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John George, Xin-cheng Yao; Lateral interactions in outer retina disclosed by high resolution dynamic optical imaging of neural activation. Journal of Vision 2007;7(9):322. doi: https://doi.org/10.1167/7.9.322.
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We have recently demonstrated the feasibility of dynamic near-infrared imaging of fast intrinsic optical changes directly associated with the electrophysiological response in isolated frog retina activated by visible light. High Resolution CCD image sequences acquired with transmitted light (bright field) illumination disclosed large fractional responses and showed evidence of multiple response components with both negative and positive-going signals with different timecourses. Dark field imaging further enhanced the contrast and sensitivity of optical measures of neural activation. High resolution imaging disclosed optical responses in single pixels often exceeding 5%, of background light, allowing dynamic imaging at the resolution of single cells, in single passes. Based on cell location and response dynamics (including correspondence with identified components of the ERG), we identified responses consistent with photoreceptors, horizontal and bipolar cells. Some cells were classified according to functional criteria, including ON, OFF and ON/OFF responses. Optical responses showed complex but consistent spatiotemporal dynamics from frame to frame and trial to trial. Following photoreceptor activation we observed a rapid diffuse response of opposite polarity in the surrounding retina, distant regions of punctate activity and development of an enhanced response associated with the perimeter of the stimulated region. Our experimental results and theoretical analysis suggest that the optical responses may result from dynamic volume changes corresponding to ion and water flow across the cell membrane. Transient intrinsic optical responses associated with neural activation offer an attractive strategy for studying the computation performed by extended neural networks such as the retina. Our studies and theoretical analysis of optical responses in other neural systems suggest that such fast light optical responses can be detected with high sensitivity in reflected light and therefore might enable non-invasive methodology for diagnostic imaging of retinal function.
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