Spatial and attentional functions of the midbrain visual system
The superior colliculus of the midbrain has been implicated in spatial and attentional behaviours, in particular the redirecting of attention to novel peripheral stimuli. Paradoxically, while certain aspects of sensory and motor organisation within the structure are common to all species studied, others (eg. single unit characteristics) show wide interspecies variation. Models of intracollicular processing and possible functional subdivisions were discussed. The hippocampal formation and dorsomedial frontal cortex in the rat have also been reported as having spatial and attentional functions. Thus in the present studies, rats with both types of lesion were compared with colliculars on a variety of behavioural measures. Those with bilateral collicular lesions were grossly inattentive towards novel stimuli in a number of situations, regardless of lesion size. Elevation of open field activity occurred in animals with large lesions extensively damaging deeper collicular laminae. Unilateral lesions of colliculus or frontal cortex produced ipsiversive turning when leaving a goaldoor, and contralateral stimulus neglect, though gross locomotion was at worst temporarily affected. Rats with bilateral frontal lesions were normally attentive and tended to exhibit elevated "attentional" behaviour in an open field arena. Hippocampals showed intermittent distractability in a runway and, unlike other groups (but like visual decorticates), exhibited a severe radial maze deficit. Hippocampal "hyperactivity" in an open field was found to result primarily from decreased habituation across test sessions. It was concluded that hippocampal, dorsomedial frontal cortical, collicular and posterior cortical lesions are dissociable in terms of their behavioural consequences; also that the most likely function of the superior colliculus is the detection of novelty and shifting of attention, though the details of such a function are likely to vary with task, species, amplitude of orientation response, and the location, meaningfulness and other parameters of the stimulus concerned.