Memory consolidation of one-trial learning in chicks.

The thesis that an enduring memory trace takes time to consolidate has not only been inferred from the phenomena of retroactive inhibition in verbal learning' and from retrograde amnesia after brain injury,2 but is also supported by many direct experimental studies. Most of the experiments have been conducted on rats. The typical procedure is to subject the animal to electroconvulsive shock or some other "disruptive" treatment shortly after a learning experience, and later assess the effect on retention of the learning. In general, it has been found that retention is disturbed only if the treatments are given within a critical "consolidation period" that varied from a few seconds to a few hours in different experiments. Within this period, the sooner the treatment, the worse the retention (see recent reviews3-5). The present study investigates the kinetics of memory consolidation of one-trial learning in day-old chicks, using electroconvulsive shock (ECS) as the treatment. Newly hatched chicks were used partly for comparative purposes and partly because of some special advantages involved in the particular experimental procedure. This procedure is free from possible "punishing" effects of the ECS, a complicating factor that has been difficult to control in many earlier studies. Also, its simplicity makes feasible the use of large numbers of subjects, thus yielding more reliable data and permitting the detection and clarification of certain subtle features of the problem, as will be indicated below. Experiment 1.-Disruption by ECS of memory consolidation in chicks: The aim of the first experiment was to determine whether retention of learning in chicks can be disrupted by postlearning ECS, and if so, how retention deficit depends on the interval between the learning trial and ECS. The learning involved inhibition of the chick's natural predilection to peck at a visually attractive lure. Chicks in the experimental groups were each given a learning trial followed after various intervals by an ECS. Retention of the learning was measured 1 or 2 days later. Some control groups provided data on either the normal retention level without ECS, or the basal pecking rate without learning and ECS. Other control groups served to check whether ECS functioned as a punishment in the learning situation, or whether an ECS alone would influence the normal pecking rate. Method.-Day-old White Leghorn cockerels (S's) were obtained in the morning from a nearby hatchery and housed individually in white, cylindrical, topless cartons (each 17 cm high, 9 cm in diameter) in the experimental room where the temperature was 85-95° and the humidity was 30-50%6. S's that would later receive an ECS were anesthetized with halothane, and two electrodes were implanted under the skin, one on each side, behind the eye and above the ear. The electrodes were made of no. 16 Mustad-Viking hooks each connected with about 30 cm of fine, insulated no. 8340 Belden wire that led to the outside of the carton. Implantation of the electrodes was performed between 9 and 11 A.M., after which