| Summary: | The role of several subcortical structures in visual intensity discrimination was examined by comparing the effects of localized lesions on a variety of intensity discriminations. In Experiment 1 light avoidance was unimpaired after lesions of the ventral lateral geniculate nucleus (LGNv), nucleus lateralis posterior (TLP), nucleus posterior of Gurdijian (NPG), dorsal pretectum (PTd), and ventral pretectum (PTv). The LGNv, TLP, NPG and PTv, but not the PTd, groups were impaired on a simultaneous black versus white (BW) discrimination in Experiment 2. None of these groups was impaired on a horizontal versus vertical discrimination (HV). The TLP group showed a transient impairment on a successive light versus dark discrimination, not present with the LGNv and NPG groups (Experiment 3). In Experiment 4 all three groups were impaired on a successive BW discrimination. In Experiment 5 rats with LGNv lesions but not with TLP lesions had elevated relative brightness thresholds. Both groups had normal absolute thresholds. The results are related to the possibility that information about intensity and pattern is coded in separate visual pathways.
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