The effects of distributed training on retention of operant conditioning in human infants.

Long-term retention of operant footkicking acquired in the mobile conjugate reinforcement paradigm was assessed as a function of the distribution of training time. In the first study, 3-month-old infants were trained for either one 18-min session or for two 9-min or three 6-min sessions separated by 24-hr intervals. All infants exhibited retention during a test administered immediately after training, but only those trained in a single session continued to perform the conditioned response during cued-recall tests 7 or 14 days later. Infants trained in three sessions showed no evidence of remembering the contingency even after a week. A warm-up decrement, seen in the day-to-day performance of infants in the distributed conditions, was eliminated in the second study by the interpolation of a nonreinforcement period at the outset of daily sessions. This procedure also enhanced long-term retention such that infants trained in three 6-min sessions now remembered the contingency for 14 days and did not differ from infants who had received a single 18-min session. Whether distributed training facilitates or impairs long-term retention appears to depend on the opportunity for infants to acquire a sufficient number (or kind) of effective retrieval cues during original learning.

The economics of food choice in chicks.

Growing chicks offered diets containing either surfeit or inadequate protein in the day but adequate protein at night adopted a pattern of nocturnal feeding but did not eliminate diurnal meals. Their weight gain surpassed that of corresponding dietary controls who received continuous access to surfeit or inadequate protein but was less than that of chicks with continuous access to a standard diet. Chicks receiving the standard diet at night with no food available in the light phase also grew but did so at a slower rate than the diurnally feeding control group. The shift in feeding patterns was not accompanied by a shift in body temperature, nor was nocturnal antipredator behavior, characterized by sustained motoric inhibition, disrupted by nocturnal feeding. These data demonstrate that feeding specializations which have been selected over a species' evolutionary history are not rigidly fixed but can be modified by the economic relations in the current habitat.

Organization of infant memory.

The differential retrieval of specific details and general features was studied in a conditioning paradigm with 3-mo.-olds. Infants learned to move a crib mobile by foot kicking and produced high response rates during cued-recall tests with the same components after retention intervals of 24, 48, 72, and 96 hr. Use of a novel mobile during retention tests significantly reduced responding for as long as 3 days after training. As time since training increased, however, response rate gradually increased until, after 96 hr., it was high and indistinguishable from response to the original mobile. The latter was interpreted as (a) the failure to detect specific details as novel after 96 hr. and (b) the use of general features of the novel mobile as an instance of the general class "mobiles" as retrieval cues. The data demonstrate that access to general visual features persists longer than access to specific visual details.

Immediate and long-term memory for reinforcement context: the development of learned expectancies in early infancy.

Infants were pretrained to move crib mobiles containing 6 or 10 identical objects by means of footkicking and were then exposed to a reinforcer containing only 2 objects. Relative to infants with no prior history of reinformcement with either of the "larger mobiles, infants shifted to the "smaller" mobile had higher kick rates, and, in addition, their visual attention decreased and negative vocalizations increased. These effects were not transient but persisted 24 hours later in infants experiencing the larger (10- to 2-object) shift. The results suggested that infants develop reward-expectation habits which continue to influence behavior for periods of at least 24 hours.

Reactivation of infant memory.

Three-month-old infants learned to activate a crib mobile by means of operant footkicks. Retention of the conditioned response was assessed during a cued recall test with the nonmoving mobile. Although forgetting is typically complete after an 8-day retention interval, infants who received a reactivation treatment--a brief exposure to the reinforcer 24 hours before retention testing--showed no forgetting after retention intervals of either 2 or 4 weeks. Further, the forgetting function after a reactivation treatment did not differ from the original forgetting function. These experiments demonstrate that (i) "reactivation" or "reinstatement" is an effective mechanism by which early experiences can continue to influence behavior over lengthy intervals and (ii) memory deficits in young infants are best viewed as retrieval deficits.

Positive behavioral contrast in 3-month-old infants on multiple conjugate reinforcement schedules.

Positive behavioral contrast was assessed in two experiments with young infants using multiple conjugate reinforcement schedules. Reinforcement was produced by footkicks which activated the objects of an overhead crib mobile in a manner proportional to the vigor and rate of responding. Distinctive color/pattern cues on the sides of the objects served as discriminative stimuli for components of the multiple schedule. In Experiment 1, infants were trained with one cue (S+) only before insertion of S+ into a multiple schedule with an extinction component. A control group received S+ throughout all sessions. In Experiment 2, a multiple schedule was introduced at the outset, and responses in both components were reinforced before the introduction of extinction in the second component. In a final phase, reinforcement was reintroduced into the second component. Positive behavioral contrast occurred in both experiments. Response reduction in the extinction component was seen only in individual relative response curves. In both experiments, negative emotional behaviors accompanied the extinction component, and in Experiment 1, cooing accompanied presentations of S+.

A conditioning analysis of infant long-term memory.

The long-term retention of conditioned operant footkicks by 3-month-old infants was assessed in 2 studies. In both, infants were trained in a conjugate reinforcement paradigm in which footkicks produced conjugate activation of the components of an overhead crib mobile. After 2 training sessions, retention (cued recall, savings) was assessed cross-sectionally in a third session scheduled after varying intervals. In experiment 1, 32 infants were tested after intervals of 48, 72, 96, or 120 hours; in experiment 2, 24 infants were tested after 96, 144, 192, or 336 hours. No evidence of forgetting was observed for as long as 192 hours following original training. Although both retention measures indicated a significant memory deficit in the group tested after 336 hours (2 weeks), some individuals continued to exhibit substantial recall and savings after this retention interval. A conditioning analysis was viewed as a logical means by which to bridge the gap between animal and adult human models of memory.