Anatomical Distribution of Arm-movement-related Neurons in the Primate Superior Colliculus and Underlying Reticular Formation in Comparison with Visual and Saccadic Cells

We recorded from 389 "reach" neurons (two monkeys) in the superior colliculus (SC) and underlying reticular formation (RF) or adjacent periaqueductal grey, whose activity was related to visually guided arm movements. Reach neurons were present from approximately 0.7 mm down to a depth of 6 mm below the surface of the SC (mean 3.7+/-1.3, n=389). Although this mean distribution was different from that of cells with visual (mean depth 1.7+/-1.4 mm, n=283) or saccadic responses (mean depth 2.0+/-1.4 mm, n=232), there was a large amount of overlap. Fifty-five per cent of all reach cells (213/389) were assumed to be located inside the SC. The others were considered to be located in the underlying RF. The characteristics of visual responses and saccadic bursts (e.g. response latencies, discharge rates, burst durations) of arm-movement-related neurons were not different from those of typical visual or saccade cells in the SC. Although reach neurons could be recorded in a large area of the SC, they were found more often in the lateral than in the medial parts (chi-squared=19.3, P<0.001). Possible pathways by which arm-movement-related neuronal activity in and below the SC might gain access to spinal motor structures are discussed. The location of arm-movement-related neurons described in this study is in accordance with the known target areas of skeletomotor-related corticotectal projections and with the sites of origin of tectofugal pathways. It is concluded that this population of reach cells is in a position to relay and transmit limb movement information to the spinal motor system, where it may influence (or interact with) motor commands coming from other motor centres.