Learning and the Concept of Critical Periods in Infancy

Develop. Med. Child Neitrol. 1912, 14, 105-114

Learning and the Concept of Critical Periods in Infancy

Kevin Connolly

Introduction The notion of development concerns a

directional pattern of changes in structure and function associated with growth and with ageing processes. The terms ‘growth’, ‘maturation’ and ‘development’ have some- times been used synonymously and this can be seriously misleading. Growth emphasises changes which are due to tissue accretion; a further and related term, that of ‘differentiation’, refers to variations and changes in structural aspects with increas- ing age. The concept of development en- compasses both of these plus behaviour, and places emphasis on progressive changes in the organisation of an individual considered as a functional adaptive system throughout its life history.

Gesell’s (1954) notion of human ontogeny, in which early behaviour emerges through the innate processes of growth (which he termed maturation), implies a process of gradual unfolding. Development is neither a process of unfolding, directed solely by intrinsic forces, nor is it a moulding process directed only by extrinsic forces-it is a combination of both (Connolly 1970). Behavioural development was ascribed by Coghill (1929) to the maturation of central nervous system tissues, the nervous system being

represented as a carrier of the innate determinants of behavioural patterns pre- sumed to arise through passive development. A preformationist theory, or a theory of genetic predetermination, cannot readily account for data now available to us from developmental genetics (Con- nolly 1971).

The problem of development is one of understanding what changing integrations underlie the successive functional stages which are characteristic of a given species. In order to deal with this problem, Schneirla (1966) postulated two concepts which stand for the complex system of intervening variables. They are : maturation, which he defined as ‘the contributions to development of growth, and of tissue differentiation together with their organic trace effects surviving from earlier development’ ; and experience, which he defined as ‘. . . the contributions to development of stimulation from all available sources (external and internal) including their functional trace effects surviving from earlier development’. Trace effects refer to organic changes which affect further development. The developmental contributions of these two com- plexes must be viewed as fused at all stages of ontogenesis. Schneirla’s definition

Department of Psychology, University of Shefield, Sheffield S10 2TN.

705

DEVELOPMENTAL MEDICINE A N D CHILD NEUROLOGY. 1972, 14

706

of experience is extremely broad-ranging, from the stimulative effects of biochemical and physiological processes to the ideas of learning and conditioning as used by psychologists.

Early Learning and Early Experience At one level we all know what is meant

by the term ‘learning’, and yet it is astonishingly difficult to define adequately. When we speak of learning we refer to relatively permanent changes in behaviour which are a consequence of the organism’s experience and commerce with its environment. However, the inadequacy of this definition is apparent on reflection. Let us take an extreme example to illustrate the point. Consider a bipedal animal which looses its legs in an accident. Its behaviour will certainly be changed by this experience, but we should not consider such a change to be an example of learning, though it may learn to adapt its behaviour to its new circumstances. Defined in this way, learning is too crude and too general an entity. What we imply by the term is a set of dynamic processes concerned with the formation of structures and modes of operation. We are concerned with the ways in which an organism constructs and modifies models of its world in order to adapt to the demands made upon it by its environment. The processes can and are being analysed at many levels-biochemical, physiological, behavioural and cognitive.

Early experience has been considered as particularly important by many developmental psychologists because they argue that it lays the foundations of an individual’s characteristic behaviour. In this paper I propose to discuss some of the recent work in the field of early learning and early experience, with particular emphasis on the concept of critical periods of behavioural development.

Early experience refers to all effects of

stimulation in infancy, both immediate and long-lasting. In contrast, early learning has a narrower connotation, because certain kinds of early experience involve no learning but all learning implies early experience. Whether early learning differs from learning which occurs later in life has been the subject of some controversy. Hebb (1949) believed that later learning builds upon, rather than replaces, initial learning, and this in itself would make the characteristics of the two different. He argued that much early learning was foundational in that it provided the organism with essential perceptual and motor skills and formed the framework within which subsequent learning took place. If you consider an animal as a self- organising system, then early inputs will be responsible for determining the basic organisational structure.

Work reported since Hebb published his extremely influential theory has tended to modify but, in general outline, support his conception.

Effects of Early Experience on Later Behaviour

In the course of the past fifteen years, a very extensive literature has grown up on the effects of early experience on later behaviour (Newton and Levine 1968). We now know that genetically based characters may be extensively modified by early experience, and we know also that early experiences are one of the principal sources of individual differences in behaviour. We know, too, that early experiences have multiple effects and that the age at which they are administered is crucial. Let me illustrate these points by citing some experiments.

It is a common characteristic of most strains of mice that two adult males will fight when placed together in a restricted environment. Strain differences are well known and provide evidence of a genetic

KEVIN CONNOLLY

component. Dennenberg and co-workers (1964) fostered four day-old mice pups on to lactating rats until weaning. Mice so reared show an almost zero incidence of fighting in adulthood.

Cooper and Zubeck (1958) reported important findings on the effects of early experience in lines of rats selectively bred for maze brightness and maze dullness. At weaning, rats from these two strains were placed in one of three environments: normal, enriched, or restricted. The normal environment was the usual laboratory arrangement; the restricted environment was a cage containing only food and water receptacles ; and the enriched environment contained ramps, mirrors, swings, balls, barriers, slides, tunnels and so on, in addition to food and water containers. The animals were maintained from weaning at day 25 to day 65 in these conditions and were then tested in a Hebb-Williams maze. The results, i.e. number of errors made in learning the maze, are startling. In the normal environment, the maze dull rats made a little over 30 per cent more errors than the maze bright animals. When reared in an enriched environment, the number of errors made by the two groups did not differ significantly. Similarly, no differences were obtained between animals reared in restricted environments.

Harlow’s (1962) studies of monkeys raised in social isolation from birth to adulthood showed that these animals had great difficulty in performing the basic steps involved in copulation. Females coached by Harlow to mate did become pregnant but turned out to be very bad mothers (Arling and Harlow 1967).

A study by Landauer and Whiting (1964) revealed an interesting correlation between stress in infancy and the adult height of males. They compared boys subjected to two stressors with control subjects who were not so treated. The stressors were piercing of the nose, lips or

ears and moulding of the arms, legs or head. Their results revealed that in societies where the infants were subjected to stress during the first two years of life, the average height of males was over two inches greater than boys who did not experience this stress. A similar pattern was found by Whiting and colleagues (1968) when they compared the adult height of American children inoculated before the age of two years with children not inoculated. In this study it proved possible by various statistical procedures to correct for parental height. It is possible, of course, that those parents who have their children inoculated early differ in many ways from those who do not; for example, they may practice other forms of medical care more assiduously.

These studies and many others point to the long-term and, in some cases, dramatic effects which may be brought about in an animal’s behaviour by appropriate inter- vention in early life. Dennenberg (1969) has suggested that the human analogue of pre-weaning stimulation in the rat (handling) is represented by the first 4-24 weeks of the baby’s life, and that the analogue to successful use of an enriched environment in the case of the rat begins at roughly six months in the human.

Developmental Changes in Learning The way in which an organism’s learn-

ing capacity develops with age is by no means fully understood, and it is difficult to decide whether there are developmental changes in learning over and above the effects of growth, neuromuscular maturation, experience and motivation. Some studies by Vince (1958, 1959) on the learning of young and adult passerine birds (finches and titmice) are illuminating in this context. Vince trained birds on tasks involving the pulling of string and lifting the lids of pots, etc. Basically, this author considered two features of the learning

707

DEVELOPMENTAL MEDICINE AND CHILD NEUROLOGY. 1972, 14

process; the acquisition of the appropriate response when reinforced, and the sup- pression of the tendency to respond in the absence of reinforcement. On the whole, she found that juvenile birds were more generally responsive and capable of acquiring the correct response more rapidly than mature birds. However, adult birds were found to be better able to inhibit incorrect responses, that is, to learn not to respond inappropriately. Depending on the exact nature of the task, the young or old birds could learn more quickly. What is important is not that early learning is less or more important than later learning, but that early and later learning may differ in certain features. A single criterion of learning efficiency may, in fact, be seriously misleading.

Luria (1932) has made similar observa- tions on the development of certain simple reactions in young children. The tasks with which the children were faced involved such things as pressing a button when a signal was received and making delayed movements. On testing children from two to seven years of age he found that the capacity to make these responses develops slowly. A child aged four to five years could produce a reaction to a signal but each signal tended to produce many movements, so that gaps between signals were filled with extra responses. In some cases, the extra responses could be inhibited in children aged three to four years, but this might disrupt the habit so that the occurrence of a signal inhibited the reaction. Delayed movements were found by Luria to be almost impossible in children up to the age of seven. Luria maintained that the excitatory aspects are no problem for the young child and that the difficulty lies in controlling the excitation. Paramenova (1955) reported that 3-year- old children can learn to press a key in responses to a signal, such as a red lamp, but usually give the same response to

other signals, such as a blue or a green lamp. It appears that at this age it is extremely difficult for the child to refrain from making a generalised response even though he well understands the in- structions. The problem of inhibiting responses is not usually overcome until the child had reached the age of six or so.

These studies of young birds and children do suggest that different aspects are important in the young as compared with the adult organism.

Critical Periods in the Development of Behaviour

The concept of critical periods is not new either to biology or to psychology. The origins of the idea are to be found in experimental embryology. From his experiments on the effects of various in- organic compounds on the development of Fundulus eggs, Stockard (1921) origin- ally thought that the production of one-eyed monsters was caused specifically by the magnesium ion. However, further experiments showed that almost any substance would produce the same effect if administered at the appropriate time during development. In embryology, therefore, the concept refers to time periods of maximum sensitivity or indifference to chemical forces acting on the cellular mass.

Freud’s ideas about the origin of human neurosis relied heavily upon the importance of early childhood experiences and implied that certain periods or stages in the life of an infant are times of particular sensitivity. The concept of critical periods in behavioural development was emphasised by Lorenz (1935) in his work on the formation of primary social bonds in precocial birds-the following and ap- proach behaviours which he called ‘imprinting’. Lorenz commented upon the similarity of these behavioural phenomena to the critical periods in the development

708

KEVIN CONNOLLY

of the embryo. Following this, McGraw (1946) argued that there was a critical period for the optimal learning of motor skills in the human infant. The all-em- bracing notion of educational readiness and stage-dependent theories of development, such as that elaborated by Piaget, are implicitly related to the critical period hypothesis. Relating as it does to the learning process, the idea of readiness differs very little from the hypothesis advanced by many working on imprinting.

Three principal kinds of critical period phenomena have been described in behavioural studies; optimal periods for learning, for infantile stimulation and for the formation of basic social relationships. Dealing first with the establishment of social relationships, there is now an extensive literature on imprinting which has been reviewed by Sluckin (1964) and by Bateson (1966). I do not propose to discuss this in any detail; suffice to say that there is good evidence for the existence of sensitive periods for the establishment of primary social bonds in many species. The dog has been studied from the view- point of socialisation more extensively than any other mammal (Scott 1958). From 3 weeks up to about 10 weeks, the puppy is susceptible to socialising influences. During this time, primary socialisation determines to what living things, including humans, the animal will become attached. Work on humans has explored the question of whether there are periods during which the human infant is maxi- mally sensitive to social contacts with its mother-contacts which will lead to the cementing of an affectional bond. For obvious reasons this is a difficult problem to tackle experimentally, and most of our knowledge comes from clinical cases where the mother-infant relationship either did not develop or where it was disrupted. However, the work of Bowlby (1951,

1958, 1969), Yarrow (1960), Ambrose (1963), Scott (1963) and Klaus et al. (1972) all provide evidence in support of the basic contention for sensitive periods.

Turning to infantile stimulation, Levine (1956) found that handling rats after weaning did not have the same consequences as handling during the first three weeks of life. In a subsequent study, Levine and Otis (1958) report differential effects of pre- and post-weaning stimulation on adult resistance to deprivation. Dennenberg and co-workers (1968) found that the enhanced free-environment experience during the second three weeks of life had a greater effect on learning than did the same experience during the first three weeks. Levine and Mullins (1966) have suggested that early experience may serve to set or ‘tune’ the neuroendocrine system, so that later in the organism’s life history, the threshold and duration of emotional stress reactions are altered. There is evidence, too of early stimulation having morphological and physiological consequences. Shapiro and Vukovich (1970) investigated the effects of a wide range of sensory stimulation on the development of cortical pyramidal cell synaptic loci in rats. Stimulation given 3-5 times per day was found to increase the number of dendritic spines measured at days 8 and 16. This effect of early afferent input on brain development may provide a neuroanatomical basis for the influence of early experience.

From her now famous studies on the twins Jimmy and Johnny, McGraw (1935, 1946) concluded that there are critical periods for learning which vary from activity to activity. To one member of the twin pair, McGraw gave extra training. The results showed that some activities, such as walking, were not affected. Others, notably roller skating, showed a substantial effect in the trained twin. In some activities, performance was actually worsened

709

DEVELOPMENTAL MEDICINE A N D CHILD NEUROLOGY. 1972, 14

by the acquisition of inappropriate responses.

Forgays and Forgays (1 952) have demonstrated that pet rats, raised in the rich environment of a home, performed much better on learning tasks than rats reared in laboratory cages.

Bernstein (1957) provided evidence in- dicating that discrimination learning could be improved by gentle handling beginning at 20 days. In this context, the effects may well be on fear, which affects the capacity for future learning.

Thorpe (1961) has described a critical period for song learning in the chaffinch. If the young male is isolated three or four days after hatching, it produces an in- complete song only. However, if, as a fledgling of two to three weeks old, or in early juvenile life before he himself sings, the young bird hears adult chaffinches singing, he will, during the following year, produce the song characteristic of the species even if he has been kept in isolation. In nature, the fine details of the song are added at the time of competition over territory, within a period of 2-3 weeks when the bird is about a year old. At this time, the bird learns the ‘local song’ and never learns any others in subsequent years. The critical period for song learning is thus quite long but is certainly over by the end of a year.

Money and his colleagues (1957) studied the development of hermaphroditic children who had been reared as one sex and then changed to the other. They found that if this occurred before the age of two-and-a-half years very little emotional disturbance resulted. On the basis of this data it has been suggested that there is a critical period for learning the sex rBle.

The Nature of the Period The critical period concept has been used

in two quite distinct ways (Caldwell 1962). Some authors have spoken of a critical

period beyond which a given phenomenon will not appear, that is, a point in time which marks the onset of total indifference or resistance to certain patterns of stimulation. Another way in which the term has been used is to refer to a period of time during which the organism is especially sensitive to various developmental modi- fiers which, if introduced at a different point in the life-cycle, would have little or no effect. In other words, a period of maximum sensitivity.

At the embryological level there are data to support both versions of the critical period hypothesis. Cell transplants done prior to a particular critical time will develop in conformity with the region into which they are transplanted. Beyond this terminal point, transplantation will result in the misplaced development of the organ. The ‘during’ version of the hypothesis has also been demonstrated by embryologists in the experimental production of congenital abnormalities. The teratogen used may vary, but, in order to produce an effect, the timing must be precise. Periods exist, therefore, in which organisms are vulnerable, while, at other times, they are relatively immune to external factors. From the embryological studies we know that organ systems are particularly sensitive during their period of maximum cellular proliferation (Ham- burger and Habel 1947).

The ‘beyond’ version of the hypothesis, with respect to the development of social behaviour, is considered by Scott (1 958) to be an established generalisation. From his studies with dogs, Scott has provided a great deal of evidence supporting the contention that there is a period early in the animal’s life during which primary social relationships become established and after which they rarely do so. Clear evidence relating to human behaviour is lacking, and for obvious reasons is very difficult to obtain, though Bowlby (1951)

710

KEVIN CONNOLLY

has asserted that there is a parallel between the ‘during’ version of the chemical organiser and maternal care.

The basic timing mechanisms for developmental periods are biological processes ongoing through time. Time as such does not explain the changes which have been described. For various reasons these processes are not precisely correlated with age from birth. For example, birds often retain newly formed eggs in their bodies overnight, thus incubating them for several hours before laying. Gottlieb (1961) chilled duck eggs prior to incubating them (this process kills embryos except those in the earliest stages of development) and found that the critical period for imprinting in these birds showed a much reduced variance.

Studies investigating critical periods for imprinting in precocial birds have shown that there is not so much a critical as a more extended sensitive period during which the animal is particularly sensitive to forming a primary social bond. The period for imprinting, claimed by Hess (1959) to be between 13 and 16 hours post-hatch, can be extended by administer- ing tranquilliser drugs such as chlor- promazine (Hess 1960), or shortened by social rearing (Guiton 1959). Freedman and co-workers ( I 961) have shown that although the optimum period for socialising dogs is seven to nine weeks, adequate socialisation is possible up to thirteen weeks. The distinction between the terms ‘critical’ and ‘sensitive’ in speaking of periods of development is very largely semantic. Whilst not wishing to imply categorically that critical periods do not exist, on the basis of our present knowledge it would be more prudent to use the slightly weaker term ‘sensitive period’.

In searching for causal mechanisms, we should ask whether a period is sensitive because of the events which occur within it, because of the organism’s physiolo-

gical state at that time, and because of the sequence in which developmental events occur. In one child the boundary may occur at x months whilst in another it may occur at x+l months. This suggests that there is a danger of emphasising the period too strongly. Sensitive periods for attachment behaviour would be expected to show a general uniformity from one child to another in much the same way as most children learn to walk at twelve to eighteen months or to talk at about two years. A similar uniformity might also be expected for children becoming dry at night, and indeed there is evidence to support this (MacKeith 1968). Here, of course, parental expectation will play a part, as is clear from the cross-cultural studies of Whiting and Child (1953). How- ever, it is important to remember that wide individual differences exist both in maturation and in learning significant life functions.

Those of us who study the behaviour of children often tend to assume that what we are currently observing is crucial for that child. A child who lacks a warm relationship during infancy will not neces- sarily become an affectionless psychopath. There may well be adaptive characteristics which come into play later and, on the whole, evolution has built many modifying mechanisms into the processes of ontogeny. Feeding, which is initially sucking and swallowing, must be replaced by chewing and swallowing and eventually will involve using tools. Similarly, intense filiative behaviour must give way to other attach- ments. The failure of an intense mother- infant relationship to weaken and change (Harlow and Harlow 1965) is maladaptive ontogenetically and phylogenetically, as shown by studies on mother-infant bonds in monkeys.

To speak of a critical or a sensitive period is in essence to describe certain temporal characteristics of a phenomenon.

71 1


Mac Keith‘s (1964, 1968) hypothesis re- garding a sensitive period for learning nocturnal bladder control provides a relevant example. Whilst this is a useful generalisation it is not in itself an explanation. It is essential that we pursue our enquiries further in order to uncover the causal mechanisms involved in behavioural change. From a knowledge of the mechanisms involved and important factors affecting these, more effective diagnosis and treatment programmes will be possible.

Conclusions (1) Development is an extremely com-

plex set of processes which are often over- simplified.

(2) Learning is difficult to define adequately.

(3) Early experience has far reaching consequences for an organism’s development.

(4) Learning does take place in very young animals and in the human newborn.

( 5 ) There are significant developmental changes in the nature of learning.

(6) Certain periods in an organism’s life history appear to be specially sensitive for the development of certain behaviours.

(7) We should concentrate less on ‘critical periods’ and more on critical events in trying to understand the mechanisms involved in the so-called ‘sensitive periods’.

This article is based on a paper presented to a colloquium on Recent Advances in Knowledge of Bladder Control in Children, Department of Psychological Medicine, University of Newcastle upon Tyne, July 1971.

SUMMARY The importance of early experience in the development of behaviour is amply docu-

mented by empirical research. This paper discusses learning in infancy and the critical period hypothesis as an explanation of behavioural development. The use of the term ‘sensitive period’ is advocated rather than ‘critical period‘ and examples of such sensitive periods associated with quite different behavioural parameters are given. Two quite separate ways in which the term ‘critical period’ has been used are distinguished and examples given of each. The view is advanced that attention should be devoted primarily to the events and processes occurring within the sensitive period rather than to its temporal characteristics.

RESUME Apprentissage et concept de ptriodes critiques durant I’enfance

L‘importance des exptriences prCcoces dans le developpement d’un comportement est largement suggtrCe par des recherches empiriques. Cet article discute de l’apprentissage dam l’enfance et de l’hypothkse de ptriode critique pour expliquer le dtveloppement comportemental. L’emploi du terme ‘pCriode sensible’ parait prtftrable celui de ‘ptriode critique’ et des exemples de telles ptriodes sensibles assocites a des paramktres de comportement trks variCs sont indiqub. Deux facons difftrentes selon lesquelles le terme ‘pCriode critique’ a Ctt utilise sont distingutes et des exemples donnts pour chacune d’elles. Les auteurs suggttrent d’apporter avant tout une attention aux tvenements et aux organisa- tions survenant durant la ptriode sensible plutbt que d‘analyser ces caracttristiques temporelles.

712

KEVIN CONNOLLY

ZUSAMMENFASSUNG Lernen und der BegrifS der kritischen Perioden im Kindesalter

Die Bedeutung fruhzeitiger Erfahrung in der Verhaltensentwicklung ist durch empirische Forschung hinreichend dokumentiert. I n der vorliegenden Arbeit wird der Lernprozerj im Kindesalter und die Hypothese der kritischen Periode als Ausdruck der Verhaltensent- wicklung diskutiert. Es wird empfohlen, den Begriff ‘kritische Phase’ durch ‘sensible Phase’ zu ersetzen und es werden Beispiele solcher sensibler Perioden im Zusammenhang mit sehr verschiedenen Verhaltensparametern gegeben. Zwei ganz unterschiedliche Formen, in denen der Begriff kritische Periode gebraucht wird, werden beschrieben und fur jede werden Beispiele angefuhrt. Man sollte darauf achten, da13 die Aufmerksamkeit in erster Linie auf die wahrend der sensiblen Phase vorkommenden Ereignisse und Vorgange zu richten ist, als auf das zu erwartende zeitliche Auftreten.

RESUMEN Aprendizaje y el concepto de periodos criticos en la infancia

La importancia de la experiencia temprana en el desarrollo del comportamiento esti ampliamente docurnentada por la investigacion empirica. Esta comunicacidn discute el aprendizaje en la infancia y la hipotesis de 10s periodos criticos corno una explication del desarrollo del comportamiento. El us0 del tkrrnino ‘periodo sensitivo’ se prefiere a1 de ‘periodo critico’ y se dan ejemplos de tales periodos semi tivos, asociados con parametros de comportarniento completamente diferentes. Se distinguen dos vias completamente distintas en las cuales se ha utilizado el ttrmino ‘periodo critico’ y se dan ejemplos de cada una. Se adelanta la idea de qua la atencion debe aplicarse en primer lugar a 10s aconteci- mientos y procesos que ocurren en el periodo sensitivo, mas bien que a sus caracteristicas temporales.

REFERENCES Ambrose, J. A. (1963) ‘The concept of critical periods for the development of social responsiveness.’ in

Arling, G . L., Harlow, H. F. (1967) ‘Effects of social deprivation on maternal behavior of rhesus monkeys.’

Bateson, P. P. G . (1966) ‘The characteristics and context of imprinting.’ Biological Reviews, 41, 177. Bernstein, L. (1957) ‘The effects of variations in handling upon learning and retention.’ Journal of Physio-

Bowlby, .I. (1951) Maternal Care and Mental Health. London: H.M.S.O. - (1958) ‘The nature of the child’s tie to his mother.’ Internstional Journal of Psycho-analysis, 39, 350. - (1969) Attachment and Loss. Vol. 1. Attachment. London: Hogarth. Caldwell, B. M. (1962) ‘The usefulness of the critical period hypothesis in the study of filiative behavior.’

Merrill-Palmer Quarterly of Behaviour and Development, 8, 229. Coghill, G. E. (1929) Anatomy and the Problem of Behaviour New York: Hafner. (Reprinted 1963). Connolly, K. (1970) ‘Skill development: problems and plans.’ in Connolly, K. (Ed.) Mechanisms of Motor

Skill Development. London: Academic Press. - (1971) ‘The evolution and ontogeny of behaviour.’ Bulletin of the British Psychological Society, 24,

93. Cooper, R. M., Zubek, J. P. (1958) ‘Effects of enriched and restricted early environments on the learning

ability of bright and dull rats.’ Canadian Journal of Psychology, 12, 159. Dennenberg, V. H. (1969) ‘Animal studies of early experience: some principles which have implications for

human development.’ in Hill, J . P. (Ed.) Minnesota Symposia on Child Psychology, Vol. 3. Minneapolis: University of Minnesota Press.

- Hudgens, G. A,, Zarrow, M. X. (1964) ‘Mice reared with rats: modification of behavior by early experience with another species.’ Science, 143, 380. - Woodcock, J . M., Rosenberg, M. K. (1968) ‘Long-term effects of preweaning and postweaning free

environment experience on the rat’s problem solving behavior.’ Journal of Physiological and Comparative Psychology, 66, 533.

Forgays, D. G., Forgays, J. W. (1952) ‘The nature of the effect of free-environmental experience in the rat.’ Journal of Physiological and Comparative Psychology, 45, 322.

71 3

Foss, B. M. (Ed.) Determinants of Infant Behrrviour, Vol. 2. London: Methuen. p. 201.

Journal of Physiological and Comparative Psychology, 64, 371.

logical and Comparative Psychology, 50, 162.


Freedman, D. G., King, J. A., Elliot, 0. (1961) ‘Critical period in the social development of dogs.’ Science, 133, 1016.

Gesell, A. (1954) ‘The ontogeny of infant behavior.’ in Carmichael, L. (Ed.) Manual of Child Psychology. New York: John Wiley.

Gottleib, G. (1961) ‘Development age as a baseline for determination of the critical period in imprinting.’ Journal of Physiological and Compara five Psychology, 54,422.

Guiton, P. (1959) ‘Socialization and imprinting in Brown Leghorn chicks.’ Animal Behaviour, 7, 26. Hamburger, V., Habel, K. (1947) ‘Teratogenic and lethal effects of influenza-A and mumps viruses on

early chick embryos.’ Proceedings of the Society for Experimental Biology and Medicine, 66, 608. Harlow, H. F. (1962) ‘The heterosexual affectional system in monkeys.’ American Psychologist, 17, 1 . - Harlow, M. K. (1965) ‘The affectional systems.’ in Schrier, A. M., Harlow, H. F., Stollnitz, F. (Eds.)

Behavior of Nonhuman Primates, Vol. 2. London: Academic Press. Hebb, D. 0. (1949) The Organization of Behaviour. London: John Wiley. Hess, E. H. (1959) ‘Two conditions limiting critical age for imprinting.’ Journal ofPhysioloaical and Com- - - . -

parative Psychology, 52, 515. - (1960) ‘Effects of drugs on imorinting behavior.’ in Uhr. L.. Miller. J. G. (Eds.) Drum and Behaviour. - , , ~ , -

London: John Wiley.- Klaus, M. H., Jerauld, R., Kreger, N. C., McAlpine, W., Steffa, M., Kennell, J. H. (1972) ‘Maternal

attachment. Importance of the first post-partum days.’ New England Journal of Medicine, 286, 460. Landauer, T. K., Whiting, J. M. W. (1964) ‘Infantile stimulation and adult stature of human males.’

American Anthropologist, 66, 1007. Levine, S. (1956) ‘A further study of infantile handling and adult avoidance learning.’ Journal of Personality,

25, 70. - Otis, L. S. (1958) ‘The effects of handling before and after weaning on the resistance of albino rats

to later deprivation.’ Canadian Journal of Psychology, 12, 103. - Mullins, R. F. (1966) ‘Hormonal influences on brain organization in infant rats.’ Science, 152, 1585. Lorenz, K. (1935) ‘Der Kumpen in der Umwelt des Vogels.’ Journal of Ornithology, 83, 137, 289. Luria, A. R . (1932) The Nature of Human Conflicts. New York: Liveright. McGraw, M. B. (1935) Growth: A Study ofJohnny andJimmy. New York: Appleton-Century. - (1946) ‘Maturation of behavior.’ in Carmichael, L. (Ed.) Manual of Child Psychology. New York:

John Wiley. Mac Keith, R. (1964) ‘A new concept in development.’ Developmental Medicine and Child Neurology, 6,

1 1 1 . - (1968) ‘A frequent factor in the origins of primary nocturnal enuresis: anxiety in the third year.’

Developmental Medicine and Child Neurology, 10, 465. Money, J., Hampson, J. G., Hampson, J . L. (1957) ‘Imprinting and the establishment of gender role’.

Archives of Neurology and Psychiatry, 77, 333. Newton, G., Levine, B. (Eds.) (1968) Early Experience and Behavior. Springfield, 111.: Charles C . Thomas. Paramenova, M. P. (1955) ‘On the question of the development of the physiological mechanism of move-

ment.’ Voprosy Psichologii, 1, 51. Schneirla, T. C . (1966) ‘Behavioural development and comparative psychology.’ Quarterly Review of Biology,

41, 283. Scott, J. P. (1958) ‘Critical periods in the development of social behaviour in puppies.’ Psychosomatic

Medicine, 20, 42. - (1963) ‘The process of primary socialization in canine and human infants.’ Monographs of the Sociefy

for Research in Child Development, No. 85. Shapiro, S., Vukovich, K. R. (1970) ‘Early experience effects on cortical dendrites: a proposed model for

development.’ Science, 167, 292. Sluckin, W. (1964) Imprinting and Early Learning. London: Methuen. Stockard, C. R. (1921) ‘Developmental rate and structural expression: an experimental study of twins,

“double monsters” and single deformities and their interaction among embryonic organs during their origins and development.’ American Journal of Anatomy, 28, 115.

Thorpe, W. H. (1961) Bird-song. Cambridge: Cambridge University Press. Vince, M. A. (1958) ‘String-pulling in birds. 2. Differences related to age in greenfinches, chaffinches and

canaries.’ Animal Behaviour, 6, 53. - (1959) ‘Effects of age and experience on the establishment of internal inhibition in finches.’ British

Journal of Psychology, 50, 136. Whiting, J. W. M., Child, I. L. (1953) Child Training andPersonality. London: O.U.P. - Landauer, T. K., Jones, T. M. (1968) ‘Infantile immunization and adult stature.’ Child Development,

Yarrow, L. J. (1960) ‘Maternal deprivation: toward an empirical and conceptual re-evolution.’ Psycholo- 39, 59.

gical Bulletin, 58, 459.

714

Documents

Learning and the Concept of Critical Periods in Infancy