THE COLLICULAR CODE OF SACCADE DIRECTION DEPENDS ON THE ROLL ORIENTATION OF THE HEAD RELATIVE TO GRAVITY

This paper investigates the influence of static head tilt on the relat ion between activity in the motor layers of the superior colliculus (m SC) and saccadic oculomotor output. Based on single-unit recordings an d electrical microstimulation in awake rhesus monkeys, we report that head roll changes the direction of the saccade vector generated by the mSC, with respect to a head-fixed coordinate system. Typically, the v ector rotates in a direction that is opposite to the head roll directi on. This rotation exceeds the amount of ocular counterroll that is obs erved as a result of difference in static head roll positions. We find that the mSC works in an oculocentric coordinate system that is biase d in the direction of gravity. It is argued that the modification is a result of processes that occur downstream of or parallel to the mSC.