Neuronal activity in primary motor cortex differs when monkeys perform somatosensory and visually guided wrist movements

Abstract

This study was designed to investigate how activity patterns of primary motor cortical (MI) neurons change when monkeys perform the same movements guided by somatosensory and/or visual cues. Two adult male rhesus monkeys were trained to make wrist extensions and flexions after holding a steady position during an instructed delay period lasting 0.5-2.0 s. Monkeys held against a 0.07 Nm load that opposed flexion movements. Wrist movements were guided by vibratory cues (VIB-trials), visual cues (VIS-trials), or both in combination (COM-trials). Extracellular recordings of 188 MI neurons were made during all three paradigms. Individual neurons were counted twice, once for each movement direction, yielding 376 cases. All neurons had significant task-related activity (TRA) changes relative to delay period activity during at least one of the three paradigms. TRA was analyzed to determine if it was different as a function of the sensory cue(s) that initiated movement and that specified movement endpoints. Cases were grouped by whether the TRA changes were greater in VIB- or VIS-trials; this defined their "bias". One hundred and eighteen cases (31.4%) had greater TRA changes in VIB-trials (Vb-neurons), whereas 185 (49.2%) showed greater TRA changes in VIS-trials (Vs-neurons). The remaining 73 cases (19.4%) had similar TRA changes in VIB- and VIS-trials (Nb-neurons). For Vb- and Vs-neurons, earlier TRA onsets and greater TRA changes were observed in the trials for which these neurons were biased. During the COM-trials, the TRA was intermediate. During the trials for which the activity was not biased, the TRA was the least. For Nb-neurons, no significant TRA differences were observed across paradigms. TRA changes of MI neurons may represent movement planning-related inputs from other central, presumably cortical, sources as well as contribute to motor outflow from the cortex. These data suggest that Vb- and Vs-neurons are affected differently by somatosensory- and visually related central inputs, resulting in different TRAs, even for essentially identical movements. Such differences may depend not only on the type of sensory information that initiates movement but also whether that information specifies movement endpoints or might interfere with movement monitoring.