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W. H. Swanson, H. Sun, B. B. Lee, D. Cao; Responses of Retinal Ganglion Cells to Perimetric Stimuli. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3862.
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© ARVO (1962-2015); The Authors (2016-present)
Perimetry is used clinically to assess damage to retinal ganglion cells in patients with glaucoma by measuring psychophysical thresholds throughout the central visual field. Psychophysical modeling suggested that ganglion cell responses begin to saturate when the stimulus luminance exceeds 25 dB (Sun et al., 2006, Optom Vis Sci 83:455-465). However, relatively little is known about primate ganglion cells’ contrast responses for perimetric stimuli. To assess glaucomatous deficits, it is essential to know the pattern of responses in the healthy retina. Here we investigated the responses of macaque retinal ganglion cells to perimetric stimuli.
Stimuli were circular luminance increment pulses 200 msec in duration, 26 minutes of arc (minarc) in diameter, on a uniform 10 cd/m2 background. Stimulus location varied in steps of 15 minarc across the cell’s receptive field. Data were recorded from macaque retinal ganglion cells, via an in vivo preparation, at eccentricities of 5º-15º. For data analysis, a linear receptive field combined with a Michaelis-Menten function was fit to each cell’s contrast responses. There were four variable parameters: half-saturation contrast, baseline firing rate, maximum response, receptive field width. Contrasts were expressed in perimetric dB units: a stimulus with 100% Weber contrast is referred to as "25 dB" , and 1 dB is equal to 0.1 log unit. Our stimuli ranged from 38 dB (5% contrast) to 18 dB (500% contrast).
At low contrasts the firing rates of both M and P retinal ganglion cells increased linearly with stimulus contrast. M cells were more responsive than P cells to perimetric stimuli, and tended to saturate earlier, with average ± SD for half-saturation contrast of 24 dB (126%) ± 1 dB for eight M-on cells, 23 dB (158%) ± 3 dB for eight M-off cells, and 18 dB (501%) ± 2 dB for eight P cells. For most contrasts both M and P cells were only responsive when the stimulus fell within 15 minarc of the cell’s receptive field center; but at high contrasts (18-25 dB, 500%-100%), for all of the M cells and one of the P cells substantial responses were obtained for stimuli offset by 30 minarc. This may be due to scattered light.
For perimetric stimuli, the M cells were more responsive and saturated at lower contrasts than the P cells. The values obtained for the half-saturation contrast confirm the prediction by Sun et al. (2006) that saturation occurs for perimetric stimuli of 0 to 25 dB. Further investigation in the glaucomatous retina is warranted.
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