Scattered-light imaging in vivo tracks fast and slow processes of neurophysiological activation

We imaged fast optical changes associated with evoked neural activation in the dorsal brainstem of anesthetized rats, using a novel imaging device. Th e imager consisted of a gradient-index (GRIN) lens, a microscope objective, and a miniature charged-coupled device (CCD) video camera. We placed the p robe in contact with tissue above cardiorespiratory areas of the nucleus of the solitary tract and illuminated the tissue with 780-nm light through fl exible fibers around the probe perimeter. The focus depth was adjusted by m oving the camera and microscope objective relative to the fixed GRIN lens. Back-scattered light images were relayed through the GRIN lens to the CCD c amera. Video frames were digitized at 100 frames per second, along with tra cheal pressure, arterial blood pressure, and electrocardiogram signals reco rded at 1 kHz per channel. A macroelectrode placed under the GRIN lens reco rded field potentials from the imaged area. Aortic, vagal, and superior lar yngeal nerves were dissected free of surrounding tissue within the neck. Se parate shocks to each dissected nerve elicited evoked electrical responses and caused localized optical activity patterns. The optical response was mo deled by four distinct temporal components corresponding to putative physic al mechanisms underlying scattered light changes. Region-of-interest analys is revealed image areas which were dominated by one or more of the differen t time-course components, some of which were also optimally recorded at dif ferent tissue depths. Two slow optical components appear to correspond to h emodynamic responses to metabolic demand associated with activation. Two fa st optical components paralleled electrical evoked responses. (C) 2001 Acad emic Press.