Electrophysiological measurement of temporal integration in listeners with normal hearing
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Temporal integration refers to the phenomenon whereby the detection threshold of the stimulus decreases (improves) as the signal duration increases. The majority of studies of temporal integration have relied on behavioral methods. As a result temporal integration can be influenced by the subject’s physical and psychological status and these factors may affect signal detection. In the present study, the measurement of the auditory steady-state response (ASSR) was used to investigate temporal integration in listeners with normal hearing. The stimuli were sinusoidally amplitude modulated (SAM) signals varying in the modulation frequency (40 Hz and 80 Hz) and duration (50 ms, 100 ms, 200 ms, 300 ms, 400 ms, and 800 ms). The carrier was 1-kHz tone, 4-kHz tone, or white noise. The ASSRs were analyzed across different stimulus conditions in terms of amplitude, phase, signal-to-noise ratio (SNR), and percentage of detected responses. The results showed that temporal integration was more clearly revealed when the ASSR was recorded with the 40-Hz modulation frequency as compared to 80-Hz modulation frequency. For 40-Hz modulation frequency, the amplitude of the ASSR increased over the first 200 milliseconds after the stimulus onset until reaching a steady-state plateau, and then dropped rapidly after the stimulus offset. Conversely, the phase strength (i.e., variability) decreased over the first 400 milliseconds and remained relatively stable after that. For 80-Hz modulation frequency, the ASSR amplitude did not increase until approximately 200 milliseconds, beyond which the ASSR amplitude increased at the same rate as for 40-Hz modulation frequency. In addition, the ASSR phase was less stable across subjects, which suggests weaker responses overall. An exponential model fit the electrophysiological data best; however, a significant frequency effect on the time constant was not observed. These results suggest that both auditory midbrain and brainstem are able to integrate auditory information over the first 200 milliseconds of stimulus.