Representation of Horizontal head-on-body position in the primate superior colliculus.

Movement-related activity within the superior colliculus (SC) represents the desired displacement of an impending gaze shift. This representation must ultimately be transformed into position-based reference frames appropriate for coordinated eye-head gaze shifts. Parietal areas that project to the SC are modulated by the initial position of both the eye-re-head and head-re-body and SC activity is modulated by eye-re-head position. These considerations led us to investigate whether SC activity is modulated by the head-re-body position. We recorded activity from movement-related SC neurons while head-restrained monkeys performed a delayed-saccade task. Across blocks of trials, the horizontal position of the body was rotated under a space-fixed head to three to five different positions spanning +/-25 degrees . We observed a significant influence of body-under-head position on SC activity in 50/60 neurons. This influence was expressed predominantly as a linear gain field, scaling task-related SC activity without changing the location of the response field (linear gain fields explained >/=20% of the variance in neural activity in approximately 50% of our sample). Smaller nonlinear modulations were also observed in roughly 30% of our sample. SC activity was equally likely to increase or decrease as the body was rotated to the side of neuronal recording and we found no systematic relationship between the directionality or magnitude of the linear gain field with recording location in the SC. We conclude that a signal conveying head-re-body position is present in the SC. Although the functional significance remains open, our findings are consistent with the SC contributing to a displacement-to-position transformation for oculomotor control.