Rats housed in social isolation show heightened levels of object-contact in an open-field and are slower than socially-housed controls to emerge from a small enclosure into an unfamiliar environment. Isolation between 25 and 45 days of age produced an irreversible effect upon object contact but had no lasting effect if between 16 and 25 days or after 45 days. In contrast to object contact, emergence was affected by isolation at any age and the effect was reversed by subsequent social housing. Thus the effects of isolation upon object contact and upon emergence apparently do not depend upon a single underlying variable.

Juvenile rats were allowed short daily periods of social contact to see if this would reduce the known effects of isolation rearing upon habituation of locomotor activity and object contact in the open field. Animals totally deprived of social experience (ISOL) were slower to habituate than animals living in small social groups (SOC). Rats allowed 1 hr of social contact (partial isolates, PI), but living otherwise in isolation, were intermediate between ISOL and SOC animals. In further experiments the quality of social interactions in the daily period was altered by drugging one of the partners, either with amphetamine or with chlorpormazine. In later tests in the open field, the rats that had interacted with amphetamine-injected or chlorpromazine-injected partners differed from PI animals in the direction of resembling complete isolates (ISOL); this was particularly true of those paired with amphetamine animals. Observation revealed that injection of 1 of the partners considerably altered social interactions in the pair. A further test showed that 1 hr of contact a day considerably alleviated the deleterious effects of isolation rearing upon response reversal. We conclude that normal development in the rat may depend upon the flexibility of behavior encouraged by the early social situation.Recent reports (Rosenzweig, 1971; suggest that some of the behavioral effects of keeping rats in impoverished conditions may be caused by social isolation prior to 50 days of age. One such behavior is the slow reversal of a previously learned discrimination. Rosenzweig found that rats that were isolated for 30 days from 25 days of age were slow to reverse a previously learned visual discrimination, but that reversal was not affected if isolation occurred after 60 days of age. Furthermore, he has reported that although environmental enrichment of social rats affected brain development it did not affect reversal, suggesting that it is specifically social factors that are implicated in t h s deficit. Similarly, we have found that hyperactivity and habituation deficits which are found following early isolation cannot be produced by isolation initiated after 45 days of age, nor can they be reversed in rats isolated between 25 and 45 days of age by subsequent housing in social groups for up to 90 days in press).

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Rats reared in social isolation made more errors on a spatial memory task and made errors earlier in each trial than socially reared rats. The difference in performance only occurred when rats were isolated prior to 50 days of age, and it survived IOO days of subsequent social housing. IOO days of isolation after 50 days of age did not influence performance on the spatial memory task. Subsequent experiments suggest that spatial abilities may not differ between groups but that isolates are slower to learn to make a particular response and to locate a particular arm when spatial and response cues are irrelevant. In contrast to previous experiments, clear response strategies were seen in the present experiments. These were prevalent in the young (54-days-old) rats, were less common at 90 days and had completely disappeared by 180 days. Response strategies were more common in male rats and in socially reared rats.

Matched litter mates were reared in one of three conditions: in pairs or in isolation with or without one hour of daily playfighting experience from 20 to 50 days of age. The rats were then regrouped within condition so that they lived with identically reared cagemates for a month. This regrouping eliminated the transient effects of isolation such as increased fearfulness. When tested as adults, there was no effect of early rearing condition on the probability of intraspecific aggression or muricide, although isolation-reared rats were less likely to retrieve the mice. However, isolation rearing reduced the latency to initiate shock-induced defensive aggression and increased both its frequency and intensity. Isolated animals which had been given daily playfighting during development did not show the effects of early social deprivation. The mechanisms through which playfighting experience shapes later defensive behavior remain to be determined.