A large number of studies have shown the effect of melanopsin-dependent retinal ganglion cells on humans performing brightness discrimination tasks. These studies often utilized targets that only differ in their melanopsin activation levels, and not in their luminance or hue, which are both factors that make large contributions to brightness discrimination. The purpose of the present study was to evaluate the relative contribution of melanopsin activation to brightness discrimination when luminance and hue are also varying in addition to melanopsin activation. Using an apparatus consisting of three separate high luminance projectors, we were able to manipulate melanopsin-isolating stimulation, and L-, M-, and S-cone stimulation separately, thus allowing us to vary stimuli in their melanopsin activation, luminance, and hue category independently. We constructed three sets of target stimuli with three different levels of melanopsin activation (100%, 131%, and 167% relative melanopsin excitation) and five levels of luminance. We then had subjects do a two-alternative forced choice task where they compared the previously described target stimuli set to a set of four comparison stimuli that varied in their hue category but had identical luminances. We found that in our stimuli set the overall contribution of melanopsin activity to brightness discrimination was small (an average of 6% increase in likelihood to call a high melanopsin activity stimulus brighter compared to a low melanopsin activity stimulus) when luminance and hue also varied. However, a significant interaction showed that when the comparison was between stimuli differing only in melanopsin stimulation (with luminance and hue unchanged) the contribution of melanopsin to brightness judgments was about 3 times larger (an average of 18% increase in likelihood to call a high melanopsin activity stimulus brighter compared to a low melanopsin activity stimulus). This suggests that although luminance and hue have large effects on brightness discrimination such that the melanopsin contribution can become hard to detect, when there are minimal cone-dependent signals available, melanopsin can make a large contribution to brightness discrimination.
Equilibrium (unique) red, green, blue, and yellow stimuli look bright in a black surround, but they look dark in a bright white surround, and yellow changes to brown. We investigated differences in equilibrium-hue chromaticity between bright and dark hues to reveal changes in weighting of cone and rod signals. The largest, most consistent shifts were found between yellow and brown, with equilibrium-brown chromaticity shifted toward red compared to equilibrium yellow at both photopic and mesopic levels. Also, at mesopic levels, rod influence reversed for most observers from a green bias for yellow to a red bias for brown. Bright/dark differences for blue, green, and red were much smaller and/or less consistent. Thus, shifts of cone and rod hue biases between bright and dark hues are most prominent in L-M-cone pathways, especially those activated by yellow and brown stimuli.
Previous research from our lab has established that red-green-balanced yellow targets become greenish-brown as surround luminance increases, while red-green-balanced brown targets become reddish-yellow as surround luminance decreases. To help assess the generality and underlying processes of this contrast-dependent red-green hue shift, we investigated red-green hue shifts for target stimuli that appeared achromatic or blue as well as yellow/brown. Results confirmed that the red-green hue shift was largest for yellow/brown targets and was progressively reduced for achromatic and blue targets as target excitation of S cones increased. The magnitude of the hue shift could be predicted by the S/(L+M) excitation of the target when bright white surrounds are used. The hue shift also requires that the target and surround are presented to the same eye, consistent with processing in monocular pathways. Increased S-cone excitation by the surround was associated with red-green hue shifts for all targets equally. Thus, S-cone signals from bright white surrounds might play a role in the contrast-dependent red-green hue shift, but the source of the variation of the magnitude of the hue shift with variations in target S-cone excitation when presented on those surrounds is unknown.
A bright white surround makes a yellow long-wavelength target look both browner and darker. We explored the parallel between these two types of induction by examining their dependence on the proximity of the bright surround to the target at two different time scales with 27 ms and 1 s stimulus durations. We assessed (a) brown induction by adjustment of target luminance to perceptual brown and yellow boundaries and (b) darkness induction by a successive matching procedure. We found that brown induction is a quick process that is robust even for 27 ms stimuli. For darkness induction, there was a strong, spatially localized surround proximity effect for the 27 ms stimuli and much weaker proximity effect for the 1 s stimuli. For brown induction, proximity effects were generally weaker but still showed relatively stronger localized proximity effects for 27 ms stimuli than for 1 s stimuli. For these stimuli, darkness induction predicts the relative pattern but not the magnitudes of brown induction. Both brown and darkness inductions show the operation of quick, spatially localized processes that are apparently superseded by other processes for extended stimulus presentations.
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