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 relation between activity in the motor layers of the superior colliculus (mSC) and saccadic oculomotor output. Based on single-unit recordings and 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 vector rotates in a direction that is opposite to the head roll direction. This rotation exceeds the amount of ocular counterroll that is observed as a result of difference in static head roll positions. We find that the mSC works in an oculocentric coordinate system that is biased 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.