Unit responses to bidirectional off-vertical axes rotations in central vestibular and cerebellar fastigial nuclei

Extracellular recordings were made from tilt-sensitive units in the vestibular nuclei and the cerebellar fastigial nucleus of cats during slow constant-velocity rotations in the CW and CCW directions about different off-vertical axes (5-25°). Each unit showed modulation of discharge rate, with position-dependent discharge maximum and minimum during any 360° unidirectional rotation. The location of the CW discharge maximum differed from that obtained during CCW rotation while the response gain was not affected by the direction of rotation. Characterized directional axes of neurons could be found along the roll and pitch axes. Units were classified into either phase-lead or phase-lag groups based on the location of CW discharge maximum relative to the CCW counterpart along the rotatory locus. About two-third of the units showed phase-lead responses, while the remaining units showed phase-lag responses. In such off-vertical axis rotations, increase in the amplitude of head tilt resulted in an increase in the discharge modulation but did not result in significant fluctuation in the positions of the CW or CCW discharge maxima. It is suggested that these directional signals on head position in space would provide a framework for spatial coding in otolithic motor commands.