There is general agreement that saccades are guided by a motor error signal produced by a local feedback circuit that computes the difference between desired eye displacement and current eye displacement. The superior colliculus (SC) has been suggested to reside inside the feedback loop and its neurons may signal the dynamic motor error (DME) to saccade-related neurons downstream. Two experiments tested this possibility.; In the first, saccades were slowed by injecting muscimol into the omnipause neuron area of the pons while the activity of a SC neuron was recorded simultaneously. In 14 such experiments, eleven neurons showed robust increases in the duration of saccadic discharge as saccadic duration increased. Only five of 11 neurons exhibited a decrease in peak firing rate as saccadic velocity decreased. These data support the notion that the SC does receive feedback about the ongoing saccade but that this feedback controls the duration of SC discharge rather than DME. A new saccade generator model that explains those data is proposed.; The second experiment tested whether DME is coded as the spread of neuronal activity travelling from the caudal SC, where larger saccades are encoded, to the rostral SC, where smaller saccades are encoded. This scenario predicts that as a large saccade evolves, a particular rostral neuron begins discharging a burst at that time during the saccade when the remaining motor error equals the optimal vector for that SC neuron. Therefore, the relation of the burst timing to the timing of DME should have a slope of zero. Instead, the relations using measures of burst timing to the onset, peak and center of the burst all showed slopes that differed significantly from zero. Although the peak and center leads fell between −1 and zero, a hill of activity travelling rostrally at rates indicated by these slopes would arrive at the rostral SC much too late to terminate the saccade at the appropriate time. Thus, these data do not support the travelling activity hypothesis. Instead, the data indicate that the saccadic discharges occur almost simultaneously across the entire SC, which may help to trigger the saccade.
展开▼
