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Tyler Godat, Juliette E McGregor, Keith Parkins, William H Merigan, David R Williams; In vivo classification of macaque foveal ganglion cells through optical recording of responses to chromatic and luminance flicker. Invest. Ophthalmol. Vis. Sci. 2019;60(9):3883.
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© ARVO (1962-2015); The Authors (2016-present)
It has been challenging to characterize the physiology of retinal ganglion cells that serve the primate fovea, because the fovea’s complex topography limits accessibility with conventional microelectrode techniques. We functionally classify cells that express a genetically encoded calcium indicator (GCaMP6) in the ganglion cell layer (GCL) by optically recording responses to chromatic and luminance flicker.
GCaMP6 was expressed in the GCL of one eye in each of two female macaques. Adaptive optics scanning light ophthalmoscopy (AOSLO) was used to image GCaMP6 fluorescence (488 nm excitation, 520/35 nm emission) from a ring of displaced GCL cells driven by cones in the central 35 arcminutes of the fovea. We recorded responses to sinusoidal flicker (1.5 degree diameter, 0.2 Hz, LED 420 nm, 470 nm, 660 nm) modulated in one of three orthogonal directions in color space: 100% contrast luminance modulation, a modulation calculated to maximize the counterphase excitation of M and L cones while keeping S cone modulation constant, and a modulation to drive S cones with M and L cone excitation constant. We calculated the amplitude and phase from the fluorescence responses of 99 cells in one animal and 98 cells in the other. The criterion for deciding whether a response was different from zero was an SNR of 2.5.
Response to luminance flicker demonstrated ON and OFF dominant types of cells, while M-L cone flicker revealed two populations of M and L dominant types, and S cone modulating flicker revealed a subpopulation of S dominant types. 79% of cells in one animal and 67% in the other were responsive to the M-L cone flicker, and 4% and 3% were responsive to the S cone flicker. 8% and 13% of cells were responsive to luminance flicker, but unresponsive to either chromatic stimulus. 12% and 19% of cells were unresponsive to all three stimuli.
AOSLO calcium imaging can reveal the signed trichromatic cone inputs of large numbers of ganglion cells in the central fovea of the living primate. While we examined responses to only chromatic and luminance flicker, exploration of a larger parameter space could provide a more complete description of ganglion cells in the fovea and the extent to which, at the retinal output, the fovea is specialized relative to the better understood extrafoveal retina.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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