Detection of visual stimuli after lesions of the superior colliculus in the rat; deficit not confined to the far periphery.

The visual neglect which follows lesions of the superior colliculus in the rat is well documented, although its nature has been somewhat contentious. Recently, using large, bright stimuli, Overton et al. [Exp. Brain Res., 59 (1985) 559-569] presented evidence that collicular lesions produce a detection deficit in the far peripheral visual field (120 degrees), whilst the central field is unaffected. However, some simple tests for visual neglect by other workers suggest that a detection deficit may also exist in the central field, if the stimuli are dim (and small). This possibility was investigated psychophysically. Using a signal detection paradigm, sensitivity to small (2.5 degrees v.a.), transient (modal duration 200 ms) stimuli of varying luminance was determined at 40 degrees on either side of the vertical meridian, before and after collicular lesions or control lesions of the striate cortex. Despite extensive preoperative training, collicular animals showed a decrease in sensitivity of around 50% for all stimulus intensities postoperatively. This deficit was significantly more severe than that produced by striate lesions. Furthermore, it proved very difficult to retrain the animals to a preoperative criterion. These data suggest that collicular lesions do indeed cause a detection deficit which is not confined to the far periphery, and that this deficit may specifically affect the detection of small stimuli. The proposed detection deficit is related to recent anatomical findings in the rat.