How does one usually go about analyzing the auditory steady state response (ASSR)? I have data where speech was amplitude modulated to a 40 Hz sine tone. There's 3.5 minutes of data; should I just take the FFT or do some averaging first? What is the usual process? The data has been already preprocessed.
ASSR analysis is based on the fact that the electrophysiological responses are time locked with the stimulus repetition rate.
There is not one possible method of analysis. Generally spoken, ASSR analysis occurs in the spectral domain, and specifically the frequency of the stimulus repetition rate is analyzed, as well as its harmonics. For example, if the stimulus repetition rate is 100 Hz, the ASSR will occur at 100 Hz, 200 Hz, 300 Hz etc (Fig. 1). The base frequency (in this case, 100 Hz) will have the largest amplitude, and the harmonics decrease in amplitude. Detecting the presence of ASSR in the spectral domain means relying on amplitude and/or phase values (sometimes combined into a vector).
Fig. 1. ASSR base frequency and harmonics. Stimulus rate was 90 Hz. source: The Hearing Review