PubMed Health⌕ Search

Biomedical subjects

B A Wandell

Publications and source records attributed to B A Wandell.

At least 19 recordsLinked to original sources
PubMed1995

Photoreceptor sensitivity changes explain color appearance shifts induced by large uniform backgrounds in dichoptic matching.

Photoreceptor sensitivity changes explained the effect of large uniform backgrounds on the color appearance of small targets in a dichoptic asymmetric color matching experiment. Subjects viewed in each eye a target superimposed on a large background. The backgrounds presented to the two eyes had different spectral compositions. Subjects adjusted the target seen by the right eye to match the appearance of the target seen by the left eye. Receptor sensitivity changes explained the effect of numerous adapting backgrounds on the color appearance of many targets with high precision. Post-receptoral sensitivity changes provided a poorer account of the data. The apparent sensitivity of each receptor class varied inversely with changes in background light absorbed by that receptor class, but did not depend on background light absorbed by the other two receptor classes.

Adaptation, Ocular↗
PubMed1992

Asymmetric color matching: how color appearance depends on the illuminant.

We report the results of matching experiments designed to study the color appearance of objects rendered under different simulated illuminants on a CRT monitor. Subjects set asymmetric color matches between a standard object and a test object that were rendered under illuminants with different spectral power distributions. For any illuminant change, we found that the mapping between the cone coordinates of matching standard and test objects was well approximated by a diagonal linear transformation. In this sense, our results are consistent with von Kries's hypothesis [Handb. Physiol. Menschen 3, 109 (1905) [in Sources of Color Vision, D. L. MacAdam, ed. (MIT Press, Cambridge, Mass., 1970)]] that adaptation simply changes the relative sensitivity of the different cone classes. In addition, we examined the dependence of the diagonal transformation on the illuminant change. For the range of illuminants tested, we found that the change in the diagonal elements of the linear transformation was a linear function of the illuminant change.

Color Perception↗
PubMed1992

Linear models of surface and illuminant spectra.

We describe procedures for creating efficient spectral representations for color. The representations generalize conventional tristimulus representations, which are based on the peripheral encoding by the human eye. We use low-dimensional linear models to approximate the spectral properties of surfaces and illuminants with respect to a collection of sensing devices. We choose the linear-model basis functions by minimizing the error in approximating sensor responses for collections of surfaces and illuminants. These linear models offer some conceptual simplifications for applications such as printer calibration; they also perform substantially better than principal-components approximations for computer-graphics applications.

Algorithms↗
PubMed1990

The ellipsoidal representation of spectral sensitivity.

When plotted in three-dimensional color-space, thresholds of colored lights fall on or near the surface of an ellipsoid. Using data reported in the literature, we estimate the deviation between sets of spectral threshold measurements and the ellipsoid that passes closest to the data. Seventy-three percent of the reported spectral thresholds fall within 0.1 log units of the best-fitting ellipsoid. Our ability to distinguish one ellipsoidal fit as significantly better than another is limited by the choice of sampling directions in color-space. Spectral lights do not provide a good set of sampling directions for reducing the uncertainty about the estimated best-fitting ellipsoid. Complete characterization of visual sensitivity requires measuring thresholds to mixtures of spectral lights.

Color Perception↗
PubMed1990

Task-dependent color discrimination.

When an observer's ability to discriminate colored objects is estimated from the variability in color matches, the observer inspects adjacent visual fields carefully and makes considered judgments. Color discrimination does not always take place under such viewing conditions. When color video displays are used in time-critical applications (e.g., head-up displays, video control panels), the observer must discriminate among briefly presented targets seen within a complex spatial scene. We compare color-discrimination thresholds by using two tasks. In one task the observer makes color matches between two halves of a continuously displayed bipartite field. In a second task the observer detects a color target in a set of briefly presented objects. The data from both tasks are well summarized by ellipsoidal isosensitivity contours. The fitted ellipsoids differ both in their size, which indicates an absolute sensitivity difference, and orientation, which indicates a relative sensitivity difference.

Calibration↗
PubMed1990

Surface characterizations of color thresholds.

We evaluate how well three different parametric shapes, ellipsoids, rectangles, and parallelograms, serve as models of three-dimensional detection contours. We describe how the procedures for deriving the best-fitting shapes constrain inferences about the theoretical visual detection mechanisms. The ellipsoidal shape, commonly assumed by vector-length theories, is related to a class of visual mechanisms that are unique only up to orthogonal transformations. The rectangle shape is related to a unique set of visual mechanisms, but since the rectangle is not invariant with respect to linear transformations the estimated visual mechanisms are dependent on the stimulus coordinate frame. The parallelogram is related to a unique set of visual mechanisms and can be derived by methods that are independent of the stimulus coordinate frame. We evaluate how well these shapes approximate detection contours, using 2-deg test fields with a long (1-sec) Gaussian time course. Two statistical tests suggest that the parallelogram model is too strong. First, we find that the ellipsoid and rectangle shapes fit the data with the same precision as the variance in repeated threshold measurements. The parallelogram model, which has more free parameters, fits the data with more precision than the variance in repeated threshold measurements. Second, although the parallelogram model provides a slightly better fit of our data than the other two shapes, it does not serve as a better guide than the ellipsoidal model for interpolating from the measurements to thresholds in novel color directions.

Calibration↗
PubMed1989

Black light: how sensors filter spectral variation of the illuminant.

Visual sensor responses may be used to classify objects on the basis of their surface reflectance functions. In a color image, the image data are represented as a vector of sensor responses at each point in the image. This vector depends both on the surface reflectance function and on the spectral power distribution of the ambient illumination. Algorithms designed to classify objects on the basis of their surface reflectance functions typically attempt to overcome the dependence of the sensor responses on the illuminant by integrating sensor data collected from multiple surfaces. In machine vision applications, we show that it is often possible to design the sensor spectral responsivities so that the vector direction of the sensor responses does not depend upon the illuminant. We state the conditions under which this is possible and perform an illustrative calculation. In biological systems, where the sensor responsivities are fixed, we show that some changes in the illumination cause no change in the sensor responses. We call such changes in illuminant black illuminants. It is possible to express any illuminant as the sum of two unique components. One component is a black illuminant. We call the second component the visible component. The visible component of an illuminant completely characterizes the effect of the illuminant on the vector of sensor responses.

Artificial Intelligence↗
PubMed1988

Discrete analysis of spatial-sensitivity models.

The visual representation of spatial patterns begins with a series of linear transformations: the stimulus is blurred by the optics, spatially sampled by the photoreceptor array, spatially pooled by the ganglion-cell receptive fields, and so forth. Models of human spatial-pattern vision commonly summarize the initial transformations by a single linear transformation that maps the stimulus into an array of sensor responses. Some components of the initial linear transformations (e.g., lens blurring, photoreceptor sampling) have been estimated empirically; others have not. A computable model must include some assumptions concerning the unknown components of the initial linear encoding. Even a modest sketch of the initial visual encoding requires the specification of a large number of sensors, making the calculations required for performance predictions quite large. We describe procedures for reducing the computational burden of current models of spatial vision that ensure that the simplifications are consistent with the predictions of the complete model. We also describe a method for using pattern-sensitivity measurements to estimate the initial linear transformation. The method is based on the assumption that detection performance is monotonic with the vector length of the sensor responses. We show how contrast-threshold data can be used to estimate the linear transformation needed to characterize threshold performance.

Humans↗
PubMed1986

Color constancy: a method for recovering surface spectral reflectance.

Human and machine visual sensing is enhanced when surface properties of objects in scenes, including color, can be reliably estimated despite changes in the ambient lighting conditions. We describe a computational method for estimating surface spectral reflectance when the spectral power distribution of the ambient light is not known.

Color Perception↗
PubMed1986

Analysis of the retinex theory of color vision.

If color appearance is to be a useful feature in identifying an object, then color appearance must remain roughly constant when the object is viewed in different contexts. People maintain approximate color constancy despite variation in the color of nearby objects and despite variation in the spectral power distribution of the ambient light. Land's retinex algorithm is a model of human color constancy. We analyze the retinex algorithm and discuss its general properties. We show that the algorithm is too sensitive to changes in the color of nearby objects to serve as an adequate model of human color constancy.

Algorithms↗
PubMed1985

Color measurement and discrimination.

Theories of color-difference measurement provide a quantitative means for predicting whether two lights will be discriminable to an average observer. Consider the following color-measurement hypothesis. Suppose that two lights evoke responses from the color channels that we write as vectors, U and U'. The vector difference dU = U - U' is itself a set of channel responses that will result from the presentation of some light. I test the hypothesis that U and U' will be discriminable only if the light that gives rise to their differential, dU, is detectable. In the absence of a luminance component in the difference stimulus, dU, the vector-difference hypothesis holds well. In the presence of a luminance component, the theory is clearly false. When a luminance component is present, discrimination judgements depend largely on whether the lights U and U' are in separate, categorical regions of color space.

Biofeedback, Psychology↗
PubMed1984

A model of a single visual channel's response to weak test lights.

We describe a model of the response of a single visual channel to weak test lights. We assume that the initial channel response may be approximated as a linear filter whose output is sampled at random times. At each sample time there is some probability (increasing with the size of the filter response) that a detection event is generated. The detection events form the basis of the observer's detection and duration discrimination judgments. We derive the statistics of the detection events and empirical tests of the model. Assuming the probability of a detection event to be negligible in the absence of a signal, we derive an exact prediction of the form of the psychometric function for detection. Second, assuming that duration discrimination of weak test lights is based solely on the temporal separation of detection events, we predict the exact form of detection/discrimination performance. Third, assuming that the observer's response is initiated by the first detection event, we derive the form of the cumulative reaction time distribution.

Discrimination, Psychological↗
PubMed1984

Duration discrimination between weak test lights.

Maloney and Wandell [Vision Res. 24, 633-640 (1984)] describe a model of the response of a single visual channel to weak test signals. In the model an initial continuous visual response is randomly sampled, and each sample gives rise--with a probability that increases with the magnitude of the sample--to a discrete detection event. The authors derive a parameter-free prediction for the upper bound on the discriminability of two lights of different durations. In this paper we describe an experimental test of that prediction. We find that the model accurately distinguishes between discrimination performance under conditions where both test lights are detected by a single channel and conditions where the test lights are detected by different channels.

Discrimination, Psychological↗
PubMed1984

Reaction times to weak test lights.

Maloney and Wandell [Vision Res. 24, 633-640 (1984)] describe a model of the response of a single visual channel to weak test lights. The initial channel response is a linearly filtered version of the stimulus. The filter output is randomly sampled over time. Each time a sample occurs there is some probability--increasing with the magnitude of the sampled response--that a discrete detection event is generated. Maloney and Wandell derive the statistics of the detection events. In this paper we test the hypothesis that the reaction time responses to the presence of a weak test light are initiated at the first detection event. This permits us to extend the application of the model to lights that are slightly above threshold, but still within the linear operating range of the visual system. We test a parameter-free prediction of the model proposed by maloney and Wandell for lights detected by this statistic. The data are in agreement with the prediction.

Humans↗
PubMed1982

Adaptation in the long-wavelength pathways.

We describe and test predictions of a model of long-wavelength test sensitivity upon large, uniform backgrounds. The model explains changes in sensitivity in the red-green detection pathways strictly based upon losses of sensitivity in the receptors. We derive the prediction that field mixture data for field-mixtures of mu1 (fixed) and an addend, mu 2, must follow the same shape on different intensities of the fixed background, mu 1. This prediction is not in good agreement with the measurements.

Adaptation, Ocular↗