The role of the superior colliculus in gaze control has been well studied over the last several decades. Many studies have investigated the discharge of neurons in the deeper layers in association with saccades. These cells are characterized by high frequency bursts of spikes beginning roughly 20 ms before saccade onset. Although these bursts are most vigorous for saccades of a particular vector, the movement fields of individual neurons are often quite large. Thus, the SC evidently utilizes a population coding scheme that is able to generate precise motor responses despite the broad tuning of individual neurons. Sparks, et al. (1976) proposed that each activated SC neuron requests a saccade of the vector corresponding to the peak of its movement field. The SC would encode the precise vector of the desired saccade by taking a weighted average of the movement tendencies of the individual members of the activated population. Lee et al. (1988) and Sparks et al. (1990) tested this population coding hypothesis in head-fixed rhesus monkeys by reversibly inactivating a subset of the normally activated populations. The predicted pattern of errors was found for saccade amplitude and direction. Recently, however, it has become clear that the primate superior colliculus is involved in the generation of coordinated eye-head gaze shifts, not just saccades. Therefore, procedures similar to those used in Lee et al. (1988) and Sparks et al. (1990) will be used to determine if the population coding hypothesis can be extended to the more realistic situation where the animal's head is free to move. A detailed analysis will be performed on the resulting pattern of errors for the eye, head, and gaze.