Effects of aging on event-related potentials to single-cycle binaural beats and diotic amplitude modulation of a tone.

Aim of the study is to determine whether the auditory processing of temporal fine structure (TFS) is affected with normal aging, even in the presence of normal audiometric hearing and fine cognitive state; and, if it is, to see whether a comparable effect is also observed in the processing of a diotic change in sound envelope. The event-related potentials (ERPs) to binaural beats (BBs), which are the responses of the binaural mechanisms processing TFS of a sound, and the ERPs to diotic amplitude modulation (AM) stimuli, which are the responses of the monaural mechanisms processing the changes in its envelope, were recorded from thirteen young university students and ten senior but active university professors, all with normal hearing in low frequencies. To obtain directly the specific BB responses without confounding monaural frequency change-evoked responses, we used single-cycle BB stimuli with temporary sub-threshold frequency shifts. BBs of a 250-Hz tone and diotic AM of the same tone with similar perceptual salience were presented with 2-second stimulus onset asynchrony. The N1 components of the ERPs to both stimuli displayed notable age-dependent changes in their scalp topography and significant amplitude reduction and latency prolongation in the elderly. These amplitude and latency changes were at similar rates for the two stimulus types, implying that the auditory TFS and envelope processing mechanisms are proportionally affected by physiological aging. These results may serve as control data in future studies investigating the effect of aging-associated cognitive pathologies on auditory TFS processing.

Event-related potentials to single-cycle binaural beats of a pure tone, a click train, and a noise.

There are only few electrophysiological studies on a phenomenon called "binaural beats" (BBs), which is experienced when two tones with frequencies close to each other are dichotically presented to the ears. And, there is no study in which the electrical responses of the brain to BBs of complex sounds are recorded and analyzed. Owing to a recent method based on single-cycle BB stimulation with sub-threshold temporary monaural frequency shifts, we could record the event-related potentials (ERPs) to BBs of a 250-Hz tone as well as those to the BBs of a 250/s click train and to the BBs of a recurrent 4-ms Gaussian noise. Although fundamental components of the click train and noise stimuli were lower in intensity than the tonal stimuli in our experiments, the N1 responses to the BBs of the former two wide-spectrum sounds were recorded with significantly larger amplitudes and shorter latencies than those to the BBs of a tone, suggesting an across-frequency integration of directional information. During a BB cycle of a complex sound, the interaural time differences (ITDs) of the spectral components are all equal to each other at any time; whereas their interaural phase differences (IPDs) are all different. The ITD rather than the IPD should, therefore, be the cue that is relied upon by the binaural mechanism coding the perceived lateral shifts of the sound caused by BBs. This is in line with across-frequency models of human auditory lateralization based on a common ITD, fulfilling a straightness criterion.

Event-related potentials to single-cycle binaural beats and diotic amplitude modulation of a tone.

When two tones with slightly different frequencies are dichotically presented, binaural beats (BBs) are experienced. BBs resulting from the cycling change in interaural phase difference elicit electroencephalographic responses. Because they repeat at short periods, allowing poor recovery of the cortical responses, these steady-state responses have small amplitudes, and their various wave components intermingle and might mask each other. Using single-cycle BBs separated by relatively long inter-onset intervals would be a solution, but introducing a transient interaural frequency shift requires response subtraction which may not be acceptable for non-additive brain responses. The proposed stimulation method employs transient and monaurally subthreshold frequency shifts in opposite directions in the two ears to produce single-cycle BBs of a 250 Hz tone. These shifts are perceived as distinct BBs when presented dichotically, but remain subthreshold when presented monotically. Therefore, no frequency-shift response is elicited, and the specific BB response is obtained with no need for waveform subtraction. We recorded from 19 normal hearing participants the event-related potentials (ERPs) to single-cycle BBs and also to temporary diotic amplitude modulation (AM) with matched perceptual salience. The ERPs to single-cycle BBs presented at 2 s inter-onset intervals had N1-P2 responses with up to seven times larger amplitudes than the conventional steady-state BB responses in the literature. Significant differences were found between the scalp potential distributions of the N1 responses to BB and AM stimuli, suggesting that the cortical sites, where envelope-based level processing and temporal fine structure-based spatial processing of the stimulus take place, are not totally overlapped.