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David F. Garway–Heath, Joseph Caprioli, Fred W. Fitzke, Roger A. Hitchings; Scaling the Hill of Vision: The Physiological Relationship between Light Sensitivity and Ganglion Cell Numbers. Invest. Ophthalmol. Vis. Sci. 2000;41(7):1774-1782.
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purpose. 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° 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° (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
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
(r 2 = 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
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