A focusing image probe for assessing neural activity in vivo

We describe a compact, focusing image probe to record rapid optical changes from neural tissue. A gradient index (GRIN) lens served as a relay lens fr om tissue to a microscope objective which projected an image onto a CCD cam era. The microscope objective and camera assembly was adjusted independentl y from the GRIN lens, allowing focus changes without disturbing the probe/t issue interface; firm contact minimized movement and specular reflectance. Fiber optics around the probe perimeter provided diffuse illumination from a 780 nn laser, or 660 and 560 nm light emitting diodes. To characterize de pth-of-field, we imaged a black suture through increasing tissue thicknesse s. Light modulation by the suture remained detectable down to 900 mu m usin g 780 nm illumination. We acquired images from cardiorespiratory areas of t he rat dorsal medulla, at different depths and illumination wavelengths. Im ages illuminated at 560 nm were dominated by vasculature flow patterns, whi le 660 nm illumination revealed different spatial patterns which preceded v ascular flow by 40 ms and may represent cardiac-related neural activity. Us ing 780 nm light, image sequences triggered by the cardiac R-wave showed va scular perfusion changes with delayed and broader responses at deeper level s. Electrical stimulation within the vagal bundle caused fast optical chang es which track the electrical response, with a different spatial distributi on from hemodynamic signals. (C) 1999 Elsevier Science B.V. All rights rese rved.