T159. Hemispheric difference in temporal perception between 40- and 80-Hz auditory steady-state responses: MEG and ECoG studies

Abstract

Auditory steady-state responses (ASSRs) underlie the temporal integration of the auditory cortex (AC). However, the hemispheric asymmetry of the temporal integration has not yet been systematically studied because most of studies adopted binaural stimulation to record ASSRs. Although 40-Hz ASSRs are considered as a potential biomarker for various psychiatric diseases, physiological implications of 80-Hz ASSRs have not been well understood due to their weak EEG responses. Currently, 80-Hz oscillations are reported to be brain stem origin. Thus, we studied the functional difference between 40-Hz and 80-Hz ASSRs with monaural presentation measured by MEG and ECoG in order to clarify the hemispheric lateralization of temporal integration. MEGs were recorded from 14 healthy subjects (aged 20–46 years). The trains of clicks for 40 Hz or 80 Hz with 500 ms duration were presented 200 times monaurally with inter-train interval of 500 ms. ASSRs consisted of transient onset and offset responses intermixed with induced oscillations. The amplitudes and peak latencies of transient responses were analyzed after applying 5 to 30 Hz bandpass filter. In contrast, the induced oscillations were time-frequency analyzed in the frequency range of 40 Hz (35–45 Hz) and 80 Hz (75–85 Hz). Then, they were evaluated by the phase locking factor. In four patients with localization-related epilepsy who underwent ECoG, ASSRs were recorded to elucidate the neural sources of 80-Hz oscillations. Both the onset and offset responses were well delineated in the 80-Hz condition compared with those of 40 Hz. The onset responses were recorded in the bilateral ACs without asymmetry while the offset responses were significantly larger in the right AC by left ear stimulation. 40-Hz oscillations were induced mainly in the contralateral AC in the 40-Hz condition, while 80 Hz oscillations showed right hemispheric predominance in the 80-Hz condition irrespective of the stimulated ear. In ECoG recordings, the 80-Hz stimulation revealed clear 80 Hz oscillations in the bilateral ACs. The offset responses by 80-Hz stimulation were clearly obtained in the right AC. The 80-Hz oscillations showed the right hemispheric predominance while those of 40-Hz were recorded mainly in the contralateral AC. The discrimination among single sounds is an important part of the perceptual process. The auditory perception to 80-Hz (12.5 ms) stimulation is considered as “continuous sound”, while that to 40-Hz (25 ms) stimulation as “periodic sound”. Our findings suggest that the former is processed in the right AC whereas the latter in the contralateral AC. Our results are consistent with behavioral studies that separation time of 20–30 ms is required to distinguish sequences of auditory events and that the inflection point of amplitude modulation detection is about 50 Hz. Our findings also endorse use of 80-Hz ASSRs for the basic and clinical researches in temporal perception.