Generation of human auditory steadystate responses (SSRs). I: Stimulus rate effects

Abstract

Auditory evoked responses were recorded in 16 normally hearing subjects 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 response, MLR), 20, 30, 40, 50, 60 Hz. In each subject predictions of the responses recorded at stimulus repetition rates of 30, 40, 50, 60 Hz were synthesized by superimposing MLRs at suitable time intervals. The calculated mean amplitude/rate and phase/rate functions behaved similarly for the recorded and predicted curves, showing the highest amplitude at 40 Hz and a linear increase of phase values when increasing the stimulus rate. Nevertheless the synthetic curves closely predicted amplitude and phase values of the recorded responses only at 40 Hz. At frequencies below 40 Hz, the mean amplitude of the predicted curve was lower than that of the recorded one while at frequencies above 40 Hz the mean amplitude was higher. Predicted phase values were found lagging at 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 model based on the linear addition of transient MLRs is not able to adequately predict steady-state responses at stimulus rates other than at 40 Hz. Other mechanisms related to the recovery cycle of the activated system come into play in the steady-state response generation causing a decrease in amplitude and an increase in phase lag when increasing the stimulus repetition rate.

Generation of human auditory steady-state responses (SSRs). II: Addition of responses to individual stimuli

“…Nevertheless, with respect to latency, the only significant difference consisted in a delayed Pb component in LCRs. In our previous study, phenomena dealing with the recovery cycle have been shown to take place to determine amplitude and latency of the steady-state auditory responses (Azzena et al, 1995). Taking into account these data, one can hypothesize that the increased Pb latency may result from a rate effect taking place at 40 Hz.…”

“…The 625 point-MLR (and LCR) curve was divided into five consecutive 125 point-segments, each corresponding to 25 ms. Then five consecutive 625 point-curves were summed in such a way that each curve was shifted by 25 ms with respect to the previous one (see Fig. 1 in Azzena et al, 1995).…”

“…As discussed above, the mLCR greatly differs from MLR due to the presence of additional components which may be regarded as the result of an enhancing effect related to the resonant properties of the generating system. Moreover, our previous paper suggests that adaptation phenomena take place to determine the SSR parameters starting from stimulus rates lower than 40 Hz (Azzena et al, 1995). One would expect that these phenomena would lead to an amplitude decrease of the actual 40 Hz-SSR in comparison to the 40 Hz-SSR predicted on the basis of the transient MLRs.…”

“…As previously mentioned in the first part of our study (Azzena et al, 1995), two main hypotheses have been proposed. According to the first, the 40 Hz-SSR reflects a more general property of the sensory systems (Basar et al, 1979a;Basar et al, 1979b;Bressler and Freeman, 1980;Basar et al, 1987), behaving like neural resonators tuned to a frequency of 40 Hz (Galambos, 1982).…”

“…Nevertheless, as introduced in the first paper (Azzena et al, 1995), the effectiveness of predictive models based on the linear MLR addition does not rule out the possibility that phenomena related to the resonant frequency of the system may take place in the SSR generation. In other words, these two hypotheses may deal with two different features of the auditory system, and thus they do not mutually exclude each other.…”