Constancy Mechanisms Research Articles

The neurological basis of conscious color perception in a blind patient.

We have studied patient PB, who, after an electric shock that led to vascular insufficiency, became virtually blind, although he retained a capacity to see colors consciously. For our psychophysical studies, we used a simplified version of the Land experiments [Land, E. (1974) Proc. R. Inst. G. B. 47, 23-58] to learn whether color constancy mechanisms are intact in him, which amounts to learning whether he can assign a constant color to a surface in spite of changes in the precise wavelength composition of the light reflected from that surface. We supplemented our psychophysical studies with imaging ones, using functional magnetic resonance, to learn something about the location of areas that are active in his brain when he perceives colors. The psychophysical results suggested that color constancy mechanisms are severely defective in PB and that his color vision is wavelength-based. The imaging results showed that, when he viewed and recognized colors, significant increases in activity were restricted mainly to V1-V2. We conclude that a partly defective color system operating on its own in a severely damaged brain is able to mediate a conscious experience of color in the virtually total absence of other visual abilities.

Mechanisms of color constancy under nearly natural viewing.

Color constancy is our ability to perceive constant surface colors despite changes in illumination. Although color constancy has been studied extensively, its mechanisms are still largely unknown. Three classic hypotheses are that constancy is mediated by local adaptation, by adaptation to the spatial mean of the image, or by adaptation to the most intense image region. We measure color constancy under nearly natural viewing conditions, by using a design that allows us to test these three hypotheses directly. By suitable stimulus manipulation, we are able to titrate the degree of constancy between 11% and 83%, indicating that we have achieved good laboratory control. Our results rule out all three classic hypotheses and thus suggest that there is more to constancy than can be easily explained by the action of simple visual mechanisms.

Colour Constancy in Birds: An Alternative Mechanism?

When recognising a surface colour, the visual system discounts the illumination, apparently by using some reference surfaces (like a spectrophotometer). To recover the illuminant colour it uses signals from different, sometimes remote, parts of the scene viewed either in sequence or in parallel. As a result, humans and animals fail to recognise the colour of a patch that is locally illuminated with a narrow light beam different in colour from the ambient illumination, but show good colour constancy when the beam envelops the surrounding scene. The results obtained in birds distinguish them from humans and all animals hitherto investigated. The behaviour of hole-dwelling birds was studied in the wild by the method of alternative choice of entrance into experimental nesting-boxes having three entrances marked with coloured stimuli made from papers painted in different shades of blue, grey, or orange (see Maximov and Derim-Oglu, 1996 Perception25 Supplement, 98, and the corresponding WWW site: http://www.digipark.com/science/meta ). The spectral content of the direct sunlight illumination was changed by filters, either locally at the stimuli or over the whole front panel of the experimental box. In such artificial lighting conditions the birds proved to be incapable of using a neighbouring white surface as a sign of the illumination to discount its effect on the colour of objects. These unexpected results can be explained by the ability of animals with colour vision of rather high dimensionality to take recourse to local mechanisms of colour constancy that extract the colour information which is invariant to changes of the illumination, using certain a priori constraints on environmental light spectra. Natural daylight spectra have been shown to satisfy these constraints.

Mechanisms of velocity constancy

Human observers can compare the physical velocities of objects (cm/sec) moving at different distances quite well, although the objects' retinal velocities (deg/sec) may vary considerably. This perceptual ability is called velocity constancy. We conducted a number of experiments to investigate what mechanisms observers use to attain this constancy and if pure motion signals can also be matched according to their physical speeds. Subjects were asked to match the velocities of two moving stimuli presented at different viewing distances. The stimuli consisted of sparse random-dot kinematograms or drifting Julesz patterns. The subjects matched the true physical velocities of the stimuli provided that the two visual scenes contained identical size references. Knowledge of the actual viewing distances proved to be irrelevant for evaluating the physical velocities of the stimuli. We conclude that velocity constancy is based upon a relative scaling algorithm.

Is a Perceived Shape Based on its Retinal Image?

Although the processing of phenomenal shape might be supposed to begin at an early stage, with the shape of the retinal image of an object, it is possible that it does not begin until a later stage at which the locations of the parts of the object have been perceived. Such perceived locations are based on a compensation or constancy mechanism that takes account of eye position. Ordinarily these two possible bases of shape perception--retinal image and perceived location--are confounded. To separate them the parts of a shape were presented sequentially, during which time the eyes were in motion. The eye movement did not alter phenomenal locations of the parts vis-à-vis one another but did yield an entirely different composite retinal image of the parts. Another method employed was to change the location of each sequentially presented part with respect to a displacing frame of reference. By and large the results indicate that the composite shape perceived is based on the perceived location of the parts of the object with respect to one another, rather than on the composite retinal image.

A role for the corpus callosum in visual area V4 of the macaque.

The classically defined receptive fields of V4 cells are confined almost entirely to the contralateral visual field. However, these receptive fields are often surrounded by large, silent suppressive regions, and stimulating the surrounds can cause a complete suppression of response to a simultaneously presented stimulus within the receptive field. We investigated whether the suppressive surrounds might extend across the midline into the ipsilateral visual field and, if so, whether the surrounds were dependent on the corpus callosum, which has a widespread distribution in V4. We found that the surrounds of more than half of the cells tested in the central visual field representation of V4 crossed into the ipsilateral visual field, with some extending up to at least 16 deg from the vertical meridian. Much of this suppression from the ipsilateral field was mediated by the corpus callosum, as section of the callosum dramatically reduced both the strength and extent of the surrounds. There remained, however, some residual suppression that was not further reduced by addition of an anterior commissure lesion. Because the residual ipsilateral suppression was similar in magnitude and extent to that found following section of the optic tract contralateral to the V4 recording, we concluded that it was retinal in origin. Using the same techniques employed in V4, we also mapped the ipsilateral extent of surrounds in the foveal representation of V1 in an intact monkey. Results were very similar to those in V4 following commissural or contralateral tract sections. The findings suggest that V4 is a central site for long-range interactions both within and across the two visual hemifields. Taken with previous work, the results are consistent with the notion that the large suppressive surrounds of V4 neurons contribute to the neural mechanisms of color constancy and figure-ground separation.

A color constancy model for advanced television cameras

A color constancy mechanism is proposed to perceive the image color from the strengths of three RGB responses of a color camera or the like, independent of the color of the light illuminating the object. The first step of the method is to use a finite-dimensional linear model to estimate the color signals. It is shown that any color signal can be characterized as a linear combination of four principal component basis functions. Once the color signals have been estimated the unknown illuminant can be determined by S. Tominaga and B.A. Wandell's (1989) estimation method, which is based on a dichromatic model. In color constancy, the knowledge of surface reflectance helps in determining the canonical color descriptors despite the variations in the spectral power distribution of the ambient light. It is shown that the estimate of surface reflectance is derived from both the estimated color signal and illuminant straightforwardly. The model of a combination of the estimations of the surface spectral reflectance and the color signals can be employed in designing a color constancy electronic video camera which indeed improves the shortcomings of the traditional video camcorders. >

The Goldmeier effect in adults and children: environmental, retinal, and phenomenal influences on judgments of visual symmetry.

Adults judge that patterns symmetrical about the vertical axis are more similar to standard patterns symmetrical about both major orthogonal axes than are patterns which are symmetrical only about the horizontal axis (the Goldmeier effect). Thus, symmetry about the vertical axis is more salient for adults than symmetry about the horizontal axis. Two experiments are reported in which subjects from three age groups (preschool, 8 years old, and adult) were given Goldmeier problems under different conditions. In experiment 1 three head-tilt conditions were used (0 degrees, 45 degrees, 90 degrees); in experiment 2 there were four conditions defined by head orientation (0 degrees, 90 degrees) and phenomenal instructions (top of figure at 0 degrees or at 90 degrees). In both experiments, increasing head tilt from 0 degrees decreased the consistency with which the environmentally vertical pattern was chosen. Noncorrespondence between the three spatial frameworks (environmental, retinal, and phenomenal) failed to produce biases in favor of either retinal-egocentric or phenomenal systems. For rotated adult subjects in experiment 2, 0 degrees phenomenal instructions strengthened an environmental bias, and 90 degrees phenomenal instructions shifted responses toward a retinal bias. These findings provide strong refutation of explanations for symmetry perception that are based solely upon the anatomical symmetry of the visual system. The data also fail to support arguments for environmental or phenomenal frameworks as singular influences. The results are best explained in terms of failure of constancy mechanisms to coordinate environmental and retinal information as a function of degree of head rotation and stimulus complexity.

The State of the Art in Perception

Renewed interest in neural vector fields as the generators of visual organization is indicated by recent publications, such as Dodwell and Caelli's Figural Synthesis. Perceptual and physiological approaches are beginning to converge. Studies of induced movement and pattern integration revive problems first raised by gestalt psychologists, and a new theory of the constancy mechanisms implies a parallel to the author's own work involving visual composition in the arts. Recently, several extensive publications have reported ongoing research in shape perception with a renewed appreciation of structural organization in the total visual pattern. It is an approach appealing to the interest of those of us who are involved in the visual arts. I therefore propose to describe briefly the present state of affairs in this field in occasion of the publication of a valuable new collection of papers edited by Peter C. Dodwell and Terry Caelli entitled Figural Synthesis [1]. Concerned with psychophysical research, the book is too specialized for the needs of many Leonardo readers. It teems with mathematical formulae and experimental technicalities. Therefore, instead of reviewing the book on its own terms, I will use it to sketch an up-to-date image of some of the major problems now facing the scientific discipline in this area and the directions in which solutions are sought. The principal dilemma pervading the field is the discrepancy between what is observed in consciousness when humans or animals look at visual patterns and what can be ascertained through a study of the anatomy and physiology of the nervous system. On the one side, there is the world of visual experience organized by more or less integrated patterns according to principles that derive from the structure of the whole. Using these psychological facts as their point of departure, investigators explore the brain in search of physiological equivalents by which to explain what is being observed in perception. This approach was initiated by the gestalt psychologists and their followers. They had to struggle with neurological situations in which sums of elements rather than organized wholes seemed to be in charge. Millions of Rudolf Arnheim (psychologist), 1133 South Seventh Street, Ann Arbor, MI 48103, U.S.A. Received 3 March 1986. ? 1987 ISAST Pergamon Journals Ltd. Printed in Great Britain. 0024-094X/87 $3.00+0.00 receptor organs in the eye were found to connect with complex networks of fibers, which in turn ended up at the innumerable nerve cells of the cerebral cortex. To derive from these agglomerations of elements the kind of field process that alone seemed to do justice to what happens in perception offered technical difficulties. On the other side, the intricate anatomy of the brain was the base for the opposite approach to the problem. Here, too, the need to account for wholedetermined functioning began to be felt keenly. Some 25 years ago, evidence began to accumulate that, in the brains of higher animals, dealing with visual stimuli did not work simply as a summation of the point-sized recordings of isolated receptor cells. Rather, groups of such cells were shown to be programmed to respond together to simple shapes, directions and movements of visible objects. These inbuilt mechanisms,

Object-motion detection affected by concurrent self-motion perception: Psychophysics of a new phenomenon

Thresholds for object-motion detection are significantly raised when concurrent self-motion perception is induced by either vestibular, or visual, or cervico-somatosensory stimulation. (1) Active sinusoidal horizontal head oscillations with compensatory vestibulo-ocular reflex (VOR) and foveal or eccentrical target presentation; (2) ‘passive’ head movements with fixation suppression of the VOR; (3) pure body oscillations with the head fixed in space (cervical stimulation); (4) optokinetically induced apparent self-motion (circularvection). This new visual phenomenon of a physiological ‘inhibitory interaction’ between object- and self-motion perception seems to have a somatosensory motor analogue. It may reflect the disadvantageous side effect due to unspecificness of an otherwise beneficial space constancy mechanism, which provides us with the image of a stable world during locomotion.

Chromatic adaptation and color constancy: A possible dichotomy

AbstractDifferences between chromatic adaptation and color constancy are discussed, in order to call into question the commonly held view that chromatic adaptation is the mechanism of color constancy. Whereas chromatic adaptation requires many seconds of time and occurs for simple visual scenes, color constancy asserts itself immediately and is most powerful in complex visual scenes. Furthermore, models of chromatic adaptation are not so illuminant invariant as other models of color vision. Therefore, a new operational foundation for color constancy is proposed, and existing non‐adaptation models of color constancy are enumerated for future tests.

Perception of body weight and body mass at twice earth-gravity acceleration levels.

On Earth, when standing on two feet, we experience particular patterns of force and pressure on the soles of our feet. As we lift one foot and balance on the other, little or no increase in force or pressure is perceived on the sole of the stance foot even though the contact forces of support on that foot have doubled. The failure to perceive this increase is actually an illusion resulting from the operation of spatial constancy mechanisms serving to preserve feelings of near constant force and pressure on the support surface(s) of the body. On Earth, body weight and body mass are perceived as remaining constant regardless as to whether we are standing on two feet or one and whether we are carrying large objects. In the high force phase(2 g acceleration) of parabolic flight, body weight is perceived as doubling, and a great increase in force is perceived on the soles of our feet if we are standing. When shifting balance from two feet to one, an increase in force of approximately 0.5 mg is felt on the sole of the stance foot. The actual increase in force is 1.0 mg but perceptual compensation is only being made for a 0.5 mg increase such as would be characteristic of shifting balance on Earth; accordingly an additional 0.5 mg (1.0-0.5 mg) residue is perceived. These findings indicate that body weight is dependent on the magnitude of the gravitoinertial forces acting on the body. Variations in the contact forces supporting the body due to passive or active locomotion of the body or to objects that are being carried are monitored and disregarded in computing apparent body weight. When stepping up and down from a low platform during the high force phases of parabolic flight, aberrant motion of the body and the aircraft is experienced. These illusory motions result because the doubling of body weight in a 2 g force background alters the normal relationship between patterns of alpha and gamma activation of antigravity muscles, muscle spindle activity, and the movements of the body. Accordingly, sensory-motor control and perceptual and postural stability on Earth are dependent on an active calibration to a 1 g background force level.

Schedule-induced polydipsia and size of water dipper

Schedule-induced polydipsia as a function of six water dipper sizes was examined in four food-deprived rats under Fixed Time 1- and 2-minute schedules. Increasing dipper size increased session water intake but did not affect the number of intervals containing at least one barpress (i.e., number of bouts), nor the number of barpresses per bout, nor the temporal distribution of barpressing within the interfood interval. Previous research showing polydipsic intake to be impervious to other types of restrictions on water availability have suggested the notion of a volume constancy mechanism. The present research raises questions about the operation of the volume constancy mechanism, questions about how to conceptualize the dipper size manipulation, and questions about variables which affect the temporal locus of drinking.

Eye-hand tracking using afterimages. Evidence that sense of effort is dependent on spatial constancy mechanisms.

Oculomotor tracking of one's unseen hand is greatly enhanced when a positive afterimage of the arm is projected onto its changing apparent position. The "afterimage arm" also influences the perception of real arm motion. The nature of this influence indicates that sense of effort or will derives from spatial computations potentially involving sensory and motor information about the ongoing orientation of the entire body.

Effect of body tilt on receptive field orientation of simple visual cortical neurons in unanesthetized cats.

The receptive field (RF) orientation of 53 simple visual cortical neurons was determined by recording the activity of single cells during presentation of stationary bars of light. An RF tuning curve was constructed for each cell by averaging the neural discharge resulting from the repeated presentation of a number of slit orientations. RF curves were then determined again, following a 45 degrees roll tilt of the entire head and body, and subsequently after the return of the animal to the original horizontal position. RF tuning curves were typical of what others have found to characterize simple cells, and were highly replicable on the return to the starting position. In 73% of the cells studied, the RF orientation after tilt remained unaltered relative to the head axis (+/- 15 degrees); in the remaining 27% of the cells RF orientations either under- or over-shot the retinal tilt by more than 15 degrees, and in some cases by as much as 45 degrees. These results support the hypothesis that the well documented vestibular inputs to visual cortex play a role in modifying the RF orientation selectively of visual cortical neurons, and suggest that such information may be an important neurophysiological substrate underlying visual spatial constancy mechanisms.

Comparison of the constancy mechanisms in the cereal systems of crickets (Acheta domesticus andGryllus bimaculatus)

Characteristics of the constancy mechanisms for directional selectivity shown to exist in the terminal abdominal ganglion (TAG) of the cricket were estimated and compared in the two cricket speciesAcheta domesticus andGryllus bimaculatus. Directional properties of TAG neurons were studied using extracellular recordings from their axons in the connectives. The stimuli were sounds (400 Hz) and airstreams of various directions. Directional sensitivity diagrams (DSD) of TAG neurons were plotted for 5–10 positions of the cerci. Small shifts in DSD axis orientation and variations in DSD amplitude due to step changes in the position of the cerci were assessed.

Die sehwinkelkonstanz des subjektiven schachbrettmusters, ein hinweis auf den kortikalen entstehungsort der gröβenkonstanz

It is shown that the visual angle of the fields of the subjective checkerboard pattern, which becomes visible with flicker stimulation of both eyes, remains constant if the viewing distance is changed. This implies that the checkerboard pattern is subject to the size constancy mechanism. From this result and the cortical origin of the checkerboard pattern we conclude that the size constancy must be of cortical origin.

Color perception and the limits of color constancy.

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

An illusory depth reversal dependent upon rotary motion.

A textured rotating disk viewed obliquely was found to perceptually reverse its orientation in depth under a variety of conditions, none of which were able to eliminate the illusory depth reversal. The only necessary conditions for its production appear to be that some texture be present, that the stimulus be moving, and that the stimulus be viewed from an oblique angle. Qualitative observations indicated that changes in the perceived size and speed of the texture elements of the rotating stimulus consistent with constancy mechanisms for size and speed accompanied changes in apparent depth orientation.

Visual direction depends on the operation of spatial constancy mechanisms: the oculobrachial illusion

Subjects fixating a target light attached to their stationary hand saw it move when illusory motion of their arm was induced by muscle vibration. During the experienced visual motion and change in visual direction of the target light, their eyes maintained steady fixation. The existence of an ‘oculobrachial ilusion’ provides evidence that visual direction depends on the operation of a spatial constancy mechanism interrelating sensory information about the external environment and the moment-to-moment postural configuration of the body.