). Then, absence of reaching was manifested by inhibition replacing the second excitatory component. The first but not the second excitatory components were seen in the PSHs plotted during negative reactions to CS+ (n = 18). It is concluded that the conditioned stimuli elicit in the motor cortex preparatory activity preceding actual movement by 200-250 ms. The neuronal populations engaged in the preparatory activity and movement execution overlap.