Spatial specificity of the enhanced dip inherently induced by prolonged oxygen consumption in cat visual cortex: implication for columnar resolution functional MRI.

Since changes in oxygen consumption induced by active neurons are specific to cortical columns, the small and transient "dip" of deoxyhemoglobin signal, which indicates an increase in oxygen consumption, has been of great interest. In this study, we succeeded in enhancing and sustaining the dip in the deoxyhemoglobin-weighted 620-nm intrinsic optical imaging signals from a 10-s orientation-selective stimulation in cat visual cortex by reducing arterial blood pressure with sodium nitroprusside (a vasodilator) to mitigate the contribution of stimulus-induced blood supply. During this condition, intact spiking activity and a significant reduction of stimulus-induced blood volume changes (570-nm intrinsic signals) were confirmed. The deoxyhemoglobin signal from the prolonged dip was highly localized to iso-orientation domains only during the initial approximately 2 s; the signal specificity weakened over time although the domains were still resolvable after 2 s. The most plausible explanation for this time-dependent spatial specificity is that deoxyhemoglobin induced by oxygen consumption drains from active sites, where spiking activity occurs, to spatially non-specific downstream vessels over time. Our results suggest that the draining effect of pial and intracortical veins in dHb-based imaging techniques, such as blood oxygenation-level dependent (BOLD) functional MRI, is intrinsically unavoidable and reduces its spatial specificity of dHb signal regardless of whether the stimulus-induced blood supply is spatially specific.