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John S. George, Xin-cheng Yao; Imaging fast intrinsic optical signals for studies of retinal function. Journal of Vision 2006;6(6):1099. doi: 10.1167/6.6.1099.
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Multi-electrode arrays extend traditional single-unit electrophysiology techniques, allowing simultaneous study of dozens of units; however, they sample a limited number of neurons on the retinal surface. Here we report imaging fast intrinsic optical responses evoked by light flashes in isolated frog retina, using a CCD camera operated at high frame rates (>100fps). The method holds for promise of high-resolution dynamic functional imaging of complete populations of cells in 3D.
Optical recordings employed NIR illumination in transmitted, darkfield or cross-polarized configurations. Flashes or steps from a white LED stimulated a limited region of retina. Observed NIR response dynamics closely tracked the integral of electrophysiological activity. CCD image sequences disclose multiple response components with negative- and positive-going signals juxtaposed in a consistent spatial organization, initially limited to the region activated by the flash. Fractional responses exceeded 10% in some cases. Fast negative-going responses are correlated to a-wave of the retinal ERG and probably reflect the activation of photoreceptors. Positive-going responses are related to the b-wave, and appear to reflect the activation of ON (and OFF) bipolar cells and other postsynaptic neurons. We often observed a relatively enhanced bipolar response near the perimeter of the stimulated region as well as diffuse activation extending up to 100 microns into surrounding tissue. We observed differences in the polarity and timecourse of NIR responses as a function of depth. Spatial and temporal structure in the image sequences indicates that we captured patterns of activation in extended populations of individual neurons in single passes.
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