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salsa? Why can it take several seconds for a baby to
determine the location of a human voice?
Studies of sensory perception are among the most
difficult experiments in the field of development.
Baby animals (especially human infants) tend to be
slow, sleepy, cranky, inattentive, and forgetful. These
are generally referred to as nonsensory factors. For
example, adults pay better attention to novel stimuli
than young animals. When adult primates are pre-
sented with two images, one of which they have never
seen before, they spend about 70% of the time staring
at this novel object. In contrast, infants spend an equal
amount of time staring at the familiar and novel objects
(Bachevalier, 1990). Even though adults are more atten-
tive to novel stimuli, they can also focus narrowly on a
stimulus of interest and ignore novel stimuli that may
be distracting. For example, when taking an examina-
tion, we tend to “block out” extraneous noise. This was
demonstrated by asking people to detect a tone when
it was presented on 75% of trials. Several other tones
were presented on the other 25% of trials. Adults come
to “expect” the tone that is presented 75% of the time,
and they detect it quite well, whereas they are very
poor at detecting the other tones (i.e., those presented
on 25% of trials). Infant perception differs rather dra-
matically: they detect all of the tones equally well
(Bargones and Werner, 1994). Thus, infants and adults
experience the world in very different ways, and these
non-sensory factors lurk in the background of all
developmental studies of sensory perception.
Observer
Reinforcement
Speaker
Infant moves in
response to sound
FIGURE 10.12 Determining an infant's sensitivity to sound.
Infants will make small movements in response to a sound they can
hear, and this natural tendency has been exploited to measure the
infant's ability to hear different sound frequencies. To improve the
sensitivity of this procedure, an adult observer watches the infant
and judges whether the infant heard the stimulus based on any
response that the infant makes. To increase the infant's responsive-
ness to sound, she can be reinforced for correct responses. In this
case, the infant is rewarded with the appearance of a teddy bear.
a reward, even human infants. Thus, the head turn
that an infant makes to a sound can be reinforced by
showing her an interesting toy (Figure 10.12). The
infant will then “work” for visual stimulation (that
is, turn her head) when she hears a sound. This is
somewhat below the minimum wage, but it serves
the purpose. This procedure can be extended to
very young infants (<6 months) by having an adult
observer, who cannot hear the test sounds, watch the
baby to determine when she makes a response to
sound. The baby gets rewarded (with a viewing of the
toy bear) whenever the observer determines that
the baby has responded. In this manner, any possible
response that a baby might make to sound (for
example, an eye movement or a tongue wag) can be
conditioned. Of course, we believe that the baby is
oblivious to this process, but she nonetheless ends up
working for a reward and providing valuable infor-
mation about sensory development along the way.
Such training techniques require far more of an animal
than sensory skills, and one may well end up study-
ing the development of attention or memory rather
than the development of sensory perception.
ASKING BABIES QUESTIONS
How, then, can these uncooperative little animals
tell us about their sensory experiences? Various exper-
imental tricks have been devised to determine how
sensory information is processed in young animals. In
one scenario, the behavioral scientist watches for a
motor reflex while a sensory stimulus is presented. For
example, we often respond to an unexpected noise
with a startle, and this rapid muscle twitch provides a
reliable measure that sound has been detected. We can
also take advantage of the fact that animals tend to
stop responding, or habituate, to a stimulus when it is
presented many times. After the animal has habitu-
ated, one can present a new stimulus and ask whether
the animal responds. This is a good way of determin-
ing how well an animal notices the difference between
two similar stimuli (e.g., middle C versus C sharp).
A young animal can also be trained to produce a
stereotyped behavior, such as a head turn, when a
stimulus is detected. Most animals will work for
 
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