Contrasting effects of lateral striate and superior colliculus lesions on visual discrimination performance in rhesus monkeys.

Rhesus monkeys with lesions of lateral striate cortex, monkeys with superior colliculus lesions, and unoperated monkeys were tested for retention of a preoperatively acquired pattern discrimination. The three groups of monkeys were then tested in two-choice, color, color discrimination tests, one involving varying degrees of stimulus-response (S-R) separation and the other, administered several months later, involving various directions of S-R separations. The monkeys were also tested in a series of two-choice pattern discriminations, following each of which they were tested for relearning when the patterns were masked with bars or circles. The monkeys with lateral striate lesions were moderately retarded in retention of the pattern discrimination, whereas those with superior colliculus lesions were not. The monkeys with colliculus lesions, but not those with lateral striate lesions, were impaired in both S-R separation tests, which demonstrates that their deficit was not transient or solely due to a difficulty in shifting the gaze in one direction. The lateral striate monkeys, unlike those with colliculus lesions, were dificient in relearning discriminations between masked patterns. These findings suggest that superior colliculus and striate cortex may be involved in two different aspects of attention: respectively, shifting attention (and orientation) from one spatial locus to another and maintaining attention on fixated stimuli. Alternative interpretations of the effects of the lesions, based on their retinotopic loci, are discussed.     

Selective lesions in the temporal-hippocampal region of the rat: effects on acquisition and retention of a visual discrimination task.

The first purpose of this study was to investigate whether lesions in the temporal region may affect acquisition or retention of a discrimination task. In Experiment 1, rats with lesions of the temporal cortex (TC), the lateral entorhinal cortex (LEC), or their interconnections were tested postoperatively in simultaneous brightness discrimination. The results show that neither TC lesions nor LEC lesions affected acquisition of the task, and only LEC lesions impaired retention. TC/LEC transections impaired both acquisition and retention. The second purpose was to investigate effects of hippocampal lesions and perforant path transections on the discrimination task (Experiment 2). Both hippocampal and perforant path lesions impaired acquisition of the task, whereas retention was unaffected. It is suggested that TC and LEC are primarily involved in information storing and that hippocampal function is primarily involved in information processing.     

A comparison of the effects of inferotemporal and striate cortex lesions on the visual behaviour of rhesus monkeys

The effects of bilateral removal of inferotemporal cortex or lateral striate cortex are compared. The former operation impairs visual pattern discrimination learning, without disturbing prompt detection and retrieval of food, visual acuity or visual fields. In contrast, animals in which the macular striate projection area has been removed are significantly superior in tests of visual pattern discrimination learning despite showing impairment of visually guided reaching, visual acuity, and visual fields. The results indicate that the inferotemporal defect is not caused by deficient visual sensitivity or acuity.     

Attention effects on form discrimination at different eccentricities

Considerable disagreement exists in the visual attention literature about how attention is allocated over the visual field. One frequently expressed metaphor is that attention moves like a spotlight, and in some variants it is assumed that attention takes longer to shift to targets further from fixation. In order to test this metaphor, five experiments were conducted in which target location was precued and form discrimination accuracy was assessed. By varying the interval between the precue and the target (stimulus onset asynchrony, SOA), a time course of attention effects was obtained for targets at 2°, 6°, and 10° eccentricity. In the first three experiments, precueing effects were found, but there were no differences in performance as a function of eccentricity for very short SOAs, with either a peripheral cue or a foveal arrow cue. For long SOAs, however, performance was better for targets that were closer to fixation. In Experiments 4 (peripheral cue) and 5 (foveal cue), the targets were scaled to make them equally discriminable at all eccentricities. Again precueing effects were found, but there were no differences in accuracy as a function of eccentricity for most SOAs. These results suggest that attention shifting is not analogous to a constant-velocity moving spotlight.     

The effects of prelimbic and infralimbic lesions on working memory for visual objects in rats.

The present experiment investigated the effects of quinolinic acid (90 mM) lesions of the prelimbic-infralimbic cortices on working memory for visual objects and on acquisition of a visual object discrimination. In both tests a GO/NO-GO procedure was used. In the working memory task, rats were tested before and after surgery. A continuous recognition procedure was used to assess working memory, which involved successive exposure to different three-dimensional objects that could be displaced to receive a cereal reinforcement. Of the 12 object presentations/session, 4 objects were presented for a second time in which displacing the object did not result in a reinforcement. The number of trials between the first and second presentations of an object ranged from 0 to 3 (lags). Memory was assessed by the latency to displace an object during the second presentation. In the visual object discrimination, rats had successive exposure to two different objects. Displacement of one object resulted in a cereal reinforcement, while displacement of the other did not. The findings indicated that prelimbic-infralimbic lesions significantly impaired memory for visual objects across all lags. Prelimbic-infralimbic lesions did not impair acquisition of the visual object discrimination. The results suggest that the prelimbic-infralimbic areas are part of neural system important in the short-term memory for visual objects.     

Effects of collicular lesions in the hamster during visual discrimination. An analysis from computer-video actograms

Recent reports have suggested that lesions of the superior colliculus may impair the orienting or scanning movements of the head that are thought to serve a key function in visual discrimination. In the present investigation computer-video “actograms” were used to quantify the head movements of freely moving hamsters performing a simultaneous visual discrimination. Hamsters with collicular lesions did not differ from the controls in their head movement spectra, but there was a significant reduction in the incidence of the pauses in locomotion during which these movements are made. Intact animals showed slower and irregular progression interspersed with pauses during which they made scanning movements, whereas collicular hamsters made straighter runs. Although the rate of learning of lesioned hamsters was not impaired, our data strongly suggest that they used different orienting and learning strategies. When tested for the effects of novel stimuli, unrelated to the task, normal hamsters reacted strongly with active exploratory scanning movements, but collicular animals did not, although qualitative changes in their behaviour suggested that the novel stimuli were not being ignored.