From early embryonic stages of life, genetic influences operate in the context of specific environments. These environments place selective pressures on the embryo and influence development. The most obvious examples for humans come from studies of environmental hazards such as drug use or alcohol consumption during pregnancy. Embryonic exposure to high levels of these substances can lead to mental retardation (Johnson and Leff, 1999). As we will see later in this chapter, embryonic exposure to sex-specific steroids can alter both physical and neural aspects of sexual maturation. In bird embryos, a brief exposure to a mother's call can imprint a preference for that call upon hatching. After hatching, song birds learn to produce the father's song, and vocalization centers in its brain are strongly influenced by this process (Mooney, 1999).
Early deprivation of many kinds (visual, auditory, even emotional) is known to have permanent effects on behavioral development, just as early experiences can have profound affects on neural development. As we saw in Chapter 9, normal vision can never be restored to an adult mammal that has been blind from birth; the neural circuitry simply has not developed properly. Human infants who are blind also show delays in various motor skills, a fact that emphasizes the importance of vision as a sensory input for motor development (Levtzion-Korach et al., 2000). Lack of various aspects of education and emotional interactions early in life may also affect the ability to perform later in life because the neural substrates of these behaviors are wired in at particular stages. Thus, although we do not believe that human brains, or those of other animals, emerge as blank slates, it is nevertheless clear that experience shapes and adjusts the nervous system. An argument has been made that the brain is especially adaptive, in the Darwinian sense, simply because it is an organ that learns how to modify behavior in order to improve survival in a changing environment. In fact, learning is one of the main functions of the nervous system, and this process begins in the embryo. In dealing with the genetic and environmental influences on behavior, we must not only address how genes control cellular and molecular events to construct the neural substrate of behavior, but also how behavioral and sensory events feed back onto these molecular mechanisms.
The description of developing behavior is valuable because it provides a sensitive and fairly inclusive indicator of a successfully assembled nervous system. A multitude of human neural diseases have recognizable impacts on early behavior. For example, one of the earliest signs of fetal alcohol syndrome is the behavioral retardation of the fetus (Mulder et al., 1986). By affecting the way that neurons develop, both environmental insults and genetic mutations have an enormous impact on the emergence of the functional circuitry underlying behavior and may restrict an organism's ability to perceive the world and to respond to it with coordinated movements.
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