Scaling the hill of vision: the physiological relationship between light sensitivity and ganglion cell numbers.
OBJECTIVE
Differential light sensitivity (DLS) in white-on-white perimetry is used as a measure of ganglion cell function to estimate the amount of neuronal damage in glaucoma. The physiological relationship between DLS and ganglion cell numbers is poorly understood. Within small retinal areas, brightness 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 illuminated areas, as with a Goldmann size III target in perimetry, summation is incomplete, so that A(k) = C/L, where k is the coefficient of summation, and 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 of retinal eccentricity were taken from the literature and corrected for lateral displacement of ganglion cells from the fovea to estimate ganglion cell 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 coefficient summation (k) was measured empirically at each Humphrey 30-2 test point in 8 normal subjects. The relationship between DLS and G was investigated by plotting DLS as decibels (dB) against G and DLS as 1/L (1/Lamberts) against G(k). The physiological relationship was extrapolated to glaucomatous ganglion cell loss by calculating hypothetical cell losses for 3 and 6 dB sensitivity defects at each test point.


RESULTS
Spatial summation increased with eccentricity. The relationship between DLS (dB) and G was curvilinear. The relationship between DLS (1/L) and G(k) was linear (r2 = 0.73). The extrapolation to glaucomatous ganglion cell loss indicated that a proportionally greater loss of ganglion cells is required in the central compared with peripheral visual field for equal losses in dB sensitivity.


CONCLUSIONS
The number of underlying ganglion cells, adjusted for local spatial 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 reflects the amount of neuronal damage.
Garway-Heath
D F
DF
Glaucoma Unit, Moorfields Eye Hospital, London, United Kingdom. ted.garway-heath@moorfields.nthames.nhs.uk
Caprioli
J
J
Fitzke
F W
FW
Hitchings
R A
RA
D000328:Adult; D002452:Cell Count; D006801:Humans; D008027:Light; D012165:Retinal Ganglion Cells / Q000166:cytology* / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*; D058609:Visual Field Tests; D014794:Visual Fields