Mechanisms of interhemispheric transfer of visual information in rats

Pattern discrimination learning in functionally hemidecorticate rats leads to formation of memory traces in the intact hemisphere. The interhemispheric transfer (IHT) of such lateralized engrams is more efficient when the untrained eye rather than the trained eye is used during interdepression training (10 trials) preceding the retention test with the contralateral hemisphere, probably because the untrained hemisphere is indispensable for readout in the first case (imperative IHT) but can be completely bypassed in the second case (facultative IHT). Monocular acquisition of a pattern discrimination habit induces a strong primary engram in the contralateral and a weak secondary engram in the ipsilateral hemicortex. The primary trace is further improved with overtraining while the secondary engram is abolished. During interocular transfer, the primary trace is at first read out through commissural fibers while a secondary trace is rapidly transferred to the untrained hemisphere. Pretraining administration of 2-pyrrolidone acetamide (Piracetam, 100 mg/kg) improves the secondary trace acquired during monocular pattern discrimination learning almost to the level of the primary trace, and facilitates the imperative IHT. Uncrossed optic projections and subcortical storage may contribute to IHT of brightness but not of pattern discrimination. It is concluded that IHT of visual engrams is mainly due to transcommissural encoding activated during learning or by transcommissural readout of lateralized traces.