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cortex neurons recorded in young animals are not very
directional. In contrast, adult neurons respond to
sounds from only a narrow range of auditory space,
particularly if the sound is very quiet. To test whether
this difference is due to maturation of the external ear,
infant animals were provided with the sound cues that
are only available with an adult pair of ears (i.e., the
external ear filters sound arriving at your ear canal,
and this depends on its size and shape). The adult ears
produced a dramatic improvement in the spatial
receptive fields of the infant cortical neurons, suggest-
ing that the connections responsible for spatial tuning
are quite mature at the outset of hearing. The result
also raises an interesting prospect: Developmental
plasticity may permit the central auditory system to
remain properly sensitive to the changing sound cues
as the ears grow.
Auditory processing may also be limited by the low
discharge rates that are generally reported for young
animals. Furthermore, the sound-evoked response
fatigues rapidly during a period of stimulation in
young neurons. What does this mean for the perform-
ance of a developing nervous system? First, neurons
have a poorer resolution: they devote few action
potentials to a given change in the stimulus. For
example, adult LSO neurons can devote twice as many
action potentials to a given change of interaural level
compared to juvenile animals (Sanes and Rubel, 1988).
Therefore, either young animals make decisions based
on less neural information (e.g., fewer action poten-
tials), or, more likely, they are not able to perform at
adult levels because they have less neural information
to work with. Striking as this result is, we still have no
direct knowledge about the relationship between
amount of neural activity and perception.
A
Auditory
Somatosensory
20 days
42 days
115 days
B
300
Receptive Field Size
auditory
200
visual
100
05 0
15
20
adult
Age (weeks)
FIGURE 10.17 Single neuron receptive field sizes can decrease
dramatically during development. A. Recordings were made from
single neurons in the superior colliculus that respond to more than
one sensory modality. The auditory (green) and somatosensory (red)
receptive fields are shown for neurons from cats of increasing age.
At 20 days, the neuron responded to auditory stimuli located any-
where in space,and to touch on any area of skin. At 115 days, the
neuron responded to a small area of auditory space (green) and to
touch on a small area of skin (red) under the right ear. B. The rela-
tive size of auditory (green) and visual (blue) receptive fields are
plotted for neurons recorded throughout development. There is a
dramatic decrease during the first eight weeks, and mature proper-
ties are attained by the seventeenth week. (Adapted from Wallace
and Stein, 1997)
SEX-SPECIFIC BEHAVIOR
The many differences between male and female
behavior are a popular subject of conversation. They
are also a source of considerable controversy, and the
political stakes can be quite high. We primates tend
to debate whether sex-specific behaviors are due to
our “biology” or to the social environment in which
we are raised. While the debate is seductive, the
relationship between brain development and sexual
behavior varies tremendously from species to species.
Since mating and maternity have been most thor-
oughly explored at the neural level, we will mostly
focus on these behaviors. However, it is worth
mentioning some complex behaviors that differ
between male and female animals. These differences
auditory space is not apparent until postnatal day 32,
even though hearing begins in utero (Withington-
Wray et al., 1990).
It is essential to recognize how the development of
peripheral structures, such as the external ears, influ-
ence the response properties of central neurons. An
interesting demonstration of this comes from a study
of spatial receptive fields in the developing ferret audi-
tory cortex (Mrsic-Flogel et al., 2003). Single auditory
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