Stimulus dependence of contralateral dominance in human auditory cortex.

The auditory system is often considered to show little contralateral dominance but physiological reports on the contralateral dominance of activity evoked by monaural sound vary widely. Here, we show that part of this variation is stimulus-dependent: blood oxygen level dependent (BOLD) responses to 32 s of monaurally presented unmodulated noise (UN) showed activation in contralateral auditory cortex (AC) and deactivation in ipsilateral AC compared to nonstimulus baseline. Slow amplitude-modulated (AM) noise evoked strong contralateral activation and minimal ipsilateral activation. The contrast of AM-versus-UN was used to separate fMRI activity related to the slow amplitude modulation per se. This difference activation was bilateral although still stronger in contralateral AC. In magnetoencephalography (MEG), the response was dominated by the steady-state activity phase locked to the amplitude modulation. This MEG activity showed no consistent contralateral dominance across listeners. Subcortical BOLD activation was strongly contralateral subsequent to the superior olivary complex (SOC) and showed no significant difference between modulated and UN. An acallosal participant showed similar fMRI activation as the group, ruling transcallosal transmission an unlikely source of ipsilateral enhancement or ipsilateral deactivation. These results suggest that ascending activity subsequent to the SOC is strongly dominant contralateral to the stimulus ear. In contrast, the part of BOLD and MEG activity related to slow amplitude modulation is more bilateral and only observed in AC. Ipsilateral deactivation can potentially bias measures of contralateral BOLD dominance and should be considered in future studies.