The present study used single-unit recording and antidromic activation techniques in alert rhesus monkeys to examine the static and dynamic behavior of 21 parasympathetic, preganglionic neurons of the Edinger-Westphal nucleus (EW) during ocular accommodation. 2. All identified EW neurons were active when viewing at optical infinity with an average firing rate of 11.6 spikes/s. During near viewing, there was a linear 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 during viewing of targets with conflicting vergence and accommodative demands to dissociate their vergence and accommodation responses. With normal viewing these responses are so closely matched that it cannot be determined if the activity of a cell is related to vergence or to accommodation, but with dissociated viewing these relationships can be determined. Under this viewing condition, six preganglionic EW neurons showed the same relationship to accommodation as they did during normal viewing. However, the activity of two cells could not be explained solely by accommodation, and they showed some activity related to vergence. 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 response. This latency was essentially the same in both animals tested. 5. The activity of identified EW neurons is related to the velocity of accommodation as well as to static accommodation. The relationship between accommodation velocity and firing rate was studied for 15 identified EW neurons during sine-wave tracking of targets moving in depth. All of these cells showed a clear relationship between firing rate and accommodation velocity. Overall, this group of identified EW neurons showed a velocity sensitivity of 1.2 (spikes/s)/(diopter/s) and an estimated 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 statistically higher gains with respect to the near response; indeed, there was no overlap between the gains of these neurons and the gains of preganglionic EW neurons. Upon dissociation of vergence from accommodation, they were found to be related to either vergence or to vergence and accommodation but not solely to accommodation.
