Characteristics of cross-modal negative BOLD responses in the human sensory subcortex and cortex.

Functional magnetic resonance imaging (fMRI) is a noninvasive method for measuring human brain activity based on blood oxygenation level-dependent (BOLD) responses. Although many studies have reported positive BOLD responses evoked by sensory stimuli, others have reported negative BOLD responses (NBRs) in the sensory cortex when stimuli from different sensory modalities are presented (i.e., cross-modal NBRs). We conducted an fMRI experiment to better understand the characteristics of cross-modal NBRs in subcortical and cortical regions. Auditory and visual stimuli were presented unilaterally to one ear and to either the left or right visual field, respectively. The lateral geniculate nucleus and medial geniculate nucleus did not show a significant cross-modal NBR. In contrast, the primary auditory cortex showed a significant cross-modal NBR when visual stimuli were presented in either the contralateral or ipsilateral visual fields. Finally, we found that the cross-modal NBR in the early visual cortex was highly variable across subjects and did not exhibit consistent trends. However, each subject's data exhibited considerable split-half reliability. Our results suggest that cross-modal NBR in the auditory cortex likely reflects mechanisms such as interhemispheric suppression, rather than those coordinated within the same hemisphere.NEW & NOTEWORTHY This study demonstrated that the human primary auditory cortex showed a significant cross-modal negative BOLD response bilaterally, regardless of the visual field in which the visual stimuli were presented. This result suggests that the cross-modal negative BOLD response is not an epiphenomenon of visual cortex activation predominantly observed in the contralateral hemisphere, but is more likely to reflect interhemispheric suppression mechanisms.