Auditory responses to the envelopes of pseudorandom noise stimuli in humans

Averaged scalp potentials evoked by continuous pseudorandom noise can be cross-correlated with the evoking stimulus, yielding a cross-correlation function (CCF) which reflects neural phase-locking and is quite sensitive for low-frequency stimulus components [M.J. Wilson and R.A. Dobie (1987) Electroencephalogr. Clin. Neurophysiol. 66, 529-538]. However, for higher frequency signals, replicable CCFs can only be obtained at moderate to high intensities. Since auditory neurons also respond to envelopes of complex sounds, even for high-frequency carriers, we compared scalp responses evoked by band-limited complex sounds to the envelopes of these sounds; the resultant envelope cross-correlation functions (ECCFs) contained replicable response components primarily below 1,000 Hz, regardless of the evoking stimulus spectrum. ECCF thresholds for three octave-band stimuli (830-1,562, 1,611-3,125, and 3,174-6,201 Hz) were more sensitive than CCF thresholds (P = 0.006), averaging 35 dB spectrum level for 10 normal subjects. When stimuli with only odd harmonics were used, replicable odd-component scalp responses were seen only in the spectral range of the stimuli, while even-component responses (presumably to stimulus envelope) were seen only in low-passed scalp responses.