To shed light on the perceptual basis of the color white, we measured settings of unique white in a dark surround. We find that settings reliably show more variability in an oblique (blue-yellow) direction in color space than along the cardinal axes of the cone-opponent mechanisms. This is against the idea that white perception arises at the null point of the cone-opponent mechanisms, but one alternative possibility is that it occurs through calibration to the visual environment. We found that the locus of maximum variability in settings lies close to the locus of natural daylights, suggesting that variability may result from uncertainty about the color of the illuminant. We tested this by manipulating uncertainty. First, we altered the extent to which the task was absolute (requiring knowledge of the illumination) or relative. We found no clear effect of this factor on the reduction in sensitivity in the blue-yellow direction. Second, we provided a white surround as a cue to the illumination or left the surround dark. Sensitivity was selectively worse in the blue-yellow direction when the surround was black than when it was white. Our results can be functionally related to the statistics of natural images, where a greater blue-yellow dispersion is characteristic of both reflectances (where anisotropy is weak) and illuminants (where it is very pronounced). Mechanistically, the results could suggest a neural signal responsive to deviations from the blue-yellow locus or an adaptively matched range of contrast response functions for signals that encode different directions in color space.

We present design and first demonstration of optics for a telescopic contact lens with independent optical paths for switching between normal and magnified vision. The magnified optical path incorporates a telescopic arrangement of positive and negative annular concentric reflectors to achieve 2.8 x magnification on the eye, while light passing through a central clear aperture provides unmagnified vision. We present an experimental demonstration of the contact lens mounted on a life-sized optomechanical model eye and, using a pair of modified commercial 3D television glasses, demonstrate electrically operated polarization switching between normal and magnified vision.

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We describe a device that can display very high intensity (up to 400,000 cd/m 2 ), high-resolution visual stimuli. The device is inexpensive, is easily controlled by a conventional computer and video card, and can be calibrated for use in vision research or clinical applications. The display is capable of presenting highly saturated, near spectral colors. Unlike Maxwellian view optical systems, our display can be viewed binocularly and does not require exacting head restraint. We describe the construction, give a design example, and describe our calibration procedure. Furthermore, we report measurements of the color gamut, spatial resolution, temporal characteristics, and the dynamic range of light intensity.

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The gamut of perceived colors can be represented in a space with bright-dark, red-green and blue-yellow axes. Pre-exposure to a field that changes periodically over time in luminance or along one of the color axes reduces vividness of colors along the entire axis [Webster and Mollon (1991) Nature, 349, 235-238]. But is it possible to reduce vividness or perceived contrast selectively for half-axes in color space? We assessed such selective compression of the bright-dark axis using a task where subjects matched tests in a pre-adapted region to ones in an un-adapted region. Tests were bright or dark pinstripes on a gray background, and pre-exposure was to multiple drifting pinstripes. Matches made after pre-exposure indicate a combination of symmetric and asymmetric compression, with more compression when adapting and test stimulus were similar in contrast polarity.

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