The kinetics of neurobiological phenomena involved in 1he formation of a memory trace were examined in the first two experiments. Transcranial subconvulsive current was administered to large groups of chicks at various intervals following an aversive training experience. The resulting retrograde amnesia data indicate that immediately upon training a metastable mnemonic process becomes activated (STM) which then remains at a constant intensity. Within a minute STM induces a more permanent form of memory (Pre-LTM) to grow at a steadily declining rate, apparently as some restricted neural substrate of memory becomes exhausted. STM may continue to function as behaviorally accessible memory for the next few hours, during which time the behaviorally latent Pre-LTM trace undergoes a subsequent transition into permanent memory. An investigation of the retrograde amnesia resulting from a sequence of two training-current sessions provides support to the existence of these mechanisms and indicates that fractional engrams summate together in a simple fashion.
To examine the participation of different cerebral structures in information processing, chicks having various telencephalic lesions were tested in either a passive avoidance task or an appetitive discrimination. The hippocampus was found to be involved in reversal but not acquisition of the pattern discrimination, and in acquisition but not retrieval of the passive avoidance task. On the other hand, the amygdala seems to be important both for retrieval and acquisition of passive avoidance conditioning, but only for early stages of acquiring the pattern discrimination. Frontal ablations resulted in a deficit to retrieval but not to acquisition of passive avoidance conditioning, and caused some motivational changes independent of chicks' learning ability in the performance of the appetitive task. A comparison of these results with those following lesions in homologous mammalian limbic system structures suggests that the information processing of both classes is based upon cerebral mechanisms which have remained unchanged despite their divergent evolution.
Mechanisms of memory processing and interhemispheric transfer were further studied in chicks having extensive unilateral ablations of the dorsal telencephalon, a region critical for visual learning. Although chicks were able to acquire a passive avoidance response equally well using either the eye ipsilateral or that contralateral to the surgery, subsequent extinction conditioning could be learned only through the ipsilateral eye. Since retinal projections cross completely at birds' optic chiasma, these results suggest that anatomically distinct systems, one bilaterally represented, the other lateralized, respectively mediate the acquisition and extinction of the aversive response. An inability or lateralized memory to transfer through the commissures is indicated by the absence of interocular transfer for monocularly learned extinction.</p
