Single neurons were recorded from the superficial layers of the superior colliculus of immobilized monkeys (Macaca mulatta and Macaca irus). Two main functional types of neurons were found. The neurons of the first type (Type I neurons) responded well to simple stationary and moving stimuli such as spots, bars or slits of light. The latency of their response was 41 +/- 6 ms. They were not directionally selective and responded to a large range of velocities. The neurons of the second type (Type II neurons) responded very poorly to simple visual stimuli and their activation required real objects or certain two-dimensional patterns. The mean latency of response of these units was 66 +/- 26 ms. Habituation was always present. Type II neurons were located in the lower part of the superficial layers. The characteristics of Type II neurons suggest that in the primate superior colliculus there is a mechanism that allows the recognition of the complexity and the novelty of a stimulus and guides orienting responses to those stimuli that are worth analyzing in detail.