TOUCH RESEARCH IN PRIMATES

LANDMARK STUDIES

Now in its fourth decade, animal research on touch has extensively documented the benefits of early tactile contact-and the consequences of touch deprivation.

Evidence of the consequences of touch deprivation in humans first prompted scientific study in this area. Researchers had long noted a depression-like response by infants to the absence of parental contact. Following World War II, Spitz coined the term "anaclitic depression" to describe the clinical response of human infants to prolonged maternal separation.¹

In the 1950s, in studies that came to signify the advent of modern touch research, investigators discovered that monkey infants reared in individual cages (for reasons of hygiene and disease prevention) developed poorly. From the 1950s into the 1960s, Harlow conducted his now classic experiments.

The most memorable of these surprised the psychology world with the finding that infant rhesus monkeys preferred surrogate mother objects providing contact comfort (frames covered with a terry cloth surface) to those providing nourishment (bare wire frames featuring a nipple and milk supply). These studies established that it was touch-and not food-that promoted attachment-like behavior.²

VALUE OF THE PRIMATE RESEARCH MODEL

Nonhuman primates can offer touch researchers tremendous insights into human development, especially in areas that are difficult to study with humans. Of all animals, apes and monkeys are the most closely related to humans behaviorally, anatomically and physiologically. Some primates, such as rhesus monkeys, share over 90 percent of their genes with those of humans.

Nonhuman primates are appropriate for research especially in terms of study design, and for several reasons. (1) They can be selectively bred and reared under controlled conditions-e.g., bred for particular genetic lines, and reared in a variety of social and physical environments. (2) Primates can be observed and tested physiologically on a daily basis. (3) Rhesus monkeys and other primates offer researchers invaluable opportunities to study the longitudinal effects of touch and touch deprivation over the course of generations. These animals age from birth to maturity (onset of puberty) in three or four years, instead of 15 to 20 years, as is the case with humans.³

Harlow's milestone studies showed that touch was more important to monkey infants than anything else they could receive from their mothers or mother surrogates- including food.

Harlow found that even short-term physical separation of monkey infants from their attachment object resulted in immediate and dramatic behavioral disruption and intense physiologic arousal.⁴ He also observed that the mother or mother surrogate represented the "secure base" that infants need before they can explore their environments-and that rhesus monkeys who were denied maternal contact of any kind ceased to explore. Perhaps even more significantly, further studies showed than touch is more critical than any other form of contact in mother-infant bonding. Even when laboratory conditions enabled the infants to see, hear and smell their mothers, they still failed to explore. Only the sense of touch created the "secure base" necessary for normal development.^5,6

Early studies also hinted at the effects of touch deprivation beyond infancy. As adolescents and adults, rhesus monkeys reared in tactile isolation actively avoided most social contact. They also tended to be hyperaggressive in their infrequent social interactions, habitually exhibiting behaviors similar to the "anger" and "depression" that monkey infants normally demonstrate during a weaning period. In addition, although rhesus monkeys reared singly in cages had normal physiological capabilities for reproduction, they developed gross abnormalities in sexual behavior.^7,8

More recent research has revealed that aberrant behaviors stemming from early touch deprivation are sustained, repeated and reinforced over the long term, from generation to generation. Female rhesus monkeys with a history of depressive response to separations in infancy and childhood are at high risk for neglecting or abusing their first-born offspring in the absence of social support. Furthermore, in a number of primate species, longitudinal studies of cross-generational phenomena show that the best predictor of the amount of time a young mother will spend with her newborn infant is the amount of time she herself spent in contact with her mother when she was an infant.^9,10

Recent studies also indicate that primate responses to touch deprivation may be highly heritable and that behavioral reactions to a lack of touch may be at least partly genetic. Some primate individuals, when contact-deprived, are passive and withdrawn, do not eat adequately and exhibit other behaviors that provide an animal model for human depression.² In other words, for a significant segment of the population with predisposition, or vulnerability, to these social abnormalities, contact deprivation can be devastating.

"THERAPEUTIC" TOUCH IN PRIMATE STUDIES

Data show that returning touch contact to touch-deprived monkey infants can, in some cases, largely reverse their behavioral problems. The benefits of such "therapeutic" touch appear to correlate directly with its nature and duration.

Suomi et al. reared animals in mother-only and peer-only conditions. Both environments provided infants with more tactile stimulation than they would get in complete isolation, but more limited contact than they would normally have with parents, family and peers. Not surprisingly, the negative behavioral effects observed were less severe than those stemming from complete isolation-but were nevertheless present.

However, when the researchers allowed these deprived animals several months of tactile contact with other monkeys, the abnormal behaviors diminished considerably. This reinstituted contact was administered by several types of monkey "therapists": mothers, younger peers, and even foster-grandparent monkey couples.¹¹

PHYSIOLOGICAL EFFECTS OF TOUCH DEPRIVATION

Researchers have identified, at least in part, a physiological basis for the behavioral effects of touch-deprived animal infants. Stress hormones, in particular, appear to play a key role.

Schanberg and Field found that even short-term interruption of mother-pup interaction in rats markedly affected several biochemical processes in the developing pup: a reduction in ornithine decarboxylase (ODC) activity, a sensitive index of cell growth and differentiation; a reduction in growth hormone release (in all body organs, including the heart and liver and throughout the brain, including the cerebrum, cerebellum and brain stem); an increase in corticosterone secretion; and suppressed tissue ODC responsivity to administered growth hormone.¹²

Other studies have supported the physiological underpinnings of tactile isolation. Higley et al., for instance, reported that rhesus monkeys reared by peers rather than their mothers showed abnormal stress-hormone responses to a variety of stressors.¹³

Conversely, "therapeutic" tactile stimulation can favorably reverse the detrimental biochemical effects of touch deprivation. In the studies by Schanberg and his colleagues, a dampened paintbrush was used to mimic rat mothers' heavy licking patterns in frequency and pressure. The physiological results were promising: growth hormone rose, ODC rose, and corticosterone dropped.¹⁴ Meaney et al. found that the "environmental stimulation" of handling young rats-removing them briefly and then reuniting them with their mothers-affected the pups' neurochemical development markedly. And as adults, rats that researchers had handled as infants exhibited not only less fear in novel environments, but also a less pronounced increase in adrenal glucocorticoids in response to a variety of stressors, and greater memory.¹⁵

Monkey infants deprived of maternal touch often exhibit abnormal, self-stimulating behaviors, such as thumb sucking and rocking.

In another facet of touch research in animals, investigators have discovered both direct and indirect effects of skin stimulation on immunologic functioning. Skin, of course, often serves as an immunoresponsive organ¹⁶; thus the immunological consequences of touch and touch deprivation, though not yet widely recognized, are not surprising.

Laudenslager et al. found that monkey infants separated from their mothers demonstrated less antibody production in response to an initial injection of an antigen.¹⁷ Coe et al. reported that monkeys raised away from their mothers for the first few months of life had a variety of immunological deficits that persisted long after the behavioral problems caused by the separation were reversed.18

Investigators have measured a direct, positive relationship between the amount of contact and grooming an infant monkey receives during its first six months of life and its ability to produce antibody titer (IgG and 1gM) in response to an antibody challenge (tetanus) at a little over one year of age.¹⁹

Trying to identify a mechanism for the "immunology of touch," some investigators point to modulations of arousal and associated CNS-hormonal activity. Touch deprivation may cause stress-induced activation of the pituitary-adrenal system, which, in turn, leads to increased plasma cortisol and adrenocorticotropic hormone. Likewise, researchers suggest, regular and "natural" stimulation of the skin may moderate these pituitary-adrenal responses in a positive and healthful way.²⁰

Longitudinal studies of rhesus monkeys indicate that mother-infant bonding practices are repeated and reinforced from generation to generation.

Whatever the mechanisms are for these responses to tactile stimulation and deprivation, the scientific data have led animal researchers to declare "the therapeutic power of touch." At the same time, these experts acknowledge that good "contact relationship" between parent and child is the best source of preventive medicine.