Fast optical recording of light-flash evoked neural activation in amphibian retina

Abstract

Imaging of fast intrinsic optical responses closely associated with neural activation promises important technical advantages over traditional single and multi-channel electrophysiological techniques for dynamic measurements of visual processing and early detection of eye diseases. We have developed a fast, no-moving-parts optical coherence tomography (OCT), system based on an electro-optic phase modulator, and used it to record dynamic near infrared (NIR) light scattering changes in frog retina activated by a visible light-flash. We also employed transmitted light for highly sensitive measurement and imaging of neural activation, and to optimize illumination and optical configuration. Using a photodiode detector, we routinely measured dynamic NIR transmitted optical responses in single passes. When the whole retina was illuminated by a visible light-flash, a positive peak was typically observed in transmitted light measurements. CCD image sequences disclosed larger fractional responses, in some cases exceeding 0.5% in individual pixels, and showed evidence of multiple response components with both negative- and positive-going signals with different timescales and complex but consistent spatial organization. The fast negative-going signals are highly correlated with the a-wave of the electrophysiological signals, and may reflect the activation of photoreceptors. The fast positive-going responses are related to the b-wave of the electrophysiological signals, and may result from the activation of ON bipolar cells. Slow optical responses may signal metabolic changes of retinal tissue. Our experimental results and theoretical analysis suggest that the optical responses may result from dynamic volume changes associated with neural activation, corresponding to ion and water flow across the cell membrane.