Bilateral surgical lesions within occipital and temporal regions of the posterior neocortex and visual relational learning in the rat
Sirotkin, Laura Brewster Williams
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Research concerning the modular structure of the mammalian brain has sought to clarify how anatomically discrete regions of the neocortex underlie specific cognitive and behavioral functions. Historically, the laboratory rat has served as a valuable model organism in the study of cortical organization at the anatomical, chemical, physiological, and behavioral levels as a basis for comparative investigations. Findings from such studies suggest that the functional neuroanatomy of exclusive posterior neocortical regions in the rat resembles to a considerable degree that observed in more developed animals. Clinical observations and neurobehavioral experiments in primates indicate that injury to temporal regions results in marked deficits in visual analysis. The goal of the present study was to investigate the contributions of a purportedly homologous cortical region in the rat to such functions. The question of whether lesions in the rat temporal cortex impair performance on behavioral tasks that necessitate the use of visual relational learning was examined. In preoperative training, rats (n = 24) mastered a simple discrimination problem based on the relative luminance of two distinct visual cues and, after a 9-week hiatus, demonstrated retention of the acquired task. In the intervention phase, rats were separated into three surgical lesion groups defined as bilateral sham control (CON), partial striate (PS) or temporal (TE). In post-surgical retraining, rats (n = 21) re-mastered the original problem. In final testing, CON and PS rats generalized the original task to multiple, presumably more demanding, discrimination problems based on the relative luminance of novel, paired visual stimuli. In contrast, the TE group failed to attain behavioral criterion, exhibiting performance deficits on all measures compared to the other groups. The experimental data from the present study in the rat are consistent with research findings in primates that have suggested the presence of a specialized ventral occipitotemporal pathway subserving functions of higher visual processing.