Changes of amplitude and topographical characteristics of event-related potentials during the hypnagogic period

The auditory steady-state response is not a suitable monitor of anesthesia

Previous studies show that the human 40-Hz auditory steady-state response (ASSR) disappears on induction of general anesthesia, suggesting that it may be a good candidate for a monitor of anesthesia. In this study, we aimed to learn whether all normal alert adults display ASSRs with adequate signal-to-noise ratio. Clicks were presented at a series of frequencies between 35 and 70 Hz and electroencephalographic records taken at the vertex were Fourier transformed. ASSRs were observable as sharp peaks in the electroencephalograph spectrum at the frequency of the clicks. Initial results showed that a discernible ASSR could not be obtained from about half the subjects studied at any click frequency used. Further investigation revealed that in subjects whose ASSR was undetectable in the alert state, induction of a drowsy mental state resulted in appearance of an observable ASSR. This was attributable to an increase in signal in the drowsy state, not to a decrease in noise. We conclude that, because a significant proportion of subjects do not display easily recordable ASSRs when alert, it is not practical to use disappearance of the ASSR as a routine test for adequacy of anesthesia.

IMPLICATIONS

Auditory steady-state responses (ASSRs) are brain waves evoked by auditory stimuli. Because they reportedly disappear under general anesthesia, they have been suggested as potential indicators of anesthetic depth. However, in this study, we show that about half our normal adult subjects did not produce measurable ASSRs when awake. This suggests that ASSRs are not good candidates for use in monitoring anesthetic depth during surgery.

Comparison of the event-related potentials between tonic and phasic periods of rapid eye movement sleep

The event-related potentials (ERP) of eight young healthy volunteers were recorded during wakefulness and rapid eye movement (REM) sleep using an auditory discrimination task. REM sleep was classified into phasic and tonic periods according to the presence or absence of REM. In wakefulness, deviant stimuli elicited a P300 that was maximal over parieto-central areas. During the tonic period of REM sleep, deviant stimuli elicited a P200 and a P400. The P200 was distributed more anteriorly and the P400 was distributed more posteriorly than the P300; however, no prominent ERP components were observed during the phasic period. The study's findings suggest that the brain is less sensitive to external stimuli during the phasic period than during the tonic period of REM sleep.