Imaging of hippocampal neural activity in freely behaving animals

We examined intrinsic activity of neurons in the cat dorsal hippocampus following stimulation of the posterior medial hypothalamus and following a 2.5 mg/kg intravenous administration of cocaine. Posterior hypothalamic stimulation elicited banding of optical activity in the dorsal hippocampus, suggestive of spatial organization of neural activity underlying synchronous rhythmical slow wave activity. Cocaine administration resulted in a major decline in reflected light at 700 nm, possibly representing diminished oxygenation to the underlying tissue. The activity was examined by a new technique which uses principles of reflectance at specific bandwidths of light during cellular activity, and allows acquisition of images in the freely behaving animal. The imaging device consists of a 1.6 mm diameter coherent optic fiber bundle of 25 microns fibers attached to a 128 x 128 pixel charge-coupled device (CCD) array which can be cooled to reduce thermal noise. The target is illuminated by a light emitting diode array producing light at a number of operator-selectable narrow bandwidths which permits assessment of physiological characteristics of neural tissue. The CCD output is digitized, stored by computer, and the resulting images are averaged to obtain a greater signal-to-noise ratio. The procedure shows considerable promise for providing on-line, real-time indications of activity in localized brain regions.