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These behavioral measures of auditory processing
are relevant to language development because human
speech sounds are composed of rapid changes in fre-
quency and intensity, including discrete periods of
silence. In fact, children with learning disabilities that
are due primarily to a difficulty with spoken language
also perform poorly on simple auditory discrimination
tasks that require temporal processing. For example,
when normal children are exposed sequentially to two
tones, they can report the correct sequence with delays
as small as 8 ms. In contrast, the language-impaired
group required a silent interval of 300 ms in order to
report the correct sequence. Recently, it has been found
that performance can be improved when language-
impaired children are trained to recognize speech
sounds that are slowed down. Apparently, once the
nervous system has learned to recognize this slower
speech, it is better able to recognize the rapid tempo-
ral variations in normal speech (Tallal and Piercy, 1973;
Tallal et al., 1996).
Several mechanisms may explain poor temporal
processing in young animals. For example, we have
seen that synaptic potentials are usually of much
longer duration in young animals (see Chapter 8). We
might suppose that long PSPs effectively limit the
“clock speed” of the organism, or the fastest rate at
which information can be processed. Thus, it will be
interesting to learn more about the neural basis of tem-
poral processing, particularly in the auditory system.
Perhaps the most useful information that a devel-
oping animal gets from its ears is the location of
significant objects, such as mother or a predator.
Although infants can tell whether a sound source is
coming from the left or the right (sound lateralization),
they are not able to make fine discriminations. Adult
humans can detect a 1° change in the position of a
speaker (recall the “rule of thumb”), but newborns can
only detect a change of about 25°. In fact, even sound
lateralization is fairly challenging to a newborn infant
(Figure 10.16). The sound stimulus must remain on for
about one second if the infant is to make an appropri-
ate head orientation response, whereas adults need
only about a millisecond of sound, such as a finger
snap (Clarkson et al., 1989). The ability of nonhuman
mammals to lateralize sounds is also present even as
the animal first experiences sound, yet we know little
about the sensitivity of the system. For example, rat
pups suddenly begin to turn their heads toward a
noise at 14 days postnatal, a few days after the ear
canal opens. However, the percentage of correct turns
toward the sound continues to increase over the next
seven days (Kelly et al., 1987).
The response of central neurons to sound is known
to change during this period of development, and
A
Adult
rapid response
Sound
(minimum duration)
Head turn
(latency & duration)
0
2
4
6
8
10
seconds
B
Newborn
long latency
slow response
Sound
(minimum duration)
Head turn
(latency & duration)
0
2
4
6
8
10
seconds
C
60
Newborn Infants
40
20
0
0.5 s
1 s 5 s
Duration of rattle sound
10 s
20 s
FIGURE 10.16 Infants are poor at sound lateralization. A. When
presented with a sound located to one side, adult humans turn their
head toward the sound source within a fraction of a second. B. When
human infants are presented with a sound to one side, they may take
several seconds to respond, and the head movement can be quite
slow. C. For infants, a sound stimulus must be presented for a long
time period in order to get accurate lateralization. Whereas adults
can localize sounds that last only a millisecond, newborn infants
require at least 1 s of sound. (Adapted from Clarkson and Clifton,
1991)
some of these alterations could help to explain imma-
ture sound localization. Maps of space are found in
the superior colliculus (SC) of several mammals, and
single SC neurons are selectively activated by sound
(or visual and somatosensory stimuli) from a specific
location. During the course of development, these
SC neurons respond to a smaller part of the sensory
world. In cats, the average size of a receptive field
decreases about fourfold during the first two months
after birth (Figure 10.17). Furthermore, the visual
receptive fields become adult-like a few weeks earlier
than the auditory receptive fields in kittens (Wallace
and Stein, 1997). In the guinea pig, an orderly map of
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