Interhemispheric transfer of visual pattern discriminations: evidence for a bilateral storage of the engram

The present experiment was carried out to determine whether the memory trace of a pattern discrimination learned with one hemisphere is also transmitted to the second hemisphere via the corpus callosum or whether the trace is limited to the trained hemisphere and becomes accessible to the second during recall via this route. Split-chiasma cats learned two pattern discriminations with one eye (and hemisphere), then were subjected to a mid-sagittal transection of their corpus callosum, followed by learning with the other untrained eye (and hemisphere). Ten cats were separated into two groups: one group learned the discrimination to criterion (the non-overtrained group) while the other received 1600 overtraining trials over and beyond those needed to reach criterion (the overtrained group). Results indicated that there was little bilateral storage in the non-overtrained group (as determined by the number of trials needed to attain criterion with the second eye). Most subjects from the overtrained group showed chance performance during the first transfer session but learned the pattern discrimination much more rapidly with the second eye than with the first. These results are taken as indicating that memory transcription is possible through the callosum but that this route is slower and its readout is possibly contaminated by secondary non-specific factors which affect the initial utilization of the trace.