Slow off responses in the rat superior colliculus neurons may contribute to the predator detection.

A slowly moving dark spot imitating the shadow of a hovering bird of prey has been shown to induce freezing in rodents. Such visually triggered behaviours are usually initiated in the superior colliculus (SC); therefore, it is likely that such slowly moving dark spots can produce responses in SC neurons. In SC, two types of visual responses are typically distinguished: ON responses are produced by an increase in image brightness, and OFF responses are produced by a decrease in image brightness. Typically, OFF responses are very brief, lasting only a few hundred milliseconds, and may be poorly suited for the detection of slowly moving dark spots. Here, we report that, in the majority of SC neurons of urethane-anaesthetized rats, in addition to these brief OFF responses, very slow OFF responses lasting over 5 s were present; thus, OFF responses that occurred > 1 s after the stimulus offset were termed 'slow OFF' response, while brief, less than 1 s long OFF responses were called 'fast OFF' response. Although the slow OFF responses were of similar amplitude as the fast OFF responses (∼5 Hz), the optimal size (producing the maximal response) was larger for the slow OFF responses (20° for the slow and 10° for the fast OFF responses). Correlation analysis revealed that both the fast and the slow components of the OFF response contribute to the response to a slowly moving spot. Elimination of visual cortex inputs increased the amplitude and duration of the slow OFF responses, indicating that they originate in the retina. It is concluded that in rodent SC, a new type of OFF response that is well suited for predator detection is present.