Scaling the hill of vision: The physiological relationship between light sensitivity and ganglion cell numbers

PURPOSE. Differential light sensitivity (DLS) in white-on-white perimetry i s used as a measure of ganglion cell function to estimate the amount of neu ronal damage in glaucoma. The physiological relationship between DLS and ga nglion cell numbers is poorly understood. Within small retinal areas, brigh tness information is summated, so that A *: L = C, or A = C/L;, where A is target area, L is threshold luminance, and C is a constant. In larger illum inated areas, as with a Goldmann size LII target in perimetry, summation is incomplete, so that A(k) = Cn, where it is the coefficient of summation, a nd 0 < k < 1. This study tests the hypothesis that the target area (A) can be represented by the number of underlying ganglion cells (G) to give G(k) = C/L;. METHODS. Normative human data for ganglion cell density within 30 degrees o f retinal eccentricity were taken from the Literature and corrected for lat eral displacement of ganglion cells from the fovea to estimate ganglion cel l receptive field density (g). The number of ganglion cell receptive fields within a Goldmann size III target (G) was calculated from target area (A) and receptive field density (g) [G = A (g)]. Normative data for DLS in the central 30 degrees (Humphrey 30-2) were taken from the literature. The coef ficient summation (k) was measured empirically at each Humphrey 30-2 test p oint in 8 normal subjects. The relationship between DLS and G was investiga ted by plotting DLS as decibels (dB) against G and DLS as 1/L; (1/Lamberts) against G(k). The physiological relationship was extrapolated to glaucomat ous ganglion cell loss by calculating hypothetical cell losses for 3 and G dB sensitivity defects at each test point. RESULTS. Spatial summation increased with eccentricity. The relationship be tween DLS (dB) and G was curvilinear. The relationship between DLS (1/L) an d G(k) was linear (r(2) = 0.73). The extrapolation to glaucomatous ganglion cell loss indicated that a proportionally greater loss of ganglion cells i s required in the central compared with peripheral. visual held for equal l osses in dB sensitivity. CONCLUSIONS. The number of underlying ganglion cells, adjusted for local sp atial summation, is better reflected by the DLS scale of 1/L than by dB. If spatial summation is unchanged in glaucoma, this scale more accurately ref lects the amount of neuronal damage.