Developmental psychology.pdf

132 Modes of Awareness
Figure 5.8
Comparison of Place and Volley
Theories. This table indicates that
the theories explain pitch and
loudness differently.
Theory Pitch Loudness
Place
Volley
place in the cochlea
frequency of volleys
spread of disturbance in the
cochlea
number of impulses per volley
Theories of Hearing In hearing, just as in color vision, there are two major
theoretical positions, one of which is place theory. According to place theory, different
regions of hair cells in the cochlea are attuned to different frequencies of vibration,
and the impulses aroused in different hair cells go to different regions of the auditory
cortex. Pitch depends on the place in the cochlea that is most activated and also the
place in the auditory cortex that receives the resulting nerve impulses. The assumption
that there are precise cortical localizations for high-pitched and low-pitched sounds
has yet to be verified, however.
Loudness, according to this theory, depends on the spread of disturbance in
either direction from the activated area of the cochlea. Two tones of the same pitch
and loudness would activate the same fibers, but if the tones differ in intensity, the
range of activated fibers would be greater for the louder sound.
No single fiber in the auditory nerve can respond more frequently than 1,000
times per second, however, which raises a complication in place theory. How can we
readily hear sounds up to 4,000 hertz?
The volley theory attempts to answer this question, suggesting that nerve fibers
work in groups. One group of fibers is discharged by one sound wave, another
group by the next. The first group, on account of the absolute refractory period of .001
second, cannot respond to the second wave, but with different groups of fibers involved,
there are always some fibers available to respond to any given wave. This group activity
is the central concept in volley theory (Wever, 1949).
For a tone of 4,000 hertz, a spurt of neural activity called a volley occurs in
the auditory nerve every .004 second, with different groups of fibers contributing to
different spurts. Also, some fibers, because of their greater excitability, contribute to
more spurts than do other fibers. Pitch, according to volley theory, depends on the
frequency of volleys rather than the frequency carried by the individual fibers.
Loudness is thought to increase as more impulses occur in each volley, for an
increase in the intensity of stimulation causes more fibers to respond. A large sound
wave might activate 100 instead of 50 fibers, producing more impulses per volley without
changing the frequency of the separate volleys (Figure 5.8).
As in vision, the place and volley theories are not necessarily incompatible.
The volley factor may play a major role in mediating frequencies up to 4,000 hertz,
and the place factor may be more significant in higher frequencies. Recent research
suggests that both phenomena may be involved in human hearing, to which the neural
processes in the brain add further complexity.
Sense of Smell
In contrast to vision and hearing, the senses of smell, taste, and touch are the "lower
senses" at the human level. This description does not mean that they are necessarily
less acute, for it is impossible to make comparisons across the modalities. Rather, it
means that they are generally less important in survival. The sense of smell does not
alert us to predators, as it does with animals, although it puts us on guard when certain
foods are unfit to eat and warns of such dangers as fire, gasoline, and ammonia. Smell
might be of greater significance if we did not move in an upright position.