Generation of auditory steady-state responses: linearity assessment.

The linear summation of the responses elicited by individual stimuli can be considered the main mechanism underlying SSR generation. Moreover other phenomena become involved so that individual responses elicited at fast repetition rates (LCRs) are different from those recorded at stimulus rates preventing their overlapping (MLRs). First, phenomena related to the recovery cycle can determine parameters of individual responses within the SSR causing a reduction in amplitude and an increase in latency with increasing repetition rates. Second, other mechanisms related to the resonant frequency of the auditory system come into play enhancing the contribution of individual responses during 40 Hz steady state stimulation. Assuming linearity and admitting that the system behaves as a damped harmonic oscillator having resonant frequency of 40 Hz and damping ratio between 0 and 1, one can speculate that, for a given damping value, the output modulus depends on the frequency components of the input as well as on the system resonant frequency. The increase in the input of the frequency components close to the system resonant frequency causes the output modulus to increase and this make more likely the appearance of further oscillations in the system output. In conclusion it seems that the surface recorded SSR results from the linear summation of the responses to individual stimuli and these responses are different from MLRs since they can be enhanced or depressed by resonant and adaptation phenomena. The interplay between these effects is strictly dependent on the stimulus repetition rate.