Responses of auditory-nerve fibers to stimuli producing psychophysical enhancement

A form of auditory "enhancement" can be demonstrated by omitting a component from a harmonic series for a few hundred milliseconds and then replacing it: the replaced component stands out perceptually. Psychophysical experiments have shown that components generate more forward masking when enhanced than when present but not enhanced. This result has been interpreted as demonstrating that enhancement involves an increase in gain in the frequency region of the replaced component. The present experiments sought physiological evidence of enhancement in the responses of auditory-nerve fibers in the guinea pig. In one condition a 200-Hz harmonic series lacking components near 2 kHz preceded another series with the 2-kHz component present (the "test" series). In this condition the mean discharge rate to the 2-kHz component was larger than the adapted responses to the other components of the test. In a second condition the test series was preceded by silence. In both conditions the 2-kHz component caused the same increase in firing rate. The average discharge rate sychronized to the 2-kHz component was also the same in both conditions. However, the proportion of the total discharge rate which was locked to 2 kHz was larger when the test followed the harmonic series than when it followed silence. Thus the contrast, in terms of both mean and synchronized rates, between the responses at 2 kHz and those at other frequencies, was increased when the test was preceded by the harmonic series. However, there was no evidence of an increase in gain (i.e., absolutely larger responses) in the 2-kHz region. It seems likely therefore that the mechanisms responsible for this aspect of auditory enhancement are located more central than the auditory nerve.