Approach and avoidance systems in the rat

Current models of the mammalian superior colliculus focus on its role in the production of saccadic eye movement. In these models a single command, related to desired change in eye position, is passed down a single output channel to the brainstem, where it is transformed into an appropriate signal for the oculomotor neurons. In rodents, however, both anatomical and behavioural findings point to the existence of multiple functional output channels. These appear able to command a wide range of responses appropriate to an unexpected sensory stimulus, including not only orienting and approach, but a set of defensive reactions that vary from freezing to rapid flight, together with appropriate physiological changes in cortical EEG, blood pressure and heart rate. It may therefore be possible to use the mammalian superior colliculus as a preparation for studying and modelling a number of basic issues in sensorimotor control. These new models need not be concerned only with avoidance or defence. For example, in models of saccade generation, the trajectory of the movement is formed downstream of the superior colliculus: in rodents, however, it appears that the colliculus can control head-movement trajectory directly. Data concerning this collicular control system might therefore be of use in testing models of how the brain computes movement trajectories.