Discharge properties of neurons in the rostral superior colliculus of the monkey during smooth-pursuit eye movements

The intermediate and deep layers of the monkey superior colliculus (SC) comprise a retinotopically organized map for eye movements. The rostral end of this map, corresponding to the representation of the fovea, contains neurons that have been referred to as "fixation cells" because they discharge tonically during active fixation and pause during the generation of most saccades. These neurons also possess movement fields and are most active for targets close to the fixation point. Because the parafoveal locations encoded by these neurons are also important for guiding pursuit eye movements, we studied these neurons in two monkeys as they generated smooth pursuit. We found that fixation cells exhibit the same directional preferences during pursuit as during small saccades-they increase their discharge during movements toward the contralateral side and decrease their discharge during movements toward the ipsilateral side. This pursuit-related activity could be observed during saccade-free pursuit and was not predictive of small saccades that often accompanied pursuit. When we plotted the discharge rate from individual neurons during pursuit as a function of the position error associated with the moving target, we found tuning curves with peaks within a few degrees contralateral of the fovea. We compared these pursuit-related tuning curves from each neuron to the tuning curves for a saccade task from which we separately measured the visual, delay, and peri-saccadic activity. We found the highest and most consistent correlation with the delay activity recorded while the monkey viewed parafoveal stimuli during fixation. The directional preferences exhibited during pursuit can therefore be attributed to the tuning of these neurons for contralateral locations near the fovea. These results support the idea that fixation cells are the rostral extension of the buildup neurons found in the more caudal colliculus and that their activity conveys information about the size of the mismatch between a parafoveal stimulus and the currently foveated location. Because the generation of pursuit requires a break from fixation, the pursuit-related activity indicates that these neurons are not strictly involved with maintaining fixation. Conversely, because activity during the delay period was found for many neurons even when no eye movement was made, these neurons are also not obligatorily related to the generation of a movement. Thus the tonic activity of these rostral neurons provides a potential position-error signal rather than a motor command-a principle that may be applicable to buildup neurons elsewhere in the SC.