A distributed model of the saccade system: Simulations of temporally perturbed saccades using position and velocity feedback

Interrupted saccades, movements that are perturbed in mid-flight by pulsatile electrical stimulation in the omnipause neuron region, are known to achieve final eye displacements with accuracies that are similar to normal saccades even in the absence of visual input following the perturbation. In an attempt to explain the neurophysiological basis for this phenomenon, the present paper describes a model of the saccadic system that represents the superior colliculus as a dynamic two-dimensional, topographically arranged array of laterally interconnected units. A distributed feedback pathway to the colliculus from downstream elements, providing both eye position and velocity signals is incorporated in the model. With the help of a training procedure based on a genetic algorithm and gradient descent, the model is optimized to produce both the normal as well as slow saccades with similar accuracy. The slow movements are included in the training set to mimic the accurate saccades that occur despite alterations in alertness, as well as following various degenerative oculomotor diseases. Although interrupted saccades were not included in the training set, the model is able to produce accurate movement of this type as an emergent property for a wide range of perturbed eye velocity trajectories. Our model demonstrates for the first time, that by means of an appropriate feedback mechanism, a single-layered dynamic network can be made to retain a distributed memory of the remaining ocular displacement error even for interrupted and slow saccades. These results support the hypothesis that saccades are controlled by error feedback of signals that code efference copies of eye motion, and further, suggest a possible answer to a long standing question about the kind of the feedback signal, if any, that is received by the superior colliculus during saccadic eye movements.