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D. Y. Tso, J. Schallek, Y. Kwon, R. Kardon, M. Abramoff, P. Soliz, J. Pokorny; Blood Flow Dynamics Contribute to Functional Intrinsic Optical Signals in the Cat Retina in vivo. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1951.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the anatomical and biophysical origins of functional intrinsic optical signals in the mammalian retina, with special attention to a possible hemodynamic origin.
Adult cats were anesthetized, paralyzed and mounted in a stereotaxic. Using a modified fundus camera, the retina was stimulated with visible (550nm) patterned stimuli and illuminated in the near infrared (700-900nm), while intrinsic optical signals were recorded with a CCD camera. Stimuli with different spatial frequencies were presented to examine possible spatial frequency tuning of the intrinsic signals. To determine the blood volume related changes in reflectance, blood contrast agents were injected systemically. Nigrosin (Acid black 2) was injected in a single bolus dose of 34mg/kg into the cephalic vein. In other sessions, indocyanine Green (ICG) was injected with an initial bolus of 1.5mg/kg followed by a constant infusion of 4.4-8.2mg/kg/hr. Signals at various wavelengths before and after Nigrosin and ICG injections were compared.
Imaged signals were not tuned to the set of spatial frequencies presented. Signals were, however, dependent on stimulus energy rather than spatial contrast. The lack of spatial frequency tuning of the signal amplitude suggests minimal ganglion cell contribution to the dominant signals observed.When we injected the blood contrast agent Nigrosin, we observed remarkable signal magnitude increases (nearly a 2-fold increase at the peak absorption wavelengths) while at other wavelengths signal amplitude remained unchanged. Corroborating this finding in an alternate experiment, systemic injections of ICG also dramatically increased the intensity of the negative signal. The time course of the signal after Nigrosin or ICG injections showed the same signature as the normal imaging conditions.
We did not observe signal amplitude dependency on the spatial frequency of the stimulus. This finding is consistent with our earlier work suggesting a minimal ganglion contribution to the signals observed. Additionally, when either blood contrast agent was injected, the signal amplitude was enhanced at wavelengths mirroring the absorption spectra of each contrast agent. Furthermore, the time course of the signals were unaltered by these agents, indicating that blood volume dynamics contribute a substantial component to the functional optical intrinsic signals.
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