Ontogeny of passive avoidance: role of task demands and development of species-typical behaviors

Preweaning rats 10 to 15 days of age were given passive avoidance training in a circular chamber in which the floor comprised one shock electrode and the wall the other. All ages showed acquisition of the response, but both rate of learning and asymptotic performance increased with age. Yoked-shock controls, adaption controls, and naive controls did not demonstrate passive avoidance on subsequent test trials, ruling out alternative interpretations based on sensitization to shock, fatique, familiarity with the apparatus, and handling. From 10 to 15 days of age, improved performance was due in part to the transition from stereotyped shock-elicited behavior in the younger pups to discrete withdrawal from shock in the older pups. A paradoxical deterioration of performance with observed from 20 to 25 days of age, attributable to the development of species-typical defense reactions which were incompatible with the avoidance response. Retention of the avoidance response was found to increase from about 6 hr in the 10-day-old to at least 5 days in the 15-day-old. The data suggest that maturation of storage and/or retrieval mechanisms, rather than perceptual or experiential changes induced by development, is responsible for improved retention during ontogeny.