A regularized approach to color constancy

Color constancy is the ability of color perception independent of the spectrum of the ambient illumination. We present an algorithm that makes use of biologically plausible assumptions concerning the spectra of illumination and surface reflectance in order to solve the undetermined problem of color constancy. We test the proposed algorithm by means of computer simulations and examine its range of performance.     

Spatial constancy mechanisms in motor control

The success of the human species in interacting with the environment depends on the ability to maintain spatial stability despite the continuous changes in sensory and motor inputs owing to movements of eyes, head and body. In this paper, I will review recent advances in the understanding of how the brain deals with the dynamic flow of sensory and motor information in order to maintain spatial constancy of movement goals. The first part summarizes studies in the saccadic system, showing that spatial constancy is governed by a dynamic feed-forward process, by gaze-centred remapping of target representations in anticipation of and across eye movements. The subsequent sections relate to other oculomotor behaviour, such as eye-head gaze shifts, smooth pursuit and vergence eye movements, and their implications for feed-forward mechanisms for spatial constancy. Work that studied the geometric complexities in spatial constancy and saccadic guidance across head and body movements, distinguishing between self-generated and passively induced motion, indicates that both feed-forward and sensory feedback processing play a role in spatial updating of movement goals. The paper ends with a discussion of the behavioural mechanisms of spatial constancy for arm motor control and their physiological implications for the brain. Taken together, the emerging picture is that the brain computes an evolving representation of three-dimensional action space, whose internal metric is updated in a nonlinear way, by optimally integrating noisy and ambiguous afferent and efferent signals.     

Color perception and the limits of color constancy

Summary A summary of colorimetry is given and the limits of color constancy mechanism under changing illuminations are discussed.     

On the effect of scene motion on color constancy

A series of experiments with human subjects have shown that color constancy improves when an object moves. It has been hypothesized that this effect is due to some kind of influence of high-level motion processing. We have built a computational model for color perception which replicates the results qualitatively which have been obtained with human subjects. We show that input from high-level motion processing is not required. In our model, the dependence is an effect of eye movement in combination with neural processing. Depending on the type of stimulus used, the eye either tracks the object or the background. When the object moves but is tracked by the observer, the background appears to move when considering the stimulus with respect to eye coordinates. Hence, the retinal input is different for the two conditions leading to a difference in color constancy performance.     

Color constancy: the role of low-level mechanisms

Color constancy (CC) is an important psychophysical phenomenon, which has been studied extensively. However, it is not clearly understood. This study presents a novel biological model for the contribution of low level mechanisms to CC. The model is based on two chromatic adaptation mechanisms in the color-coded retinal ganglion cells, 'local' and 'remote', which cause a 'curve-shifting' effect at each receptive field subregion. Simulations are employed for calculating the perceived image and measuring the degree of CC using both 'human-perception' and 'machine-vision' indices. The results indicate that the contribution of adaptations to CC is significant, robust and in agreement with experimental findings. The model is successful in performing CC under multiple chromatic illumination sources, a condition which mimics common natural environments.     

Mathematical biophysics of color vision: III. Color constancy

A mechanism is presented which can account for certain aspects of the phenomena of color constancy. The mechanism involves interaction between a given region and the remaining field. Each region is represented by a color center having the structure previously introduced (Landahl, 1952,Bull. Math. Biophysics,14, 317–25) to account for a number of phenomena of color vision. The trichromatic, symmetric mechanism is introduced for simplicity. The interaction is such that collaterals from each of the primaries representing the background send elements to each of the centers corresponding to the primaries representing the spot. However, the collaterals impinging upon unlike centers are excitatory while the collaterals impinging on like centers, corresponding to the same primary colors, are inhibitory. With proper choice of coefficients, the result is that for small changes in illumination, the resulting apparent color is unchanged. However, for greater changes in the color of the illumination, there results a distortion of the apparent color. A number of examples are illustrated numerically. This research was supported in whole or in part by the U. S. Air Force under Contract AF 49(638)-414 monitored by the Air Force Office of Scientific Research.     

Physiological mechanisms of temperature biofeedback

Research on the physiological mechanisms of finger temperature biofeedback with normal subjects and Raynaud's disease patients is reviewed. Studies conducted in the author's laboratory have shown that feedback-induced vasodilation is mediated through a non-neural, -adrenergic mechanism rather than through reductions in sympathetic nervous system activation. In contrast, feedback-induced vasoconstriction is mediated through the traditional, sympathetic nervous pathway. When used with primary Raynaud's disease patients, feedback-induced vasodilation has achieved reductions in reported symptom frequency ranging from 66% to 92% in controlled investigations. Future research directions are discussed.Research conducted by the author was supported by research grants Nos. HL-23828, HL-30604, and AG-05233 from NIH. I am grateful for the collaboration of the following colleagues during the 14 years of work reported here: Peter Ianni, Dena Norton, Paul Wenig, Subhash Sabharwal, Maureen Mayes, Nagraj Desai, Michael Morris, Peter Migály, and Stewart Vining.     

Physiological color change in squid iridophores

Summary Cephalopods generally are thought to have only static iridophores, but this report provides qualitative and quantitative evidence for active control of certain iridescent cells in the dermis of the squidLolliguncula brevis. In vivo observations indicate the expression of iridescence to be linked to agonistic or reproductive behavior. The neuromodulator acetylcholine (ACh) induced dramatic optical changes in active iridophores in vitro, whereas ACh had little effect on passive iridophores elsewhere in the mantle skin. Bath application of physiological concentrations of ACh (10^-7M to 10^-6M) to excised dermal skin layers transformed the active iridophores from a non-reflective diffuse blue to brightly iridescent colors, and this reaction was reversible and repeatable. The speed of change to iridescent in vitro corresponded well to the speed of changes in the living animal. Pharmacological results indicate the presence of muscarinic receptors in this system and that Ca⁺⁺ is a mediator for the observed changes. Although ACh is present in physiological quantities in the dermal iridophore layer, it is possible that ACh release is not controlled directly by the nervous system because electrophysiological stimulation of major nerves in the periphery resulted in no iridescence inL. brevis; nor did silver staining or transmission electron microscopy reveal neuronal elements in the iridophore layer. Thus, active iridophores may be controlled by ACh acting as a hormone.     

Physiological color change in squid iridophores

Evidence is presented that changes in the optical properties of active iridophores in the dermis of the squid Lolliguncula brevis are the result of changes in the ultrastructure of these cells. At least two mechanisms may be involved when active cells change from non-iridescent to iridescent or change iridescent color. One is the reversible change of labile, detergent-resistant proteinaceous material within the iridophore platelets, from a contracted gel state (non-iridescent) to an expanded fluid or sol state when the cells become iridescent. The other is a change in the thickness of the platelets, with platelets becoming significantly thinner as the optical properties of the iridophores change from non-iridescent to iridescent red, and progressively thinner still as the observed iridescent colors become those of shorter wavelengths. Optical change from Rayleigh scattering (non-iridescent) to structural reflection (iridescent) may be due to the viscosity change in the platelet material, with the variations in observed iridescent colors due to changes in the dimensions of the iridophore platelets.     

Color categorization and color constancy in a neural network model of V4

We develop a neural network model that instantiates color constancy and color categorization in a single unified framework. Previous models achieve similar effects but ignore important biological constraints. Color constancy in this model is achieved by a new application of the double opponent cells found in the blobs of the visual cortex. Color categorization emerges naturally, as a consequence of processing chromatic stimuli as vectors in a four-dimensional color space. A computer simulation of this model is subjected to the classic psychophysical tests that first uncovered these phenomena, and its response matches psychophysical results very closely.     

Necessary and sufficient conditions for Von Kries chromatic adaptation to give color constancy

Necessary and sufficient spectral conditions are presented for Von Kries chromatic adaptation to give color constancy. Von-Kries-invariant reflectance spectra are computed for illuminant spectral power distributions that are arbitrary linear combinations of the first three daylight phases. Experiments are suggested to test models of color constancy using computed spectra (either exact or approximate) within the illuminant-invariant framework.