Production, Control, and Visual Guidance of Saccadic Eye Movements
Figure 1
Neural networks for the guidance and control of visually guided saccades. Consider visual search for a red “” among randomly oriented red and green “L”s. The color and shape of the objects are specified in feature maps that could also represent motion, depth, and other visual features. These feature maps converge on a map that represents the evidence for salience at each location. This salience map is also informed by a target template in working memory. The timecourse of the salience evidence representation at the target location (, solid line) and a distractor location (, dotted line) is plotted. According to the gated accumulator model, this evidence is integrated by a network of mutually inhibitory units that will produce a saccade to the target (, solid line) or to a distractor (, dotted line). A gate (orange box) prevents integration of noise by requiring the salience evidence to be of sufficient magnitude. A saccade is produced when the activation of a GO unit reaches a threshold (gray horizontal line) at which point inhibition is imposed on omnipause (OPN) neurons (red line) that releases inhibition of burst neurons (BNs) that innervate motor neurons (MNs) to produce a pulse of force to rotate the eye rapidly. The eye velocity signal from the BNs is integrated by a network of tonic neurons (TNs) that also innervate the MN to establish a step of force necessary to maintain eccentric fixation of the target. The activation of the GO units is also influenced by gaze-holding STOP units that release inhibition on the GO units while saccade preparation transpires. If a stop signal of some kind occurs, then the STOP units potently interrupt the GO unit activation from reaching the threshold; this interruption occurs within the theoretical interval known as stop signal reaction time (SSRT) (rightmost columns). An executive control network (yellow) comprised of neurons sensitive to errors, reward, and the conflict arising from coactivation of mutually incompatible response processes signals the consequences and conditions of an action. This executive control network may influence the level of the gate that systematically changes the beginning of the accumulation process to emphasize either speed or accuracy in task performance.
