Interhemispheric transfer: Monocular input and varied sensory conditions

The interhemispheric transfer of a visual discrimination was explored in rats. On Day 1 they were trained the task with one cortical hemisphere rendered non-functional by cortical spreading depression (CSD). This typically has the effect of limiting the information to the functional (trained) hemisphere. On Day 2 interdepression trials (IDT) were given with visual input limited to one eye, which has the effect of largely limiting the input to one hemisphere, either the trained hemisphere (the direct-access case, DA). or the untrained hemisphere (the indirect-access case, IA). On Day 3 savings were measured with either of the hemispheres depressed. Three basic IDT conditions were explored; (1) normal IDT, where IDT is identical with Day 1 training; (2) reversed IDT, where the stimuli during IDT are the reverse of that during Day 1 training; and, (3) non-specific IDT, where totally new stimuli are used during IDT. It was found that normal IDT facilitated transfer only under IA conditions, while reversed and non-specific IDT facilitated transfer only under DA conditions. It was further suggested that transfer under IA conditions is highly specific, while under DA conditions it involves perhaps two components, and is not specific to the training on Day 1. The results were discussed within the framework of a model of interhemispheric transfer involving two main components: (1) a trans-commissural write-in from the trained to the untrained hemisphere; and, (2) a comparison of the information transferred from the trained to the untrained hemisphere with the direct sensory input to the untrained hemisphere during IDT.