The superior colliculus and visual neglect in rat and hamster. I. Behavioural evidence.

Lesions of the superior colliculus in rats and hamsters produce a severe visual neglect. Three questions are asked concerning the nature of this impairment. Is the neglect a specific deficit, or part of a general disorder? It appears that the impairment is a relatively specific one, because, for example, collicular animals learn many visual discriminations as fast as controls. This pattern of behaviour leads to the second question. What is tested in neglect tasks but not in discrimination problems? Two answers have been proposed. (a) Orienting responses are required in tests of neglect but not in conventional visual-discrimination tasks. Accordingly, it has been suggested that damage to the superior colliculus interferes specifically with the orienting response. However, analysis of recent evidence indicates that rats and hamsters with collicular damage usually make no detectable response of any kind in tests of neglect, and that in some situations they do not respond to visual stimuli that produce a variety of behaviours in normal animals, such as freezing or fleeing, or activation of the EEG unaccompanied by any gross movement. Collicular neglect cannot therefore be explained solely as a response-specific impairment. (b) The stimuli used on tests of neglect are usually small, moving and presented in the peripheral visual field. In contrast, visual discriminanda are typically large, stationary and can be viewed with the central field. Recent experiments provide direct demonstrations that rats with lesions of the superior colliculus can orient to small flashing lights in central regions of the visual field, but unlike control animals may fail to respond if the lights are made dimmer, or are moved into the periphery. It appears that rats and hamsters with collicular damage fail to register particular kinds of visual stimulus. The final question concerns the nature of this stimulus-specific impairment: Do rats and hamsters with lesions of the superior colliculus neglect certain stimuli because, as has been proposed, they have difficulty in attending to the stimuli, or because they actually are incapable of detecting them? The fragmentary evidence currently available suggests that attentional factors are not important for stimuli that are very small, or that are presented in the far periphery of the visual field: such factors may be more important for large transient stimuli in the central field.(ABSTRACT TRUNCATED AT 400 WORDS)