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N.K. Dhingra, R.G. Smith; Contrast Discrimination Performance of a Mammalian Ganglion Cell . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3249.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To measure a ganglion cell's contrast discrimination performance (the ability to discriminate a contrast increment) to allow a comparison to the performance of other retinal neurons, cortical cells and psychophysics. Methods: A brisk-transient ganglion cell (visual streak, intact guinea pig retina, in vitro) was recorded with a loose patch extracellular or sharp intracellular electrode and stimulated with a spot that optimally matched cell's receptive field center. The spot was flashed for 100 ms at 2 or 4 Hz and at least 200 responses were recorded at each of several contrasts. To discriminate between the responses to a pair of contrasts (the lower contrast called "basal", and a higher contrast), we employed an ideal observer (using a single-interval forced-choice procedure) that chose which contrast had most likely been presented. The pair of contrasts that gave 68% correct choices was considered just discriminable. Results: We measured the contrast discrimination performance of 15 brisk-transient ganglion cells, based on their spike or membrane potential responses. For both spikes and membrane potential, the discrimination threshold at zero basal contrast (also called detection threshold) was 1-3%. However, the discrimination threshold at a slightly higher basal contrast dipped ~40% below the detection threshold. The basal contrast at which the discrimination threshold was lowest coincided with the detection threshold. The discrimination threshold was relatively invariant up to ~20% contrast, but for higher basal contrasts it rose sharply, so that at 40-50% basal contrast it was 10 to 20-fold higher than the detection threshold. This effect coincided with saturation in the ganglion cell response at contrasts above ~20%. Conclusions: 1) Our results suggest that the classical "dipper" observed in the discrimination functions of both cortical cells and human observers originates in the retina, presynaptic to the ganglion cells. The dipper effect in ganglion cells is correlated with their near-zero response at low contrasts. 2) The sharply rising discrimination threshold of ganglion cells at high contrasts suggests that a source of noise that limits ganglion cell sensitivity is located more proximal than the saturation mechanism. 3) A similar increase in discrimination threshold found at high contrast in cortical cells and human observers may originate in the response saturation of ganglion cells.
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