Characterising the Diffusion Functional Signature of Negative BOLD With Interleaved TMS-fMRI in the Human Brain

The coupling between brain excitatory activity and positive blood oxygen level-dependent (BOLD) responses is well-established. Although often associated with inhibition, negative BOLD remains partially understood. Moving away from neurovascular coupling, apparent diffusion coefficient (ADC)-fMRI provides a more direct measure of excitatory activity, possibly mediated by transient cellular deformations. Diffusion-weighted fMRI (dfMRI), from which ADC-fMRI derives, combines vascular and microstructural contributions. While decreases in ADC align with positive BOLD, the possible translation of negative BOLD into positive ADC and the ability of ADC-fMRI to capture inhibitory activity remain unexplored in humans. In this study, we used transcranial magnetic stimulation (TMS)-fMRI on the right primary motor cortex (M1) to selectively induce contralateral negative BOLD responses, whilst acquiring interleaved fMRI. TMS was applied at 5 Hz, 90% resting motor threshold, for both BOLD-fMRI and ADC-fMRI contrasts, in n = 12 and n = 11 healthy participants, respectively. We replicated previously reported negative BOLD clusters in the contralateral M1 and primary somatosensory cortex (S1). This was accompanied by a negative dfMRI response, but no ADC-fMRI response, indicating minimal microstructural fluctuations. In ipsilateral M1/S1, no BOLD response was detected while dfMRI revealed a positive cluster, suggesting different sensitivity to the excitatory/inhibitory balance. Overall, combining the findings from BOLD-fMRI and ADC-fMRI provides new insights into the vascular and neuronal responses underlying subthreshold TMS and negative BOLD.