Neural correlates of single-vessel haemodynamic responses in vivo

Functional imaging techniques use changes in blood flow to infer neural activity, but how strongly the two are correlated is a subject of debate; here, vascular and neural responses to a range of visual stimuli are imaged in cat and rat primary visual cortex, revealing that vascular signals are partially decoupled from local neural signals. Haemodynamic signals and neural function Functional imaging techniques exploit changes in blood flow to infer neural activity, but how strongly the two are correlated is a subject of debate. Prakash Kara and colleagues imaged vascular and neural responses to a range of visual stimuli in cat and rat primary visual cortex. Responses of parenchymal vessels in cat visual cortex displayed similar orientation preference as both spiking activity and synaptic activity in the surrounding tissue, but showed less orientation selectivity. Accordingly, parenchymal vessels in cat visual cortex could respond to sensory stimuli that evoked little or no neural activity in the surrounding tissue. Surface arteries in cat and rat visual cortex or parenchymal arteries in rat visual cortex (which does not contain orientation maps) did not show any orientation selectivity. This study indicates that vascular signals are partially decoupled from local neural signals. Neural activation increases blood flow locally. This vascular signal is used by functional imaging techniques to infer the location and strength of neural activity 1 , 2 . However, the precise spatial scale over which neural and vascular signals are correlated is unknown. Furthermore, the relative role of synaptic and spiking activity in driving haemodynamic signals is controversial 3 , 4 , 5 , 6 , 7 , 8 , 9 . Previous studies recorded local field potentials as a measure of synaptic activity together with spiking activity and low-resolution haemodynamic imaging. Here we used two-photon microscopy to measure sensory-evoked responses of individual blood vessels (dilation, blood velocity) while imaging synaptic and spiking activity in the surrounding tissue using fluorescent glutamate and calcium sensors. In cat primary visual cortex, where neurons are clustered by their preference for stimulus orientation, we discovered new maps for excitatory synaptic activity, which were organized similarly to those for spiking activity but were less selective for stimulus orientation and direction. We generated tuning curves for individual vessel responses for the first time and found that parench