Just as the speech
signal can be viewed as either a change in amplitude over time
or a spectrum of frequencies, so can the auditory evoked potentials
be considered in either the time domain or the frequency domain.
Steady-state responses are evoked potentials that maintain a stable
frequency-content over time. Steady-state evoked potentials are
usually evoked by stimuli that occur at rapid rates. The response
then shows a spectrum with energy only at the rate of stimulation
and its harmonics.
Amplitude-modulated
and frequency-modulated tones evoke clear steady-state responses.
Amplitude-modulated tones have excellent frequency-specificity
since they contain energy only at the frequency of the carrier
tone and at two sidebands, separated from the carrier by the frequency
of modulation. The most widely recorded auditory steady state
response is the 40 Hz potential. This can be used to measure audiometric
thresholds in waking adults and older children. However, the response
is difficult to measure in infants and young children. Furthermore,
the 40-Hz response is very susceptible to changes in the level
of arousal, showing small amplitudes during sleep and even smaller
amplitudes during anesthesia. Recently, it has been shown that
the auditory steady-state responses can be recorded at stimulus
rates between 75 and 110 Hz.
These
responses can be readily recorded in infants and are unaffected
by sleep. The responses to several amplitude-modulated stimuli
presented simultaneously can be independently assessed if each
stimulus is modulated at a different rate. The recorded activity,
when viewed in the frequency-domain, shows a response to each
carrier-frequency at its signature modulation-frequency. Since
amplitude-modulated tones are not significantly distorted by free-field
speakers or hearing aids, they can be used to test how well a
hearing-aid is working. Responses evoked by frequency-modulated
stimuli may become helpful in assessing supra-threshold auditory
processes, such as those necessary for speech perception.
