Steady-state auditory evoked potentials (SSAEPs) in the rabbit. Contribution of the inferior colliculus

Potentials evoked by chirp-modulated tones: a new technique to evaluate oscillatory activity in the auditory pathway

OBJECTIVE

Steady-state potentials are oscillatory responses generated by a rhythmic stimulation of a sensory pathway. The frequency of the response, which follows the frequency of stimulation, is maximal at a stimulus rate of 40 Hz for auditory stimuli. The exact cause of these maximal responses is not known, although some authors have suggested that they might be related to the 'working frequency' of the auditory cortex. Testing of the responses to different frequencies of stimulation may be lengthy if a single frequency is studied at a time. Our aim was to develop a fast technique to explore the oscillatory response to auditory stimuli, using a tone modulated in amplitude by a sinusoid whose frequency increases linearly in frequency ('chirp') from 1 to 120 Hz.

METHODS

Time-frequency transforms were used for the analysis of the evoked responses in 10 subjects. Also, we analyzed whether the peaks in these responses were due to increases of amplitude or to phase-locking phenomena, using single-sweep time-frequency transforms and inter-trial phase analysis.

RESULTS

The pattern observed in the time-frequency transform of the chirp-evoked potential was very similar in all subjects: a diagonal band of energy was observed, corresponding to the frequency of modulation at each time instant. Two components were present in the band, one around 45 Hz (30-60 Hz) and a smaller one between 80 and 120 Hz. Inter-trial phase analysis showed that these components were mainly due to phase locking phenomena.

CONCLUSIONS

A simultaneous testing of the amplitude-modulation-following oscillatory responses to auditory stimulation is feasible using a tone modulated in amplitude at increasing frequencies. The maximal energies found at stimulation frequencies around 40 Hz are probably due to increased phase-locking of the individual responses.

The effects of propofol on the 40-Hz auditory steady-state response and on the electroencephalogram in humans

The auditory steady-state response (ASSR) is a nearly sinusoidal electrical response of the brain to auditory stimuli delivered at fast rates. The amplitude of the response is largest for stimulus rates near 40/s, hence the label 40-Hz ASSR. We have studied the effects of propofol (1.5 mg/kg) on the 40-Hz ASSR in 14 patients. The spectral edge frequency (SEF) and median frequency (MDF) of the electroencephalogram were recorded for comparison. The study was limited to 6 min after the injection of propofol. Recordings were obtained every minute. Consciousness, defined as responsiveness to verbal command, was assessed in all patients within 1 min. Nine patients, regained consciousness before the end of the study. Propofol caused disappearance of the 40-Hz ASSR for 2 min. Th 40-Hz ASSR reappeared afterward, reaching, at the end of the study period, an amplitude of about 65% of baseline. Recovery of the 40-Hz ASSR occurred whether or not consciousness was regained during the study, but the amplitude tended to be larger in patients who had regained consciousness compared with patients still unconscious during the same recording. The SEF was reduced by 24% within 2 min and recovered more quickly than the 40-Hz ASSR, reaching 91% of baseline within 4 min. The SEF tended to be higher in patients who had regained consciousness. The MDF was reduced by 27% within 2 min, and exceeded baseline values by 25% during recovery. The MDF was not higher in patients who had regained consciousness. We conclude that propofol transiently abolishes the 40-Hz ASSR. Recovery of the 40-Hz ASSR occurs whether or not consciousness is regained, but the 40-Hz ASSR tends to be larger after the return of consciousness. An association between higher amplitude 40-Hz ASSR and the return of consciousness could not be conclusively established, perhaps because of low signal-to-noise ratio in three patients. The 40-Hz ASSR did not offer any clear advantage over the SEF in predicting the return of consciousness.

Effect of natural sleep on auditory steady state responses in adult subjects with normal hearing

Auditory 40-Hz steady state response (SSR), auditory brainstem response (ABR), and middle-latency response (MLR) were recorded in 12 healthy adult females with normal hearing while awake and asleep. The responses were recorded with 500-Hz tone pips at 55 dBnHL. Synthesized SSR were made by superimposing the recorded ABR and MLR waveforms (ABR-MLR), and their amplitudes were compared with those of the actually recorded SSR. In the waking state, the ratio of the mean amplitude of recorded SSR to that of synthesized SSR was 0.819, whereas in the sleeping state it decreased to 0.522, a statistically significant difference. The results indicate that the SSR can be predicted from the linear superimposition of ABR and MLR in the waking state, but not in the sleeping state.