ROSTRAL FASTIGIAL NUCLEUS ACTIVITY IN THE ALERT MONKEY DURING 3-DIMENSIONAL PASSIVE HEAD MOVEMENTS

The fastigial nucleus (FN) receives vestibular information predominant ly from Purkinje cells of the vermis. FN in the monkey can be divided in a rostral part, related to spinal mechanisms, and a caudal part wit h oculomotor functions. To understand the role of EN during movements in space, single-unit activity in alert monkeys was recorded during pa ssive three-dimensional head movements from rostral FN. Seated monkeys were rotated sinusoidally around a horizontal earth-fixed axis (verti cal stimulation) at different orientations 15 degrees apart (including roll, pitch, vertical canal plane and intermediate planes). In additi on, sinusoidal rotations around an earth-vertical axis (yaw stimulus) included different roll and pitch positions (+/-10 degrees, +/-20 degr ees). The latter positions were also used for static stimulation. One hundred fifty-eight neurons in two monkeys were modulated during the s inusoidal vertical search stimulation. The vast majority showed a unif orm response pattern: a maximum at a specific head orientation (respon se vector orientation) and a null response 90 degrees apart. Detailed analysis was obtained from 111 neurons. On the basis of their phase re lation during dynamic stimulation and their response to static tilt, t hese neurons were classified as vertical semicircular canal related (n = 79, 71.2%) or otolith related (n = 25; 22.5%). Only seven neurons d id not follow the usual response pattern and were classified as comple x neurons. For the vertical canal-related neurons (n = 79) all eight m ajor response vector orientations (ipsilateral or contralateral anteri or canal, posterior canal, roll, and nose-down and nose-up pitch) were found in FN on one side. Neurons with ipsilateral orientations were m ore numerous and on average more sensitive than those with contralater al orientations. Twenty-eight percent of the vertical canal-related ne urons also responded to horizontal canal stimulation. None of the vert ical canal-related neurons responded to static tilt. Otolith-related n eurons (n = 25) had a phase relation close to head position and were c onsiderably less numerous than canal-related neurons. Except for pitch , all other response vector orientations were found. Seventy percent o f these neurons responding during dynamic stimulation also responded d uring static tilt. The sensitivity during dynamic stimulation was alwa ys higher than during static stimulation. Sixty-one percent of the oto lith-related neurons responded also to horizontal canal stimulation. T hese results show that in FN, robust vestibular signals are abundant. Canal-related responses art much more common than otolith-related resp onses. Although for many canal neurons the responses can be related to single canal planes, convergence between vertical canals but also wit h horizontal canals is common.