Task-dependence of saccade-related activity in monkey superior solliculus: Implications for models of the saccadic system

Current models assign a crucial role to the deep layers of the Superior Colliculus (SC) in the dynamic feedback control of saccadic eye movements. However, if the SC is to be part of the local feedback loop for saccades, it is expected that the movement-related firing patterns of deep layer SC cell maintain a fired relation with the instantaneous saccade trajectory, regardless of the conditions that evoked the saccade. In this paper we provide three different lines of evidence, suggesting that the movement activity of SC burst cells may change as a function of the sensory conditions evoking the saccade. First, it is shown that bimodal (visual-auditory) stimulation may markedly enhance (up to about 350%) or suppress (on average down to 70%) SC motor bursts when compared to the activity for unimodal visual stimulation. Second, the movement activity associated with auditory-evoked saccades appeared to be reduced by almost 60% relative to visually-evoked saccades of the same metrics (tested in one monkey). However, for both paradigms, these relatively large changes in movement activity went without a concomitant change in the saccade properties. Third, a short-term saccadic adaptation paradigm produced saccades with a smaller amplitude (gain about 0.7) upon presentation of the adapting visual stimulus. However, we found that the movement-related activity of SC burst cells did not change in this paradigm. These findings suggest that the SC cells do neither encode saccade kinematics, nor the precise components of the saccade vector. Rather, we propose that the motor SC issues a crude desired eye displacement signal to the brainstem that is transformed into the appropriate movement signals by downstream or parallel mechanisms.