Hemispheric asymmetry of auditory steady-state responses to monaural and diotic stimulation

Abstract

Amplitude modulations in the speech envelope are crucial elements for speech perception. These modulations comprise the processing rate at which syllabic (~3-7 Hz), and phonemic transitions occur in speech. Theories about speech perception hypothesize that each hemisphere in the auditory cortex is specialized in analyzing modulations at different timescales, and that phonemic-rate modulations of the speech envelope lateralize to the left hemisphere, whereas right lateralization occurs for slow, syllabic-rate modulations. In the present study, neural processing of phonemic- and syllabic-rate modulations was investigated with auditory steady-state responses (ASSRs). ASSRs to speech-weighted noise stimuli, amplitude modulated at 4, 20, and 80 Hz, were recorded in 30 normal-hearing adults. The 80 Hz ASSR is primarily generated by the brainstem, whereas 20 and 4 Hz ASSRs are mainly cortically evoked and relate to speech perception. Stimuli were presented diotically (same signal to both ears) and monaurally (one signal to the left or right ear). For 80 Hz, diotic ASSRs were larger than monaural responses. This binaural advantage decreased with decreasing modulation frequency. For 20 Hz, diotic ASSRs were equal to monaural responses, while for 4 Hz, diotic responses were smaller than monaural responses. Comparison of left and right ear stimulation demonstrated that, with decreasing modulation rate, a gradual change from ipsilateral to right lateralization occurred. Together, these results (1) suggest that ASSR enhancement to binaural stimulation decreases in the ascending auditory system and (2) indicate that right lateralization is more prominent for low-frequency ASSRs. These findings may have important consequences for electrode placement in clinical settings, as well as for the understanding of low-frequency ASSR generation.

FIG. 1

The time (left panel) and frequency (right panel) domain representation of the 4 Hz amplitude modulated speech-weighted noise stimulus.

FIG. 2

The EEG frequency spectrum of one representative participant recorded over an electrode at the left mastoid (M1), in response to the 80 Hz (upper row), 20 Hz (middle row), and 4 Hz (bottom row) AM stimuli. The power spectral density of response (red) and noise (black) is plotted over a range of frequencies. Left panels ASSRs recorded in response to ipsilateral left ear stimulation. Middle panels ASSRs to diotic stimulation. Right panels ASSRs to contralateral right ear stimulation.

FIG. 3

Average response strengths for left ear (LE: left, dark grey bars), diotic (BE: middle, dotted bars), and right ear (RE: right, light grey bars) stimulation for 80 Hz AM, 20 Hz AM, and 4 Hz AM. Bars represent the average response SNRs for left (LH) and right hemisphere (RH) electrodes. Error bars indicate ±1 SE: **p ≤ 0.01; ***p ≤ 0.001.

FIG. 4

Mean laterality indices (LI) for 80, 20, and 4 Hz AM, for left ear (LE), diotic (BE), and right ear (RE) stimulation. Error bars represent the 95 % confidence interval of the mean: *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001.