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AP Psychology
THEORIES OF HEARING
LOUDNESS DISCRIMINATION
Current theory holds that perceived loudness is a function of the number of individual
neurons that are firing at a given time. Thus, a very loud tone might activate 250
neurons, while a soft tone would activate only 50 neurons.
PITCH DISCRIMINATION
Scientists are fairly certain of the general way in which sound waves are converted into
neural impulses. They are less sure, however, of the mechanism which gives us
information about the frequency of the sounds we hear. Current belief is that the
mechanism for pitch discrimination is located in the cochlea. There are two general
theories of pitch perception: place theory and frequency theory.
Place Theories Place theories state that certain points along the basilar membrane are
sensitive only to certain frequencies. Early place theory felt that the basilar membrane
contained fibers of various thickness (like a piano), so that only specific fibers vibrated
at any given frequency. It was later found, though, that the entire cochlea was filled
with fluid, so that fibers could not vibrate at frequencies as high as some sound waves
we hear. A more recent place theory is the traveling-wave theory. This theory says that
since the basilar membrane decreases in thickness as it spirals toward the center of the
cochlea, different parts of the membrane itself will vibrate more at specific frequencies.
Frequency Theories Early frequency theory held that neurons in the cochlea fire at the
frequency of the sound waves heard. This simplistic view was objected to on the basis
that a single neuron is capable of firing at a maximum of only about 1,000 cycles per
second, yet we can hear sounds up to 20,000 cps. To meet this objection, the volley
theory was proposed. The volley theory says that specific frequencies are represented
by groups of neurons firing in succession. Thus, a 500-cps tone would cause one group
of neurons to fire at 500 cps. A 1,000-cps tone would cause two groups of neurons to
fire alternately at 500 cps. A 2,000-cps tone would cause four groups of neurons to fire
alternately at 500 cps (or, perhaps, 20 groups of neurons to fire alternately at 100 cps).
Source: http://userwww.sfsu.edu/~psych200/unit6/62.htm
Place-frequency Theories This theory combines the place and frequency theories. At
low frequencies, the frequency theory can be better applied, while at higher frequencies
the location of maximum amplitude on the cochlea is the primary detector as in the
place theory.
Source: http://www.gedlee.com/downloads/Chapter%202.pdf