Activity of neurons in Forel's field H during orienting head movements in alert head-free cats.

Single unit activities were recorded in Forel's field H (FFH) at the mesodiencephalic junction during orienting head movements in two alert cats under head-free conditions. Recordings were made of 63 neurons of which 20 showed phasic firing that preceded the onset of head movements by 20-100 ms and was temporally related to the dynamic phase of the orienting head movement. Nineteen of these neurons showed a preference for upward movements, while the remaining neuron preferred downward movements. Activities during orienting movements in eight different directions (each separated by 45 degrees) were systematically analyzed for 12 of the 19 upward-preferring neurons. The activities were broadly tuned; in most of the neurons, maximum activity was observed for direct upward movements (+90 degrees), but significant activity was also observed for ipsilateral and contralateral oblique upward movements (+45 degrees and +135 degrees). In these cases, the increase in activity preceded the onset of the movement. Some increase in activity was also observed for ipsilateral and contralateral horizontal, oblique downward and downward movements. However, the increase in activity in the latter cases occurred simultaneously with or lagged behind the onset of the movement and was often preceded by a decrease in activity. The same pattern of directional tuning was observed in the EMG of the biventer cervicis muscle, a target of FFH neurons. The preferred directions of the 12 upward-preferring neurons were estimated by calculating the vector sum of the activity and were distributed between +68 degrees and +108 degrees. The same amount of activity was observed for ipsilateral and contralateral oblique upward movements, suggesting that FFH neurons on both sides of the brainstem are equally activated even during oblique orienting. Input from the ipsilateral superior colliculus was investigated in 18 neurons, all of which were orthodromically activated with a latency of 0.8-1.8 ms, suggestive of a mono- or disynaptic excitatory connection. Seven neurons were identified as descending projection neurons by antidromic activation from the ipsilateral medullary reticular formation. Repetitive microstimulation of unilateral FFH induced oblique upward head movements and an accompanying torsional component, while simultaneous bilateral stimulation at comparable stimulus strength induced purely upward head movements. These results strongly suggest that the vertical component of orienting head movements is encoded by equal bilateral activation of the FFH.