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mammals (Ishida et al., 1999). Do these genetic
homologies extend to neural development in the sense
that genetically conserved programs construct similar
neural circuits that control homologous behaviors? If
so, how do the genes control how these circuits are
built, and what mechanisms modify these circuits to
give species-specific behaviors?
The nervous system is unique among organs in that
it responds to a huge variety of environmental influ-
ences by changing itself structurally and functionally,
even from an early age (Chapter 9). Neural activity
modulates the expression of many of the same gene
products that were used during neural embryogene-
sis. This interplay between the environment and the
genome continues throughout life. And because of
this, it is usually not reasonable to ascribe specific
behaviors to purely genetic or environmental determi-
nants. Genetic influences on behavior are as clear as
experiential ones, and the two are interlinked. Identi-
cal twins who are separated at birth and reared in
different families may show amazing similarities
in attitude and taste, compared to nonidentical twins
reared together. But these two human beings usually
also show an enormous array of dissimilarities, reflect-
ing lifelong interactions between the environment and
the genome. Similarly, work in mice has shown that
the different behavioral tendencies, like the willing-
ness to explore, in different strains of mice are heavily
influenced by epigeneitic mechanisms. A genetically
B6 mouse that both develops in a BALB uterus and is
reared by a BALB mother shows exploration behavior
that is more similar to the BALB than it is to the B6
strain (Francis et al., 2003).
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 interac-
tions 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 nev-
ertheless 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 func-
tions of the nervous system, and this process begins in
the embryo. In dealing with the genetic and environ-
mental 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 recogniz-
able impacts on early behavior. For example, one of the
earliest signs of fetal alcohol syndrome is the behav-
ioral retardation of the fetus (Mulder et al., 1986). By
affecting the way that neurons develop, both environ-
mental insults and genetic mutations have an enor-
mous 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.
ENVIRONMENTAL DETERMINANTS
O F BEHAVIORAL DEVELOPMENT
From early embryonic stages of life, genetic influ-
ences 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 environ-
mental hazards such as drug use or alcohol consump-
tion during pregnancy. Embryonic exposure to high
levels of these substances can lead to mental retarda-
tion (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
THE FIRST MOVEMENTS
The first simple twitch that an animal makes signals
the beginning of its functional motor circuitry. In most
species, the earliest skeletal movements are caused by
spontaneous activity of motor neurons. If these motor
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