Affect and Neural Development

            The major propagator of the evolutionary process is the press for continuity, the tendency to act in order to increase the probability of passing on genes. In order to preserve ones genes, individuals are in competition, to ensure that ones genes prevail over another’s; this can be accomplished individually or collaboratively (4). While it seems logical that animals would sacrifice themselves for their young or mates, humans seem to present a special problem to this evolutionary theory in that they sacrifice themselves for abstract principles and others that are of no biological relation. A possible key to this oddity is affect hunger, an urgent need for affective bonding, which continues after the critical development period and makes mutuality and sociality as important as competition in the evolutionary process. Affective bonding provokes certain social behaviors, in others, which are necessary for normal neural development. Furthermore, only with normal neural development would an individual be able to exhibit the social behaviors essential in forming affective bonds.

            Social and mutual interaction among members of the same species is observed in most animals. This aspect of mammalian society appears to originate with the affective ties between parents and offspring. The physiological underpinnings of this essential mammalian behavior offer clues to the physical reality of affect hunger (4). There is a physiological role in neonate and parental behavior that leads to mutual interaction.  For instance, tactile stimuli, such as grooming and caressing, social behaviors necessary in affective bonding, are necessary for the proper growth of the neurons (4). These interactions stimulate early neural development, the pathways of which provide the individual with social behaviors required for bonding.  In order to understand the connection between neural development and social behavior it is necessary to investigate the relationship between affect and neural Darwinism.

            The concept of neural Darwinism accounts for the physical process in neural development during which neural pathways are be changed in order to increase efficiency. The developing brain is highly immature at birth and is dependent upon sensory stimulation for its normal growth, development, and functional and structural organization. In the critical period of this sensory and consequently social development, the nervous system creates extensive synaptic connections between neurons. These connections are only kept based on their usefulness, where synapses are selectively eliminated or modified based on whether they are being rarely or frequently used respectively (1) (5).

           An example of such synaptic connection modifications can be seen in the Harlow’s experiments on new born monkeys which were isolated and deprived of tactile and movement stimulation. After six months to one year, the isolated subjects exhibited abnormal social behavior, such as isolating themselves in a corner and rocking back and forth (5). During early neural development, specific sensory deprivation, mainly the lack of touch, may result in incomplete development of neural systems that control affection. In terms of neural Darwinism, these pathways are rarely used and therefore modified to maintain cerebral efficiency.  For example, the formation of dendrites of the neuron is influenced by the sensory process of stimulation during early years of brain development (7). Certain kinds of sensory deprivation result in incomplete or damaged development of the neuronal systems that control affection and affect later social development. Further studies have shown that abnormal social behavior was reduced by introducing a surrogate mother to the isolated monkeys, eliciting clinging behavior. This did not completely remove all the symptoms of isolation, but simply reduced them (3).

            It is evident that there is a causal relationship between sensory deprivation and social-behavioral dysfunction. In accepting that neural systems provide a basis for psychological states and higher brain function, it can be inferred that if a certain pathway were impaired it may result in damaged emotional and social development. A proposed change in a neural pathway would presumably result in a change in that pathway’s resulting function. Referring back to the Harlow study, isolation, or lack of sensory stimulation, caused a change in social behavior.

            Sensory deprivation during the critical period of neural development can have consequences in development of the brain and behavior such as emotional and behavioral disorders. It has been well established that sensory systems require sensory input for normal growth and development, a natural extrapolation of neural Darwinism (3). Failure to receive this input can result in “developmental brain dysfunction and damage which underlies the depression, stereotypical movement disorders, hyper-reactivity to sensory stimulation, particularity to touch with paradoxically, impaired pain perception; social alienation, rage and pathological violence” (2). The somatosensory system of the cerebellum, which regulates the sense of movement, balance and the sense of touch, is believed to be the origin of the mentioned behavioral dysfunction (6).

             Experiments conducted by Sprague, Chambers and Stellar support the proposal that certain social-behavioral disorders are products of dysfunction of the cerebellar somatosensory system. In the study the somatosensory afferences of cats were surgically interrupted. They were observed to have self destructive behaviors similar to the behaviors of animals raised in isolation. These deafferentation and isolation studies have lead to the supposition that integration and regulation of “sensory-emotional” and motor processes occur in the cerebellum. Furthermore, a disorder in the cerebellar system may result in emotional or behavioral abnormalities. A proposal made by Snider supports this claim and concludes that the cerebellum has a regulatory role in “modulating responsivity in sensory systems to afferent input according to the neurological need of the organism”(3).

            Extensive study on isolation and stimulation during infancy and its illustrates the cerebellum’s role in behavioral disorders and its vulnerability to changes from sensory deprivation due to the fact that it is extremely under developed at birth. More specifically, it is the somatosensory system of the cerebellum that is most susceptible to changes in early neural structure development, and it is believed to be the origin of social and emotional abnormalities due to sensory affection deprivation (2) (3).

             Cerebellum development is directly effected by the process of neural Darwinism. When neonate sensory deprivation occurs, those pathways are modulated in response to their neurological need. These somatosensory structural alterations in turn affect the cerebellum, which is grossly plastic during infancy due to underdevelopment and particularly susceptible to change (6). The cerebellar dysfunction results in social-behavioral abnormalities of the infant during later development, most notably the ability for affective bonding.

             This does not prove the superiority of nurture over nature, nor does it propose that all depressive, hyperactive, anti social and violent aggressive behaviors are due to infant somatosensory deprivation. What it does establish is the correlation between the need for affect and normal neural development. For an individual to exhibit normal social behaviors the neural pathways that produce these behaviors must be unimpaired. Normal social-behavioral development is required in order ensure proper neural development in offspring through affective bonding and stimulation. Providing offspring with sensory stimuli aids the development of the social behaviors which are necessary for mutual interaction. Thus, it can be presumed that normal social behaviors are a result of unimpaired neural development, and that normal neural development, limited to the somatosensory development described in this paper, is a product of social interaction and affective bonding between parent and infant.    

1) Alienation of Affection – The influence of environment on brain structure

2) Birth Phsychology –The origins of human love and violence

3) Early Somatosensory Deprivation – In the abnormal development of the brain and behavior

4) Goldschmidt, Walter. The Bridge to Humanity. New York: Oxford University Press, 2006

5) Kandel, Eric R. and James H. Schwartz.  Principle of Neural Science.  New York: Elsevier North Holland, 1981

 6) Neural Development – Affective and immune system influences 7) Somatosensory Affectional Deprivation – SAD theory of drug and alcohol use