Effects of initial eye position on saccade-related behavior of abducens nucleus neurons in the primate.

Previous work suggests that when the eye starts at different orbital initial positions (IPs), the saccade control system is faced with significant nonlinearities. Here we studied the effects of IP on saccade-related firing of monkey abducens neurons by either isolating saccade variables behaviorally or applying a multiple linear regression analysis. Over a 50 degrees range of IPs, we could select 10 degrees horizontal saccades with identical velocity profiles, which would require identical control signals in a linear system. The bursts accompanying ipsiversive saccades for IPs above the threshold for steady firing were quite similar. The excess burst rate above steady firing was either constant or decreased with ipsiversive IP, and both the number of excess spikes in the burst and burst duration were nearly constant. However, for ipsiversive saccades from IPs below threshold, both peak burst rate (6.82 +/- 1.38 spikes.s(-1).deg(-1)) and burst duration (0.67 +/- 0.28 ms/deg) increased substantially with ipsiversive IPs. Moreover, the pause associated with contraversive saccades shortened considerably with ipsiversive IPs (mean 1.2 ms/deg). This pattern of results for pauses and for bursts below threshold suggests the presence of a significant nonlinearity. Abducting saccades are produced by the net force of agonist lateral rectus (LR) and antagonist medial rectus (MR) muscles. We suggest that the decreasing force in the MR muscle with IPs in the abducting direction requires a more vigorous burst in LR motoneurons, which appears to be generated by a combination of saturating and nonsaturating burst commands and the recruitment of additional abducens neurons.