FMRI correlates of the auditory 40-Hz steady-state response

Abstract

The auditory 40-Hz steady-state response (SSR) is a stimulus-synchronized component of the auditory evoked fields and potentials. Source analysis studies suggest that the SSR shares its main generators in the primary auditory cortex with the middle latency response, and subcortical generators with the brainstem response. In this study, we adopted a paradigm previously used in positron emission tomography (PET) (Reyes et al, 2004, Hear. Res. 194(1–2):73–80) to evaluate functional magnetic resonance imaging (fMRI) correlates of the SSR, and compare these results with the SSR recorded in Magnetoencephalography (MEG). 12 listeners participated in one fMRI and one MEG session. Listeners were presented with alternating 1-kHz sine and 40-Hz amplitude-modulated (AM) tones (modulation depth=100%). Both tones had a duration of 32 seconds and were presented binaurally at 75 dB SPL. We expected that the contrast of AM minus sine tones would serve as a reasonable estimate of SSR-related activity in fMRI, because both, AM and sine tones, evoke comparable long-latency responses in MEG, while the SSR is only evoked by the AM tones. FMRI results showed activity in multiple fields of the auditory cortex, for both, the sine tone and AM stimuli compared to silence. Stronger activity for the AM tones compared to the sine-tones was consistently found in medial Heschl's Gyrus, located medial to the activity observed for the tone versus silence contrasts. Moreover, enhanced activity for AM compared to sine tones was found in the inferior colliculus and the medial geniculate body. The time course of the difference response (AM minus sine tone) was sustained, and it was distinct from the transient BOLD responses observed at tone onset. These results suggest that a correlate of the SSR can be reliably studied by fMRI. The relative prominence of the related activity in fMRI compared to its small amplitude in MEG (compared e.g. to the sustained fields) adds to previous findings, which suggest that stimulus synchrony rather than response amplitude in MEG and EEG determine the relative contribution of an electrophysiological component to the BOLD response. Research supported by BMBF grant 01EV0712 (Junior Research Group Bioimaging)