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animal matures. Although kittens can hear at birth,
they only respond to extremely loud sounds, well
above the level of city traffic (>100 dB sound pressure
level), at 10 days postnatal. Their auditory thresholds
gradually decrease so that they can detect sounds at
the level of a whisper (ª30 db SPL) by one month, and
by adulthood they become even more sensitive than
humans. A similar change is found in all developing
animals. In humans, auditory thresholds drop rapidly
during the first 6 months of life and are virtually adult-
like by 2 years of age. Improved behavioral thresholds
are well correlated to a decrease in sound level needed
to evoke an electrical potential from the cochlea, sug-
gesting that the major factor limiting detection in
young animals is the ear (Werner and Gray, 1998).
As thresholds decrease, most animals also respond
to higher sound frequencies. This is probably due to a
physical change in the cochlea because a single phys-
ical position along the basilar membrane responds to
higher frequencies as the animal matures. Since the
topographic projection from the cochlea to the central
nervous system does not change significantly during
this time, one might expect to find interesting changes
in sound perception with development. In fact, 15-day-
old rat pups can be trained to suppress activity when
they hear a 8 kHz tone, but three days later they sup-
press activity to a higher frequency. That is, a higher
sound frequency apparently sounds like the 8 kHz
tone because the cochlear frequency map has shifted
(Hyson and Rudy, 1987).
Although the basic sensitivity and frequency range
mature rapidly, several features of sound remain diffi-
cult to detect. This is well illustrated for tasks in which
one must detect a very brief event, often referred to
as temporal processing (Figure 10.15). For example,
adults are easily able to detect a 5 ms gap of silence in
an ongoing sound. In contrast, one-year-old infants,
who have already begun to process and produce
speech sounds, can only detect gaps that are an order
of magnitude longer (ª60 ms), and adult-like perfor-
mance is not reached until about 5 years of age (Werner
et al., 1992).
In humans, the maturation of adult-like perfor-
mance on auditory perceptual tasks extends beyond
the first decade of life (Stollman et al., 2004). The ability
to detect small differences in the duration of a tone is
more than an order of magnitude poorer in 4-year-
olds, as compared to adults (Jensen and Neff, 1993).
The ability to detect one sound in the presence of
another may not mature until puberty. In one task, the
listener is asked to recognize a long duration tone that
is presented during an ongoing burst of noise. Even
10-year-old listeners cannot perform nearly as well as
adults. Furthermore, children identified as learning
Speech sounds
A
"ba"
"pa"
2.0
gap
1.5
1
.05
0
0
0.5
1
1.5
Time (seconds)
B
Trial
noise
(10 gaps)
silent gap
60
40
20
0
3 mos
6 mos
12 mos
Adult
FIGURE 10.15 Development of temporal processing may affect
speech perception. A. An oscilloscope record of the human speech
phonemes, /ba/ and /pa/. Below each record is a spectrogram of
the phoneme showing the sound frequencies that compose the
phoneme and their relative intensity (darker is louder). Note that the
/ba/ is a continuous sound, whereas /pa/ consists of a nonvoiced
component (in this case, the p sound), followed by a brief gap, and
then a voiced sound (the “a”). Perception of this brief gap is critical
to word recognition. B. The minimum gap that humans can perceive
was assessed with a brief silent period embedded in a white noise
stimulus. The bar graph shows that even at 12 months of age, human
infants are almost 10 times less sensitive at detecting a gap than
adults. (Adapted from Werner et al., 1992)
disabled never reach the normal adult level of per-
formance on this task, and it has been suggested that
the onset of puberty may terminate the critical period
during which neural maturation takes place (Wright
and Zecker, 2004).
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