Interhemispheric engram transfer in chick

Abstract

Monocular training on a one-trial passive avoidance task in the young chick has been shown to establish an engram in the forebrain hemisphere contralateral to the trained eye, and hence interocular transfer of this task must involve the naive hemisphere making access to the engram in the opposite hemisphere. We have studied the consequences for the untrained hemisphere of accessing a unilateral engram during two short term stages of its consolidation, by conducting interocular transfer tests at 7.5 min and 30 min after learning, prior to onset of amnesia induced by intracranial injection of ouabain or cycloheximide. Testing the naive hemisphere 24 h later indicated that engram transfer had occured in chicks receiving one access trial at 7.5 min, during the first stage of consolidation of the unilateral engram. At 30 min, during second consolidation stage, five access trials were required to achieve engram transfer. These experiments confirm the findings from the rat that engram transfer can follow from making access to the unilateral engram. However, it has not previously been recognised that interhemispheric memory transfer depends on the recency of formation of the unilateral engram.