Perturbing the Temporal Characteristics of Saccades

Saccades are very stereotyped movements. This means that saccades of a given size are carried out with a fixed duration, speed and trajectory. When neural recordings are made in central structures in the brain this fact makes it difficult to associate recorded neural signals with causality in the behavior. For example, a signal recorded in the superior colliculus (SC) that is correlated with eye speed during saccades can not be claimed to code eye velocity because saccade duration covaries with saccade speed. In standard man-made systems such covarying behavior is often distinguished by perturbing the normal movement trajectory in order to separate the various possible variables involved in the behavior. We have pioneered this technique for the saccadic system by placing stimulating electrodes in the region of the omnipause neurons (OPNs). Short pulse trains of high frequency stimulation delivered through this electrode near the onset of a targeting saccade halts the movement in midflight. When the stimulus is turned off, the movement resumes and lands near the location of similar un-stimulated movements in a block of trials. The figure shows the activity of a neuron recorded in the SC during an interrupted saccade produced by OPN stimulation. The activity of the neuron is altered drastically in trials in which the stimulation is delivered.