Effects of monocular enucleation at birth upon learning of a vertical-horizontal discrimination in hooded rats.

Previously we have demonstrated that adult albino rats with one eye removed at birth (OEB) relearn a black-white discrimination faster than those monocularly enucleated at maturity (OET) when relearning is conducted after lesioning of the visual cortex contralateral to the remaining eye (Type A experiment). This faster relearning phenomenon is considered to be one behavioral expression of the functioning of the expanded uncrossed visual pathways (expanded UXVPs) resulting from monocular enucleation at birth. However, neither OEBs nor OETs were able to master the discrimination when the experiment was conducted without previous learning following the same surgical treatment (Type B experiment). We hypothesized that this occurs because the cues to discriminate might be close to the threshold of discrimination for either the normal UXVPs or the expanded UXVPs. In order to gain insight into the hypothesis, the present study was undertaken using hooded rats as subjects which genetically possess larger and presumably more efficient functioning UXVPs. The questions addressed were as follows: 1) Whether or not the UXVPs can mediate a vertical-horizontal discrimination in OEBs and OETs. 2) If they can, is there any difference in the upper limit of discrimination capacity between the normal UXVPs and the expanded UXVPs? Three experiments were carried out. In the Type A experiment OEBs relearned discrimination of the 10-mm stripes [0.44 cycles/degree (c/d)] faster than OETs (Experiment 1), yet in the Type B experiment neither OEBs nor OETs were capable of acquiring that discrimination (Experiment 2). However, they could originally master the discrimination equally well when the width of stripes was broadened to 30 mm (0.15 c/d). And when the width of stripes was systematically reduced thereafter, the width of the smallest stripes for the expanded UXVPs to discriminate was found to be 6 mm (0.73 c/d) and that for the normal UXVPs 10 mm (0.44 c/d) [Experiment 3]. These findings were discussed in relation to the hypothesis advanced on our previous data in albino rats.