The last fifteen years has seen a prodigious amount of research which has firmly established that “early stimulation" has a profound effect on later behaviour. The precise nature of the relationship, however, is not yet certain. It has been found to depend upon the type, intensity, and age of stimulation (Denenberg, 1968a), the sex and breed of the rat (Henderson, 1968a), the nature of the adult test situation. (Henderson, 1968b), and of course on the type of behaviour being considered (e.g. Denenberg, 1966). The consensus of opinion currently held is that "stimulation" in the form of, for example, “handling” , or electric shock, or exposure to cold, during infancy (prior to weaning) has its most marked effect on later affective behaviour (Denenberg, 1966, 1968b). In contrast, “stimulation” during the juvenile period ( immediately after weaning) more often involves "gentling", or exposure to enriched or restricted environments, and has been found to have its most notable effects on such behaviours as Hebb-Williams maze performance. (Denenberg and Morton, 1962). Because of these differences, two essentially separate fields of research in early experience have arisen. One is concerned with the effects of preweaning stimulation; the other with the effects of postweaning stimulation. This study will be limited to the former, with only a brief consideration of other types of early stimulation.

Of primary interest to this investigation is the effect of different intensities of infantile stimulation on later open field behaviour. As will be shown, there appears to be an interesting relationship between "emotional behaviour" and "exploratory behaviour" in the open field (see Whimbey and Denenberg, 1967). Though the major portion of past research has dealt with the relationship between infantile stimulation and later emotional behaviour, thus necessitating a concentration on this aspect of the problem, the relationship between emotional and exploratory behaviours is also of interest here and will be discussed accordingly.