The phantom illumination illusion

A novel brightness illusion in planar patterns is reported. The illusion occurs, for example, when surfaces with a luminance ramp shaded from black to white are positioned on a black homogeneous background, so that each white end of the surfaces faces a single point of the plane of the pattern. The illusion consists of the enhancement of the brightness of the background in a relatively wide area around the white ends of the surfaces. A parametric study was conducted in which participants were asked to rate the difference in brightness between the parts of the background inside and outside a virtual circle formed by disks with different luminance ramps. The results show that mean ratings of brightness depended on the luminance of the background, the luminance range of ramps, and the kind of ramp. Discussion of these results with reference to other brightness illusions (assimilation, neon color spreading, anomalous surfaces, visual phantoms, grating induction, and the glare effect) shows that t hephantom illumination illusion derives from processes producing the perception of ambient illumination.     

When figure-ground segmentation modulates brightness: the case of phantom illumination

In the phantom illumination illusion, luminance ramps ranging from black to white induce a brightness enhancement on an otherwise homogeneous dark background. The strength of the illusion was tested with regard to the extension of the brightness inducing perimeter, surrounding the target area by manipulating the number of inducers (exp. 1) and the size of the inducers (exp. 2). Participants' task was to rate the difference in brightness between the target area and the background. Results show that the illusion occurs only when the target area is not completely segregated from the background by luminance ramps; vice versa, when the target area is delimited by a continuous gradient, it appears darker than the background. These findings suggest a major role of figure-ground organization in the appearance of the illusion. This hypothesis was tested in a rating task experiment with three types of target area shapes circumscribed by four types of edges: luminance contours, illusory contours, no contours, and ambiguous contours. Illusory contours, just as luminance contours, hinder the illusion and produce a darkening of the target area. A control experiment measured the brightness of the previous stimuli without luminance ramps: all configurations resulted in a darkening of the target area. Results from all experiments suggest that figure-ground segmentation plays a major role in the determination of both illumination and lightness in stimuli with luminance gradients.     

Relative area and relative luminance combine to anchor surface lightness values.

The anchoring of lightness perception was tested in simple visual fields composed of only two regions by placing observes inside opaque acrylic hemispheres. Both side-by-side and center/surround configurations were tested. The results, which undermine Gilchrist and Bonato's (1995) recent claim that surrounds tend to appear white, indicate that anchoring involves both relative luminance and relative area. As long as the area of the darker region is equal to or smaller than the area of the lighter region, relative area plays no role in anchoring. Only relative luminance controls anchoring: The lighter region appears white, and the darker region is perceived relative to that value. When the area of the darker region becomes greater than that of the lighter region, relative area begins to play a role. As the darker region becomes larger and relative area shifts from the lighter region to the darker region, the appearance of the darker region moves toward white and the appearance of lighter region moves toward luminosity. This hitherto unrecognized rule is consistent with almost all of the many previous reports of area effects in lightness and brightness. This in turn suggests that a wide range of earlier work on area effects in brightness induction, lightness contrast, lightness assimilation, and luminosity perception can be understood in terms of a few simple rules of anchoring.     

Relative area and relative luminance combine to anchor surface lightness values

The anchoring of lightness perception was tested in simple visual fields composed of only two regions by placing observers inside opaque acrylic hemispheres. Both side-by-side and center/surround configurations were tested. The results, which undermine Gilchrist and Bonato’s (1995) recent claim that surrounds tend to appear white, indicate that anchoring involves both relative luminance and relative area. As long as the area of the darker region is equal to or smaller than the area of the lighter region, relative area plays no role in anchoring. Only relative luminance controls anchoring: The lighter region appears white, and the darker region is perceived relative to that value. When the area of the darker region becomes greater than that of the lighter region, relative area begins to playa role. As the darker region becomes larger and relative area shifts from the lighter region to the darker region, the appearance of the darker region moves toward white and the appearance of lighter region moves toward luminosity. This hitherto unrecognized rule is consistent with almost all of the many previous reports of area effects in lightness and brightness. This in turn suggests that a wide range of earlier work on area effects in brightness induction, lightness contrast, lightness assimilation, and luminosity perception can be understood in terms of a few simple rules of anchoring.     

Perceiving the intensity of light

The relationship between luminance (i.e., the photometric intensity of light) and its perception (i.e., sensations of lightness or brightness) has long been a puzzle. In addition to the mystery of why these perceptual qualities do not scale with luminance in any simple way, "illusions" such as simultaneous brightness contrast, Mach bands, Craik-O'Brien-Cornsweet edge effects, and the Chubb-Sperling-Solomon illusion have all generated much interest but no generally accepted explanation. The authors review evidence that the full range of this perceptual phenomenology can be rationalized in terms of an empirical theory of vision. The implication of these observations is that perceptions of lightness and brightness are generated according to the probability distributions of the possible sources of luminance values in stimuli that are inevitably ambiguous.     

Attention,brightness contrast,and assimilation: The influence of relative area

A model of information transmission in the visual system which describes the effect of attention on apparent brightness is examined. This model states in part that the luminance of the portion of the visual field which captures the attention is overweighted in arriving at an overall average luminance level across the visual field. As this average must be computed with respect to both luminance and relative area, it is hypothesized that increasing the relative area of the portion of the visual field that captures the attention will result in a greater effect on the apparent brightness of all parts of the visual field. Two predictions, which involve the effect of relative area on apparent brightness, are experimentally tested and confirmed.     

An orientation illusion analog to the rod and frame: Relational effects in the magnitude of the distortion

In previous studies, we have measured individual differences in simultaneous brightness discrimination between a long-duration (500 msec or more) comparison pulse and a set of briefer test pulses of the same luminance. Some observers judge the comparison pulse to be brighter than all test pulses in a range from 20 to 320 msec. Other observers exhibittemporal brightness enhancement: a test pulse of 80 msec for our stimulus conditions is judged brighter than the comparison pulse. Individual differences depend upon the temporal asynchrony between test and comparison pulses. Observers designated Type A show temporal brightness enhancement for both simultaneous onset of test and comparison pulse and for simultaneous offset of the pulses. Type B observers exhibit brightness enhancement for simultaneous offset of pulses but not for simultaneous onset. Type C observers do not exhibit brightness enhancement effects. Here we use both short (80 msec) and long (640 msec) pulses as the comparison stimuli. We replicate our previous pattern of individual differences for the long comparison stimulus. For the short comparison stimulus, brightness discrimination functions for the three classes of observers are exactly predicted from the patterns obtained with the conventional long comparison pulse. This confirms the generality of individual differences in pulse brightness perception and shows that they do not depend upon the particular stimulus conditions selected for the discrimination task.     

Temporal brightness and darkness enhancement; further evidence for asymmetry

Abstract.— Previous studies by the authors on brightness and darkness enhancement during flicker indicate a sizeable and consistent difference in magnitude between the two effects, as defined in terms of log matching luminance. To explore whether this difference is an artifact of the logluminance representation of the perceptual effects, a psychophysical scaling experiment was performed and results from present and former flicker experiments were replotted in terms of subjective units. When the depth of luminance modulation of the flicker stimulus exceeded 50%, the difference in magnitude of temporal brightness and darkness enhancement was somewhat smaller expressed in subjective units compared to a log-luminance representation. On the whole, however, the two plots gave essentially similar results. The results are discussed with reference to the neurophysiological theory of the B- and D-systems.     

Duration,luminance, and the brightness exponent

The relation of brightness to duration and luminance has been studied by matching one brightness to another and also by matching numbers to brightnesses (magnitude estimation). The two methods concur in confirming certain well-known visual functions: Bloch’s law, the Broca-Sulzer effect, and the shift of the Broca-Sulzer enhancement to shorter durations when luminance increases. It is shown that the shift with luminance requires the exponent of the power function for short-flash brightness to be larger than the exponent for stimuli of longer duration. An attempt is made to analyze some of the reasons why the procedure advocated by Graham may not give comparable results.     

Orientation-specific luminance aftereffects

Prolonged viewing of bright vertical (horizontal) gratings alternating with dim horizontal (vertical) gratings generates negative brightness aftereffects that are contingent on the orientation of orthogonal test gratings. The effect is measured by a brightness cancellation technique, similar to the color cancellation technique used in measuring McCollough effects. Like the latter, brightness aftereffects appear to persist for long periods. The magnitude of these aftereffects is a positive monotonic function of the luminance difference between the inducing gratings, and it depends on the conditions of induction; monocular induction generates larger aftereffects than binocular induction does. The aftereffect transfers interocularly, although its magnitude in the contralateral eye is substantially attenuated; binocular measurement, following monocular induction, results in even smaller aftereffects. An attempt to understand these findings within the computational model of brightness perception developed by Grossberg and Mingolla (1985a, 1985b) is presented.     

Brightness enhancement and the Talbot level in stationary gratings

At low spatial frequencies, the perceived brightness of the light phase of a stationary square-wave grating is greater than the brightness of a solid field of equal physical luminance. That increase in the perceived brightness of a grating at low spatial frequencies is analogous to the brightness enhancement observed in a flickering light at low temporal frequencies. At or above the critical spatial frequency—the visual resolution threshold—the brightness of a grating is determined by its space-average luminance, just as the brightness of a flickering light at or above the critical flicker frequency is determined by its time-average luminance in accordance with Talbot’s law. Thus, Talbot’s law applies in the spatial as well as the temporal domain. The present study adds to the evidence that temporal and spatial frequency play analogous roles in some aspects of brightness vision.     

Orientation-specific aftereffects and illusions in the perception of brightness

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.     

A PARADOX IN THE PERCEPTION OF LUMINANCE GRADIENTS. II

The first paper in this series was a study on a brightness paradox in the perception of luminance gradients in space. This study tests the hypothesis that an inducing field of higher or lower luminance having a luminance acceleration in space is a necessary condition for the paradox to appear. A magnitude estimation and a constant-sum method were used. The main result was a falsification of the hypothesis. A luminance acceleration across the inducing field was not necessary but it enhanced the paradox. The results are discussed in relation to theories on neural inhibition.     

A PARADOX IN THE PERCEPTION OF LUMINANCE GRADIENTS. I

Under certain conditions subjects looking at a luminance gradient report a physically darker part of the gradient to be brighter than an adjacent area of higher luminance. This brightness paradox was studied in a series of experiments using a magnitude estimation method. The main results were that both the changing sign of the second derivative of the luminance function (Mach's hypothesis) and the higher or lower luminance of an adjacent area (McDougall's drainage theory) are critical conditions for the appearance of the paradox. In the present study none of these conditions per se resulted in a brightness paradox.     

Local brightness mechanisms sketch out surfaces but do not fill them in: psychophysical evidence in the Kanizsa square.

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous contrast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that "sketches out" surfaces at their surrounds but does not "fill" them "in."     

PSYCHOPHYSICAL INVARIANTS OF ACHROMATIC COLOURVISION. V

LIE, I. Psychophysical invariants of achromatic colour vision. V. Brightness as a function of inducing field luminance. Scand. J. Psychol. , 1971, 12, 61–64.– Brightness (the dim-bright dimension) of achromatic colour was investigated as a function of surrounding field luminance. It was found that brightness of a test area was relatively independent of luminance level of the surrounding area until the luminance of the latter had passed well beyond that of the test area. With further increase of surrounding luminance, the brightness of the test area increased rapidly.     

Contextual illusions reveal the limit of unconscious visual processing

The perception of even the most elementary features of the visual environment depends strongly on their spatial context. In the study reported here, we asked at what level of abstraction such effects require conscious processing of the context. We compared two visual illusions that alter subjective judgments of brightness: the simultaneous brightness contrast illusion, in which two circles of identical physical brightness appear different because of different surround luminance, and the Kanizsa triangle illusion, which occurs when the visual system extrapolates a surface without actual physical stimulation. We used a novel interocular masking technique that allowed us to selectively render only the context invisible. Simultaneous brightness contrast persisted even when the surround was masked from awareness. In contrast, participants did not experience illusory contours when the inducing context was masked. Our findings show that invisible context is resolvable by low-level processes involved in surface-brightness perception, but not by high-level processes that assign surface borders through perceptual completion.     

Perceived brightness as a function of flash duration in the peripheral visual field

Using a method of direct magnitude estimation, perceived brightness was measured in the dark-adapted eye with brief flashes of varying duration (1–1,000 msec), size (16’–116’), and retinal loci (0°–60°) for the lower photopic luminance levels covering the range between 8.60 and 86 cd/m² in steps of .5 log units. Perceived brightness increased as a function of flash duration as well as luminance up to approximately 100 msec, then remained constant above 100 msec. The enhancement of brightness at about a 50-msec flash duration has been observed not in the fovea but in the periphery. Target size also has been found to be effective on brightness.     

Local brightness mechanisms sketch out surfaces but do not fill them in: Psychophysical evidence in the Kanizsa square

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous con trast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that “sketches out” surfaces at their surrounds but does not “fill” them “in.”     

The effect of figural manipulations on brightness differences in the Benary cross

The Benary cross is a classical demonstration showing that the perceived brightness f an area is not solely determined by its luminance, but also by the context in which it is embedded. Despite the fact that two identical grey triangles are flanked by an equal amount of black and white, one of the triangles is perceived as being lighter than the other. It has been argued that the junctions surrounding a test area are crucial in determining brightness. Here, we explored how different aspects influencing perceptual organisation influence perceived figure-background relations in the Benary cross and, with that, the perceived brightness of the triangular patches in our stimuli. The results of a cancellation task confirm that the alignment of contours at junctions indeed has a strong influence on an area's brightness. At the same time, however, the Benary effect is also influenced by the overall symmetry of the cross and its orientation.