2-DIMENSIONAL SACCADE-RELATED POPULATION ACTIVITY IN SUPERIOR COLLICULUS IN MONKEY

The two-dimensional distribution of population activity in the superio r colliculus (SC) during saccadic eye movements in the monkey was esti mated using radial basis functions. To make these ensemble activity es timates, cells in the deeper layers of the SC were recorded over much of the rostrocaudal (caudal to 3.8 mm from the rostral tip): mediolate ral extent of this structure. The dynamic movement field of each cell was determined at 2-ms intervals around the time of saccades fora wide variety of horizontal and oblique movements. Collicular neurons were divided into partially overlapping dorsal and ventral cell layers on t he basis of recorded depth in SC. The pattern of presaccadic activity was used as an additional discriminant to sort the cells in the two la yers into separate burst (dorsal) and buildup (ventral) cell classes. Rostrocaudal and medioventral cell location on the colliculus was esti mated from the optimal target vector far a cell's visual response rath er than from the optimal motor vector. The former technique was more r eliable for locating some buildup neurons because it produced location s that compared better with the locations suggested by electrical stim ulation. From the movement field data and from the estimates of each c ell's anatomic location, a similar algorithm was used to compute the t wo-dimensional population activity in the two layers of the SC during horizontal and oblique saccades. A subset of the sample of neurons, lo cated near the horizontal meridian of the SC, first was used to comput e one-dimensional dynamic population activity estimates for horizontal saccades to allow partial comparison to previous studies. Statistical analyses on the one-dimensional data were limited to saccades of less than or equal to 20 degrees. The analyses indicated that while there was a small rostrally directed shift in the center of gravity of the d istributed activity in the buildup cell layer, there was little suppor t for the theory of a systematic rostrally directed spread of the lead ing edge of the activity. The two-dimensional results extend the previ ous one-dimensional estimates of collicular activity during saccades. Discharge in the burst layer was invariant in size for all saccade vec tors and symmetrically arranged about a center of gravity that did not : move during saccades. The size of the active area in the buildup lay er grew modestly with saccade amplitude, whereas the distribution of a ctivity was skewed toward the rostral end of the SC for saccades large r than 10 degrees. There was a small, but consistent shift in the cent er of gravity of the two-dimensional activity that was directed along the horizontal meridian (for horizontal movements) or an oblique merid ian (for oblique movements) of the SC. However, the spread of activity during a saccade was as large or larger in the mediolateral direction as it was in the rostral direction. The results indicate that changes in activity occur in an extended zone on the SC, and in all direction s but caudal, in the buildup layer during saccades and do not support the idea of a rostrally directed spread of activity as a dynamic contr ol mechanism for saccades. Our results and those of previous investiga tors of collicular population activity may be limited by stationarity concerns in that the cells used to estimate population activity were r ecorded in several monkeys over an extended period of time to obtain a sufficient spatial sample.