Target selection and the superior colliculus: goals, choices and hypotheses.

The intermediate and deep layers of the superior colliculus (SC) compose a retinotopically organized motor map and are known to be important for the control of saccadic eye movements. However, recent studies have shown that the functions of the SC are not restricted to the motor control of saccades. The pattern of activity observed during multi-saccade movements, combined eye-head movements, and pursuit eye movements argue that activity in the SC indicates the location of the goal, but does not specify the particular movements that will be used to acquire the goal. Prior to the onset of a movement, the SC is involved in representing possible targets, as has been shown by the effects of manipulating target probability and the changes in pre-motor activity found during visual search tasks. A major unresolved issue is how target-related activity in the SC is transformed into the signal that triggers the movement. One possibility is that the levels of activity across the SC motor map correspond to the likelihood of the possible targets, and that the movement decision is based on the neural equivalent of hypothesis testing. In one version of this mechanism, the decision to select a new goal would occur each time the null hypothesis, represented by neurons in the rostral SC, was rejected in favor of an alternative represented elsewhere in the SC.