BEHAVIOR OF IDENTIFIED EDINGER-WESTPHAL NEURONS DURING OCULAR ACCOMMODATION

1. The present study used single-unit recording and antidromic activat ion techniques in alert rhesus monkeys to examine the static and dynam ic behavior of 21 parasympathetic, preganglionic neurons of the Edinge r-Westphal nucleus (EW) during ocular accommodation. 2. All identified EW neurons were active when viewing at optical infinity with an avera ge firing rate of 11.6 spikes/s. During near viewing, there was a line ar relationship between firing rate and accommodation with an overall gain for the population of preganglionic EW neurons of 3.3 (spikes/s)/ diopter. 3. The activity of eight identified EW neurons was studied du ring viewing of targets with conflicting vergence and accommodative de mands to dissociate their vergence and accommodation responses. With n ormal viewing these responses are so closely matched that it cannot be determined if the activity of a cell is related to vergence or to acc ommodation, but with dissociated viewing these relationships can be de termined. Under this viewing condition, six preganglionic EW neurons s howed the same relationship to accommodation as they did during normal viewing. However, the activity of two cells could not be explained so lely by accommodation, and they showed some activity related to vergen ce. 4. Microstimulation at the sites of identified EW neurons produced accommodation in the ipsilateral eye. Repeated measures of the effect of microstimulation yielded a value of 75 ms for the latency of the r esponse. This latency was essentially the same in both animals tested. 5. The activity of identified EW neurons is related to the velocity o f accommodation as well as to static accommodation. The relationship b etween accommodation velocity and firing rate was studied for 15 ident ified EW neurons during sine-wave tracking of targets moving in depth. All of these cells showed a clear relationship between firing rate an d accommodation velocity. Overall, this group of identified EW neurons showed a velocity sensitivity of 1.2 (spikes/s)/(diopter/s) and an es timated neural time constant of 380 ms. 6. Eleven neurons encountered near to preganglionic EW neurons could not be antidromically activated by stimulation of the oculomotor nerve. These neurons had statistical ly higher gains with respect to the near response; indeed, there was n o overlap between the gains of these neurons and the gains of pregangl ionic EW neurons. Upon dissociation of vergence from accommodation, th ey were found to be related to either vergence or to vergence and acco mmodation but not solely to accommodation.