Saccades and multisaccadic gaze shifts are gated by different pontine omnipause neurons in head-fixed cats

Pontine omnipause neurons (OPNs) have so far been considered as forming a h omogeneous group of neurons whose tonic firing stops during the duration of saccades, when the head is immobilized. In cats, they pause for the total duration of gaze shifts, when the head is free to move. In the present stud y, carried out on alert cats with fixed heads, we present observations made during self-initiated saccades and during tracking of a moving target whic h show that the OPN population is not homogeneous. Of the 76 OPNs we identi fied, 39 were found to have characteristics similar to those of previously described neurons, "saccade" (S-) OPNs: (1) the durations of their pauses w ere significantly correlated with the durations of saccades; (2) the discha rge ceased shortly before saccade onset and resumed before saccade end; (3) visual responses to target motion were excitatory; and (3) during tracking , S-OPNs interrupted the discharge for the duration of saccades and resumed firing during perisaccadic "drifts". However, the characteristics of 37 ne urons ("complex" (C-) OPNs) were different: (1) the pause duration was not correlated with the duration of self-initiated saccades; (2) time lead of p ause onsets relative to saccades was, on average, longer than in the group of S-OPNs, and firing resumed after the saccade end; (3) visual target moti on suppressed tonic discharges; and (4) during tracking, firing was interru pted for the total duration of gaze shifts, including not only saccades but also perisaccadic "drifts". We conclude that cat OPNs can be subdivided in to two main groups. The first comprises neurons whose firing patterns are c ompatible with gating individual saccades ("saccade" OPNs). The second grou p consists of "complex" OPNs whose firing characteristics are appropriate t o gate total gaze displacements rather than individual saccades. The functi on of these neurons may be to disinhibit pontobulbar circuits participating in the generation of saccade sequences and associated perisaccadic drifts.