Saccade-related activity in the primate superior colliculus depends on the presence of local landmarks at the saccade endpoint

Saccade-related discharge in the superior colliculus is greater for saccades made to a spot of light than for saccades in complete darkness. However, it is unclear whether this enhancement is due to the discontinuity of the spot or due to its being a new object of fixation. In these experiments, we examined the saccade-related activity of intermediate-layer neurons in the primate superior colliculus during delayed saccades to the center or corner of a large, bright square, as well as for visual and memory-guided movements to small spots in isolation. The saccade-related discharge for movements made to a local visual landmark present at the time of the saccade, be it a corner of a square or a small spot, was higher than that for saccades made to the center of a square that contained no local visual landmarks within. Moreover, discharge for movements to the center of a square were very similar to that for saccades to blank, dark space. Saccade velocity was similarly dependent on the presence of such a landmark, though less dramatically. The endpoints of saccades directed toward a square's corner were slightly displaced toward the center of the square. Across all neurons, discharge and velocity for saccades to the center of a square increased as the square size was decreased, but were never greater than those for saccades to a small spot of light. These results suggest that both saccade-related discharge in the superior colliculus and saccade metrics are enhanced for movements directed to parts of the visual scene with high contrast, while shifting fixation to a new object is not itself sufficient to elevate discharge and metrics above those of saccades to blank space.