Michael D'Zmura

Mechanisms of color constancy

We develop a model of how the visual system finds the colors of objects that have unknown shapes and positions. The model relies on mechanisms of light adaptation, coupled with eye movements, to recover three descriptors of surface reflectance that are represented in the signals of an achromatic mechanism and two color-opponent mechanisms. These descriptors are transformed to yield estimates of hue, the dimension of surface color that is independent of object shape and viewing geometry.

Color in visual search

Colored targets pop out of displays under conditions in which the standard red-green, yellow-blue and black-white mechanisms cannot directly mediate detection. Experimental evidence suggests that observers possess chromatic detection mechanisms tuned to intermediate hues such as orange as well as to hues characterizing the standard color-opponent mechanisms and that these mechanisms, as a group, form a fine-grained representation of hue within the central visual field. Spatially-parallel search is mediated by a single such mechanism that is spectrally sensitive to the target chromaticity but insensitive to the distractor chromaticities; different mechanisms are used to detect a single target in a way that depends on distractor chromaticities.

Color constancy. I. Basic theory of two-stage linear recovery of spectral descriptions for lights and surfaces

Changing a scenes illuminant causes the chromatic properties of reflected lights to change. This change in the lights from surfaces provides spectral information about surface reflectances and illuminants. We examine conditions under which these properties may be recovered by using bilinear models. Necessary conditions that follow from comparing the number of equations and the number of unknowns in the recovery procedure are not sufficient for unique recovery. Necessary and sufficient conditions follow from demanding a one-to-one relationship between quantum catch data and sets of lit surfaces. We present an algorithm for determining whether spectral descriptions of lights and surfaces can be recovered uniquely from reflected lights.

Color contrast induction

We report the results of psychophysical experiments on the intensive, spatial, temporal and chromatic properties of color contrast induction. Modulating the contrast of an annulus induces an apparent modulation of the color contrast of a central disk, at isoluminance. Results of varying the size of the annulus suggest that mechanisms which control contrast gain are spatially localized. Results of varying the orientations of disk and annulus patterns, with peak spatial frequencies at about 2 c/deg, suggest that the mechanisms are spatially isotropic. Results of varying the rate at which annulus contrast is modulated shows that mechanisms which mediate contrast induction have a low-pass temporal sensitivity that cuts off at about 8 Hz. Results of an experiment on the interocular transfer of color contrast induction suggest that the induction has a cortical locus. Finally, the results of varying the chromatic properties of disk and annulus suggest that the underlying mechanisms are partially, but not fully, chromatically selective.

Color constancy. II. Results for two-stage linear recovery of spectral descriptions for lights and surfaces

Our analysis of color constancy in a companion paper [J. Opt. Soc. Am A 10, 2148 (1993)] provided an algorithm that lets one test how well linear color constancy schemes work. Here we present the results of applying the algorithm to a large parametric class of color constancy problems involving bilinear models that relate photoreceptoral spectral sensitivities, surface reflectance functions, and illuminant spectral power distributions. These results, supported by simulation and further analysis, provide a detailed classification of two-stage linear methods for recovering the spectral properties of reflectances and illuminants from reflected lights.

Spectral bandwidths for the detection of color

The spectral properties of human color detection mechanisms were measured using a noise masking technique that minimizes the possibility of off-axis looking and artifactually narrow estimates of bandwidth. Observers were induced to use a single detection mechanism throughout a spectral bandwidth measurement by using sectored noise to mask a time-varying signal of fixed chromatic properties. Sectored noise draws samples from sectors of variable width in the color plane, centered on the signal axis. Contrast thresholds for equiluminant signals that appeared yellow, orange, red and violet were found to depend on the power of the noise, projected along the chromatic axis of the signal, but not on the sector width of the noise. These results are consistent with the activity of spectrally broadband, linear detection mechanisms that are tuned to the signal color directions tested.

Shared pathways for rod and cone vision

We have used heterochromatic gratings falling on 10 deg temporal retina to measure the spatial contrast sensitivities of the isolated rod and cone systems in the mesopic range. As the level of illumination was raised within this range, the contrast sensitivity of the rod system increased, reaching a peak of about 50 (and providing an acuity of 6 c/deg) at 20 scot. td, whereupon the rod system began to saturate. Over most of the mesopic range the sensitivity of the cone system was lower than that of the rod system, although it provided better acuity (up to 15 c/deg). Within the range of spatial frequencies capable of exciting both rod and cone systems, a grating that excited only rods was indistinguishable from a grating of the same spatial frequency that excited only cones. Moreover, contrast adaptation to gratings that excited either rods or cones raised threshold for gratings that excited rods or cones. From these results we conclude that signals from rods and cones travel together in pathways subserving the detection of low spatial frequencies, while only signals from cones travel in pathways subserving the detection of high spatial frequencies.

EEG classification of imagined syllable rhythm using Hilbert spectrum methods

We conducted an experiment to determine whether the rhythm with which imagined syllables are produced may be decoded from EEG recordings. High density EEG data were recorded for seven subjects while they produced in imagination one of two syllables in one of three different rhythms. We used a modified second-order blind identification (SOBI) algorithm to remove artefact signals and reduce data dimensionality. The algorithm uses the consistent temporal structure along multi-trial EEG data to blindly decompose the original recordings. For the four primary SOBI components, joint temporal and spectral features were extracted from the Hilbert spectra (HS) obtained by a Hilbert-Huang transformation (HHT). The HS provide more accurate time-spectral representations of non-stationary data than do conventional techniques like short-time Fourier spectrograms and wavelet scalograms. Classification of the three rhythms yields promising results for inter-trial transfer, with performance for all subjects significantly greater than chance. For comparison, we tested classification performance of three averaging-based methods, using features in the temporal, spectral and time-frequency domains, respectively, and the results are inferior to those of the SOBI-HHT-based method. The results suggest that the rhythmic structure of imagined syllable production can be detected in non-invasive brain recordings and provide a step towards the development of an EEG-based system for communicating imagined speech.

Virtual Environments with Four or More Spatial Dimensions

We describe methods for displaying complex, texturemapped environments with four or more spatial dimensions that allow for real-time interaction. At any one moment in time, a three-dimensional cross section of the high-dimensional environment is rendered using techniques that have been implemented in OpenGL. The position and orientation of the user within the environment determine the 3-D cross section. A variety of interfaces can be used to control position and orientation in 4-D, including a mouse freelook interface for use with a computer monitor display, and an interface that uses a head-tracking system with three degrees of freedom and PINCH gloves in combination with a head-mounted display. The methods avoid the use of projections that require depth buffering in greater than three dimensions and can be used in conjunction with either 2-D or 3-D texture mapping. A computer graphic engine that displays 4-D virtual environments interactively uses these methods, as does a level editor and modeling program that can be used to create 4-D environments.

Spatial pooling of contrast in contrast gain control

We report psychophysical measurements of spatial pooling functions for contrast gain control. We use a nulling technique to measure the dependence of contrast induction on the spatial frequency of a sinusoidal contrast modulation. This dependence on spatial frequency, when transformed, provides the profile of a spatial pooling function. The measured profiles are fitted well by Gaussians. We confirm earlier results that the area over which spatial pooling takes place depends on the scale of the modulated pattern. We also find that pooling functions are similar for achromatic and isoluminant stimuli.