Non-invasive measurement of neural inhibition in the human visual cortex

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The brain must constantly sift through a complex world to separate relevant information from background noise. The brain achieves this feat by suppressing irrelevant signals--sometimes consciously through use of attention, but often unconsciously through low-level processes in neural circuits. As we gain a better understanding of how the brain responds to aspects of our experience, we increasingly need to understand how suppression modulates the response of the brain. This is a major challenge. While it is now possible to measure activity in the brain using non-invasive methods such as functional magnetic resonance imaging (fMRI), we do not know how to measure suppression similarly. We have recently discovered a potential index of suppression. Using a model where we know suppression occurs, we have found that the fMRI response of the brain is slightly delayed when suppression is engaged. We therefore think that this delay could serve as a measure of suppression, and seek to study it in detail. Specifically, we need to know what causes the apparent delay--is it a correlate of two spatially separate signals that we cannot see because of the resolution of our imaging method, or is it due to the neurochemical makeup of the brain at a given time? Does the delay occur in other suppression models, and do the same causes contribute to it? Regardless of its causes, does the delay carry relevant information that can help us understand the underlying neural circuits? To answer these questions, this program of research will refine high-resolution fMRI methods and, combined with natural and artificial stimulation of the brain to modulate suppression, will be able to answer these questions. The results will be transformative in both our approach to measuring and interpreting brain signals and in understanding how suppression is engaged and modulated during natural processing.