Evidence for an independent luminance channel

Measurements of chromatic adaptation and luminous efficiency while wearing colored filters

The visual system adapts dynamically to stabilize perception over widely varying illuminations. Such adaptation allows the colors of objects to appear constant despite changes in spectral illumination. Similarly, the wearing of colored filters also alters spectral content, but this alteration can be more extreme than typically encountered in nature, presenting a unique challenge to color constancy mechanisms. While it is known that chromatic adaptation is affected by surrounding spatial context, a recent study reported a gradual temporal adaptation effect to colored filters such that colors initially appear strongly shifted but over hours of wear are perceived as closer to an unfiltered appearance. Presently, it is not clear whether the luminance system adapts spatially and temporally like the chromatic system. To address this, spatial and temporal adaptation effects to a colored filter were measured using tasks that assess chromatic and luminance adaptation separately. Prior to and for 1 hour after putting on a pair of colored filters, participants made achromatic and heterochromatic flicker photometry (HFP) settings to measure chromatic and luminance adaptation, respectively. Results showed significant chromatic adaptation with achromatic settings moving closer to baseline settings over 1 hour of wearing the filters and greater adaptation with spatial context. Conversely, there was no significant luminance adaptation and HFP matches fell close to what was predicted photometrically. The results are discussed in the context of prior studies of chromatic and luminance adaptation.

Induction effects for heterochromatic brightness matching, heterochromatic flicker photometry, and minimally distinct border: implications for the neural mechanisms underlying induction

Brightness induction refers to the finding that the apparent brightness of a stimulus changes when surrounded by a black versus a white stimulus. In the current study, we investigated the effects of black/white surrounding stimuli on settings made between red and green stimuli on three different tasks: heterochromatic brightness matching (HBM), heterochromatic flicker photometry (HFP), and minimally distinct border (MDB). For HBM, subjects varied the relative luminance between the red and green stimuli so that the brightness of the two colors appeared equal. For the two other tasks, matches were made based on minimizing red/green flicker (HFP) or the saliency of a red/green border (MDB). For all three tasks, the presence of black/white surrounding stimuli significantly altered red/green settings, demonstrating the existence of induction effects. These results are discussed in terms of which underlying color pathways (L+ M versus L-M) may contribute to induction effects for the different tasks.

Temporal induction of blackness--II. Spectral efficiency and tests of additivity

The perception of blackness was investigated by measuring spectral efficiency and field additivity under conditions of temporal induction. For both purposes, observers foveally viewed a 0.75 deg, inducing stimulus for 5 sec followed immediately (or 200 msec later) by a broadband (5500 K) reference stimulus of the same size and spatial location. For spectral efficiency measurements, the inducing field was a monochromatic light between 400 and 700 nm (10 nm steps), while the additivity studies involved various wavelength mixtures. The psychophysical task was to increase the radiance of the inducing field until the reference stimulus just turned completely black. For two observers the spectral efficiency function of temporally-induced blackness more closely resembled their heterochromatic flicker photometry (HFP) function than their direct brightness-matching function. Their brightness functions were characterized by an inflection at about 580 nm which is generally ascribed to subtractive cone interactions, but their blackness-induction and HFP functions did not show this inflection. The brightness function of the third observer did not show an inflection at 580 nm, thereby making it difficult to differentiate between her three spectral efficiency functions. Overall, the subtle differences between the various spectral efficiency functions made it difficult to determine whether blackness induction was more similar to HFP or brightness matching. The results from the additivity tests of blackness induction, HFP and direct brightness matching removed this ambiguity from the spectral efficiency findings. Blackness induction and HFP were shown to be additive, whereas the results from brightness matching showed clear additivity failures of the cancellation type. These data support the view that the perception of blackness is mediated by neural mechanisms that additively combine the inputs of middle- and long-wave photoreceptors.

Temporal induction of blackness--I. Color appearance

Three color-normal observers described the appearance of colors under conditions of temporal induction. One of four inducing fields (unique blue, unique green, unique yellow, unique white) was foveally viewed for 5 sec followed immediately by a 400 msec reference stimulus of the same size (0.75 deg) and spatial location as the inducing field. The reference stimulus was 3 or 5 td and appeared achromatic to the observer when viewed without the inducing field. After presentation of this temporal sequence, the observers described the reference stimulus by assigning percentages to the terms red, green, yellow, blue, white and black. The range of inducing field intensities was -0.4-3.2 log td. Both chromatic and achromatic induction occurred at low illuminance levels of the inducing fields. As the illuminance of the inducing field increased, the reference stimulus gradually became blacker and the chromatic components less prominent. The minimum illuminance level at which the reference appeared 100% black was the same for the four different inducing fields.

Flicker-photometric spectral sensitivity in the presence of chromatic surrounds

Flicker-photometric spectral sensitivity is measured in the presence and absence of superimposed monochromatic surrounds. Although color appearance depends on surround wavelength, the shape of the flicker-spectral-sensitivity functions are unaffected. These findings indicate both a lack of cone-cone inhibition within the achromatic channel and independence of the achromatic and chromatic channels.