Chirp and click evoked auditory steady state responses

In this study, Auditory Steady State Responses (ASSR) to 100 psec clicks and 4 msec cochlear chirps are recorded in adult subjects at repetition rates of 20 to 100 Hz in 10 Hz increments. Response characteristics of ASSRs are compared in the frequency domain. Results show that response amplitudes to both stimuli peak in the proximity of 40 Hz and 80 Hz, but chirps generate responses with larger amplitudes at all repetition rates. The responses at 40 Hz repetition rates for both clicks and chirps are larger than the corresponding responses at 80 Hz stimuli in awake adults. The 40 Hz responses, however, show greater variability than the 80 Hz responses depending on the subject's state of consciousness. The predictions of the ASSR waveforms at each repetition rate were then synthesized by superimposing the suitably shifted Middle Latency Response (MLR) waveforms that were recorded during the same session with a stimulus repetition rate of 10 Hz for both click and chirp recordings. This part of the study was undertaken to investigate the validity of the hypothesis which states that the steady state responses are simply the linear additions of transient MLRs. The results showed that the synthesized waveforms resembled the real recordings to some extent, but consistently had larger amplitudes. This difference is assumed to be a result of the adaptation effects of higher stimulus rates.