GENERATION OF HUMAN AUDITORY STEADY-STATE RESPONSES (SSRS) .1. STIMULUS RATE EFFECTS

Auditory evoked responses were recorded in 16 normally hearing subject s in order to investigate the mechanisms underlying the generation of the 40 Hz steady-state response (SSR). In the first part of our study, auditory potentials were evoked by 0.1 ms clicks presented at 105 dB p.e. SPL with repetition rates of 7.9 (to obtain middle latency respon se, MLR), 20, 30, 40, 50, 60 Hz. In each subject predictions of the re sponses recorded at stimulus repetition rates of 30, 40, 50, 60 Hz wer e synthesized by superimposing MLRs at suitable time intervals. The ca lculated mean amplitude/rate and phase/rate functions behaved similarl y for the recorded and predicted curves, showing the highest amplitude at 40 Hz and a linear increase of phase values when increasing the st imulus rate. Nevertheless the synthetic curves closely predicted ampli tude and phase values of the recorded responses only at 40 Hz. At freq uencies below 40 Hz, the mean amplitude of the predicted curve was low er than that of the recorded one while at frequencies above 40 Hz the mean amplitude was higher. Predicted phase values were found lagging a t 30 Hz, and leading at 50 Hz and 60 Hz in comparison to phase values calculated on the recorded responses. Our findings suggest that a mode l based on the linear addition of transient MLRs is not able to adequa tely predict steady-state responses at stimulus rates other than at 40 Hz. Other mechanisms related to the recovery cycle of the activated s ystem come into play in the steady-state response generation causing a decrease in amplitude and an increase in phase fag when increasing th e stimulus repetition rate.