ACTIVITY OF VISUOMOTOR BURST NEURONS IN THE SUPERIOR COLLICULUS ACCOMPANYING EXPRESS SACCADES

1. We recorded visuomotor burst neurons in the deeper layers of the su perior colliculus while two monkeys (Macaca fascicularis) made short-l atency saccades known as express saccades to visual targets in order t o determine whether the visual discharge normally seen for these cells served as the premotor burst during express saccades. We then compare d saccade-related activity during express saccades with that recorded during repular latency D saccades and delayed saccades. 2. Saccade lat ency histograms for two monkeys during trials with a temporal gap betw een fixation-point offset and target onset showed a distinct peak of s accades around 70-80 ms. One monkey also showed an additional peak aro und 125 ms. 3. Express saccades were found on the average to have the same relationship of saccade peak velocity to saccade amplitude as reg ular latency saccades and delayed saccades. Express saccades tended to be somewhat more hypometric than the other classes of saccades. Howev er, express saccades were clearly visually guided and not anticipatory responses. 4. For most cells studied (33/40), express saccades were a ccompanied by a single, uninterrupted burst of activity beginning 40-5 0 ms after target onset and continuing until sometime around the end o f the saccade. For a smaller group of cells (7/40), two peaks of burst activity were seen, although the second peak was smaller and tended t o occur late, after saccade onset. Across all cells, the peak of visuo motor cell activity during express saccades correlated just as well wi th target onset as it did with saccade onset. 5. When considered as di scharge temporally aligned to the onset of the saccade, bursts accompa nying express saccades tended to begin at approximately the same time as that for regular and delayed saccades. However, this discharge gene rally peaked earlier for express than for regular and delayed saccades . Also, the magnitude of discharge for express saccades was higher tha n that for delayed saccades throughout the burst. 6. When considered a s discharge temporally aligned to the appearance of the target, bursts began earlier for express and regular saccade trials than for delayed saccade trials. Peak discharge tended to be greater for express sacca des than for the other classes of saccades. 7. The results of this inv estigation are consistent with the suggestion that the visual burst of visuomotor neurons in the deeper layers of the superior colliculus pl ays a role in the initiation of express saccades similar to that playe d by the premotor burst for saccades of longer latency. The elevated d ischarge for express saccades supports the idea that the superior coll iculus plays a more critical role in express saccade generation than i n the generation of longer-latency saccades. The elevated discharge al so suggests that visuomotor bursters do not code one-to-one for saccad e velocity nor for saccade dynamic motor error.