Induced contrast asynchronies may be useful for luminance photometry.

Shapiro et al. (2004) introduced a new visual effect (the induced contrast asynchrony) that demonstrates a perceptual separation between the response to a modulated light and the response to contrast of the light relative to background. The effect is composed of two physically identical disks, one surrounded by a dark annulus and the other by a light annulus. The luminance levels of both central disks were modulated in time, producing a stimulus with in-phase luminance modulation and antiphase contrast modulation. Observers primarily perceived the disks to be modulating asynchronously (i.e. they perceived the contrast), but at low temporal frequencies could also track the luminance level. Here we document that the induced contrast asynchrony disappears when the surrounds are achromatic and the center lights are modulated near the equiluminant axis. Observers viewed 1-deg-diameter disks embedded 2-deg-diameter achromatic surrounds. The chromaticity of the disks was modulated in time (1 Hz) along lines in an S versus Luminance cardinal color plane and an L-M versus Luminance cardinal color plane; observers responded as to whether the modulation appeared in phase. For all observers and both color planes, the lights appeared in phase most frequently at angles near the standard observer's equiluminant line and out of phase at angles further away from that line. Observers differed in the range of angles that produce the appearance of in-phase modulation. The results suggest that induced contrast asynchronies may be useful as a technique for equating luminance of disparate lights.

Induced contrast asynchronies.

We document a new type of perceptual effect in which asynchronous contrast signals are presented simultaneously with synchronous luminance signals. The template for the basic effect consists of two physically identical disks (.75-deg diameter, 40 cd/m2), one surrounded by a dark annulus (1.5 deg, 20 cd/m2) and the other by a light annulus (1.5 deg, 60 cd/m2). The center disks are modulated in time, with a maximum luminance of 55 cd/m2 and a minimum luminance of 25 cd/m2. With this stimulus configuration, the luminance signals of the disks modulate in phase with each other while the contrast signals relative to the surrounds modulate in anti-phase. Observers can track the contrast and luminance signals when the luminance is modulated at 1 Hz but perceive primarily the contrast signal at 2-6 Hz. We show that the asynchrony can be perceived with a thin annular surround, that the appearance of the asynchrony is dependent on the modulation amplitude, and that a decrease in the relative strength of the asynchrony at 1 Hz corresponds to the band-pass shape of the temporal contrast sensitivity function in the presence of light and dark edges. We also introduce variations of the induced contrast asynchrony principle in which a single modulated disk is surrounded by a half-light and half-dark split annulus; we refer to these configurations as the window-shade and rocking-disk illusions.