Neuronal Correlates of Negative BOLD Responses in Monkey Visual Cortex

The majority of functional brain imaging studies in humans rely on the measurement of cerebral blood flow or Blood Oxygenation Level Dependent (BOLD) signals. Recent studies have characterized the relationship between localized increases in neuronal activity and corresponding increases in cerebral blood flow and BOLD , making it possible to interpret positive functional imaging responses in terms of neuronal activity. In addition to the commonly reported increases in these signals, negative cerebral blood flow and BOLD responses are pervasive in functional brain imaging studies. In contrast to the positive cerebral blood flow and BOLD responses, however, the origin of the corresponding negative responses and their relationship to neural activity remains poorly understood. An instance of sustained negative BOLD response will be demonstrated, beyond the stimulated regions of the monkey visual cerebral cortex. Through simultaneous electrophysiological recording of neuronal activity, it will be shown that this negative BOLD response is associated with decreases in neuronal activity below spontaneous baseline activity, with comparable decreases in local field potential and spiking activity. Moreover, a tight coupling will be demonstrated between the negative BOLD response and the corresponding decreases in neuronal activity. Differences between the positive and the negative BOLD responses with respect to coupling to local field potential and spiking activity will be discussed. The findings indicate that a significant component of this sustained negative BOLD response can be attributed to concomitant decreases in neuronal activity. Part of the talk will be devoted to a different, transient negative BOLD response within the stimulated regions, namely the post-stimulation undershoot, and to analysis of changes in neuronal activity associated with it