Memory Stages and Brain Asymmetry in Chick Learning

Stages of formation of memory and the roles of different forebrain structures in memory formation were investigated by injecting various agents into the brains of chicks close to the time of peck-avoidance training. With L-glutamate injected bilaterally into the hyperstriatum 5 min pretraining, retention was good 1 min posttraining but significantly impaired at 5 min and each subsequent time point from 10 min to 24 hr. With ouabain, retention declined more slowly, showing significant impairment at 15 min and thereafter. With any of three protein synthesis inhibitors (anisomycin, cycloheximide, or emetine), retention was still good 60 min posttraining but significantly impaired at 90 min. The three time courses of decline of retention are consistent with hypotheses of three sequentially dependent stages of memory formation. Glutamate, ouabain, and emetine were found to affect only a restricted volume of tissue. Any of these three agents induced amnesia when injected into the left (but not the right) medial hyperstriatum ventrale or into the right (but not the left) lateral neostriatum; so it appears that both structures are required for formation of memory. Agents that are specific for a presumed stage of memory formation and whose action is restricted spatially should help reveal the roles of different brain structures in different stages of memory formation.