Browsing by Author "Paramei GV"

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A whiter shade of pale, a blacker shade of dark: Parameters of spatially induced blackness
(Cambridge University Press, 2006) Bimler D; Paramei GV; Izmailov CA
The surface-mode property of 'blackness' is induced by simultaneous contrast with an adjacent, more luminantsurround. As numerous studies have shown, the degree of blackness induced within an achromatic test field is afunction of the relative luminance of the adjacent chromatic inducing field, but not of its hue. But in the conversecase of chromatic test fields, susceptibility to blackening has been reported to vary with wavelength. The presentstudy investigates this possibility, that some wavelengths are more susceptible. We also questioned whether 'white'and 'black' sensory components function as opposites in blackness appearance. We recorded the appearance of acentral monochromatic test field of constant luminance (10 cd/m2), with wavelength ranging across the visiblespectrum, while a broadband white annulus was set to six luminance levels ranging across three log steps. Threecolor-normal observers followed a color-naming technique. All six opponent-hue names and their combinations were response options; blackness and whiteness in the test field could therefore be reported independently. Of primary interest were the achromatic responses. When represented within a multidimensional space, these revealed the 'white-to-black' dimension but in addition a quality (dimension) of 'desaturation.' Compared against chromatic properties of the test field, the results provide evidence that blackness is a function of inducing field brightness(not luminance). This result is in accord with observations made by Shinomori et al. (1997) using a different procedure. We conclude that blackness induction occurs at a stage of visual processing subsequent to the origin of the brightness signal from a combination of opponent-process channels.
Luminance-dependent hue shift in protanopes.
(Cambridge, 2004) Bimler DL; Paramei GV
For normal trichromats, the hue of a light can change as its luminance varies. This Bezold-Brücke (B-B) hue shift is commonly attributed to nonlinearity in the blue-yellow opponent system. In the present study, we questioned whether protanopes experience analogous changes. Two protanopes (Ps) viewed spectral lights at six luminance levels across three log steps. Two normal trichromats (NTs) were tested for comparison. A variant of the color-naming method was used, with an additional "white" term. To overcome the difficulty of Ps' idiosyncratic color naming, we converted color-naming functions into individual color spaces, by way of interstimulus similarities and multidimensional scaling (MDS). The color spaces describe each stimulus in terms of spatial coordinates, so that hue shifts are measured geometrically, as displacements along specific dimensions. For the NTs, a B-B shift derived through MDS agreed well with values obtained directly by matching color-naming functions. A change in color appearance was also observed for the Ps, distinct from that in perceived brightness. This change was about twice as large as the B-B shift for NTs and combined what the latter would distinguish as hue and saturation shifts. The protanopic analogue of the B-B shift indicates that the blue-yellow nonlinearity persists in the absence of a red-green signal. In addition, at mesopic levels (< or = 38 td), the Ps' MDS solution was two dimensional at longer wavelengths, suggesting rod input. Conversely, at higher luminance levels (76 td-760 td) the MDS solution was essentially one dimensional, placing a lower limit on S-cone input at longer wavelengths.