Sample take-home exam question and answer
Question: Different very basic aspects of hearing become mature at different ages.
Explain that statement, providing evidence for your position. Why do you think that
hearing development works that way? Would you expect complex perception, for
example of speech, to suffer as a result in early life?
Answer: The basic aspects of hearing are the representations of frequency, intensity
and the temporal characteristics of sound. Frequency representation includes the
representation of the amplitude spectrum of complex sounds, referred to as
frequency resolution, and the representation of the fine structure of sounds, which
is based on phase-locking. Intensity representation is self-explanatory: It refers to
the ability to detect low-intensity sounds and to encode intensity differences. The
temporal characteristics of sound are the changes in amplitude and frequency over
time; temporal representation refers to the ability to follow such changes. These
aspects of hearing mature at very different ages, ranging from infancy to 10 years of
age.
Frequency resolution is an early maturing capacity. It is usually assessed by
measuring masked thresholds. Thresholds for tones or noise bands masked by
broadband noise actually are not mature until around 6 years of age (Schneider et
al). That would suggest that frequency resolution is a relatively late-developing
capacity, and in fact, several studies that have measured auditory filter width
seemed to indicate that frequency resolution matured around 6 years of age or later
(Wightman et al.; Irwin et al). However, studies of psychophysical tuning curve
widths (Spetner & Olsho) and critical bandwidths (Trehub et al.) in infants
suggested that frequency resolution was mature by 6 months of age. This
discrepancy is puzzling, because it is generally assumend that relative measures of
frequency resolution, in which resolution depends on the differences across
thresholds, will not be affected by attention, memory or other cognitive processes.
Hall and Grose solved the puzzle by showing that apparently immature auditory
filter widths could result from differences between 4-year-olds and adults in
decision processes. Thus, it is currently believed that frequency resolution is mature
by 6 months of age.
The place code for frequency likely develops even earlier, during the prenatal period
(Rubel & Ryals; Lippe & Rubel). However, it is possible that the temporal code for
frequency, based on phase-locking, takes longer to develop. Two lines of evidence
suggest that is the case. First, frequency discrimination at low frequencies is not
mature until the school years (Maxon & Hochberg), although frequency
discrimination at high frequencies is mature at 6 months of age (Olsho et al). This
difference is consistent with the idea that frequency resolution, based on the place
code, is important for high frequency discrimination, while the temporal code for
frequency is important for low frequency discrimination. That high frequency
discrimination matures so early is consistent with an early maturation of frequency
resolution. The prolonged course of development of low frequency discrimination
could be explained by late maturation of the temporal code for frequency. Second,
there is evidence that phase-locking, which provides the basis for the temporal code,
is a late developing response in nonhumans (Brugge et al.) and humans (Levi et al.).
The maturation of intensity representation is also complex. Intensity representation
is reflected in absolute threshold, intensity discrimination, and loudness. Early in
life, absolute thresholds are immature (Trehub et al). Although the cochlear and
neural processes that determine absolute threshold, or threshold in quiet, seem to
be mature early (Werner et al.), immaturity of the middle ear actually leads to small
differences in threshold between school-age children and adults (Keefe et al).
Intensity discrimination may be mature by 5-6 years of age, but recent evidence
suggests that immaturity persists into the school years (Buss et al.). In contrast, the
perception of loudness is mature in 5-year-old children (Collins & Gescheider).
There are no data indicating that the growth of neural response is still immature
beyond that age. Furthermore, it is difficult to interpret age differences in intensity
representation because no relative measure of intensity resolution exists.
As noted above, the development of the ability to follow changes in a sound over
time is complicated. It is clear that when temporal resolution is measured using a
relative measure like the temporal modulation transfer function, it is adultlike by 4
years of age (Hall & Grose), and there are data suggesting that it is mature even
earlier (Werner et al). However, measures like gap detection (Wightman et al) and
duration discrimination (Abel et al) indicate a more prolonged course of
development, with mature performance only emerging during the school years.
Again, the prolonged course of development of phase-locking could be involved
here, although it is not clear how immature phase locking would lead to immature
performance on some measures, but not others. This may be another case in which
relative measures and absolute measures of hearing yield different results.
One explanation for why the auditory system develops in this irregular pattern is
that different auditory capacities require different sorts of experience with sound to
mature. According to that view, frequency resolution can reach adult status after
only a few months’ exposure to frequency specific sounds. However, both intensity
and temporal resolution depend on the integrity of neural elements like synapses,
which may require more experience with sound. It is also possible that the late
developing abilities depend on the ability to use the neural response appropriately
or on other cognitive abilities. Auditory cortex develops over a long time course
(Moore), and such abilities may depend on a mature cortex.
Despite some prolonged immaturities, children seem to be able to process complex
sounds like speech and music well enough to function at home and in the classroom.
However, naturally occurring complex sounds differ from each other in multiple
ways. For example, a particular speech contrast could be signaled by frequency,
intensity and temporal information. If children have good resolution on one of these
dimensions, then they should be able to perceive that contrast. In other words,
redundancy in naturally occurring sounds would keep immature auditory
processing from handicapping children in most conditions. It is possible, however,
that under difficult listening conditions, in which one or more distinctive speech cue
might be obscured, it is possible that children will have more difficulty than the
mature listener.