Neurovascular coupling investigated with two-dimensional optical imaging spectroscopy in rat whisker barrel cortex

Optical imaging slit spectroscopy is a powerful method for estimating quantitative changes in cerebral haemodynamics, such as deoxyhaemoglobin, oxyhaemoglobin and blood volume (Hbr, HbO(2) and Hbt, respectively). Its disadvantage is that there is a large loss of spatial data as one image dimension is used to encode spectral wavelength information. Single wavelength optical imaging, on the other hand, produces high-resolution spatiotemporal maps of brain activity, but yields only indirect measures of Hbr, HbO(2) and Hbt. In this study we perform two-dimensional optical imaging spectroscopy (2D-OIS) in rat barrel cortex during contralateral whisker stimulation to obtain two-dimensional maps over time of Hbr, HbO(2) and Hbt. The 2D-OIS was performed by illuminating the cortex with four wavelengths of light (575, 559, 495 and 587 nm), which were presented sequentially at a high frame rate (32 Hz). The contralateral whisker pad was stimulated using two different durations: 1 and 16 s (5 Hz, 1.2 mA). Control experiments used a hypercapnic (5% CO2) challenge to manipulate baseline blood flow and volume in the absence of corresponding neural activation. The 2D-OIS method allowed separation of artery, vein and parenchyma regions. The magnitude of the haemodynamic response elicited varied considerably between different vascular compartments; the largest responses in Hbt were in the arteries and the smallest in the veins. Phase lags in the HbO(2) response between arteries and veins suggest that a process of upstream signalling maybe responsible for dilating the arteries. There was also a consistent increase in Hbr from arterial regions after whisker stimulation.