Temporal and spectral information processing in the auditory cortex: a steady-state auditory-evoked potential study

We examined the temporal and spectral information processing in the auditory cortex by using steady-state auditory-evoked potentials (S-AEPs). Tone-burst stimuli at carrier frequencies of 500 and 2000 Hz were presented monaurally to elicit S-AEPs in 10 normal subjects. Modulation frequency was varied from 15 to 90 Hz at every 5 Hz. Nine recording electrodes were placed coronally, including Cz, referring to an electrode at the seventh cervical spinous process (SC7). A total of 100 responses of 1-s epoch was averaged and subjected to discrete fast Fourier transforms (FFTs), to yield the amplitude of the first harmonic (1F) component and interhemispheric coherence (Coh) value between homologous electrodes. Three amplitude peaks at around 15-20, 40-45 and 80 Hz were observed against the modulation frequency at both carrier frequencies. The interhemispheric Coh value between T3 and T4 of the right-ear stimulation was greater at around 35-45 Hz than those of the left-ear stimulation, and vice versa, from 50 to 90 Hz at a carrier frequency of 500 Hz. However, this characteristic was not observed at the carrier frequency of 2000 Hz. Three tuning peaks indicate the presence of neuronal subgroups for the temporal processing, as observed in the visual system. In addition, left-hemisphere predominance for rapid temporal processing depends on the carrier frequency, suggesting differential processing of temporal and spectral information in the auditory cortices.