A. Chaparro et al., HUMAN CONES APPEAR TO ADAPT AT LOW-LIGHT LEVELS - MEASUREMENTS ON THERED-GREEN DETECTION MECHANISM, Vision research, 35(22), 1995, pp. 3103-3118
Recent physiological evidence suggests that cones do not light adapt a
t low light levels, To assess whether adaptation is cone-selective at
low light levels, the red-green detection mechanism was isolated, Thre
sholds were measured with a large test flash, which stimulated the L a
nd M cones in different fixed amplitude ratios, on different colored a
dapting fields, Thresholds were plotted in L and M cone contrast coord
inates, The red-green mechanism responded to an equally-weighted diffe
rence of L and M cone contrast on each colored field, demonstrating eq
uivalent, Weberian adaptation of the L and hi cone signals, The L and
M cone signals independently adapted for illuminance levels as low as
60 effective trolands (e.g, M-cone trolands). Since this adaptation is
entirely selective to cone type, it suggests that the cones themselve
s light-adapt, The red-green detection contour on reddish fields was d
isplaced further out from the origin of the cone contrast coordinates,
revealing an additional sensitivity loss at a subsequent, spectrally-
opponent site, This second-site effect may arise from a net ''red'' or
''green'' signal that represents the degree to which the L and M cone
s are differently hyperpolarized by the steady, colored adapting field
. Such differential hyperpolarization is compatible with equivalent, W
eberian adaptation of the L and M cones.