Lightness effects in Delboeuf and Ebbinghaus size-contrast illusions

We examined lightness effects observed in Delboeuf and Ebbinghaus size-contrast illusions. Results of four experiments are reported. Experiment 1 was conducted with Delboeuf-like stimuli and shows that the disk that appears bigger appears either lighter or darker than the disk that appears smaller, depending on the contrast polarity between disks and background. Experiment 2 shows that the direction of these lightness effects is not influenced by the luminance of the size-contrast inducers. Experiment 3 shows that a similar lightness effect is also observed in modified Ebbinghaus size-contrast displays. Experiment 4 tested the presence of the size-contrast illusion in the stimuli used in experiments 2 and 3.     

Similarity and lightness effects in Ebbinghaus illusion created by keyboard characters

36 observers judged the size of a central S in variants of the Ebbinghaus figure having contextual Ss, $s, or Hs. When the figures were composed of similarly shaped elements, underestimation of the central S was obtained. Manipulations of lightness indicated that these underestimations were strongest for figures with gray contextual characters and a black central S and weakest for figures with black contextual characters and a gray central S. All black or all gray figures produced intermediate illusions. The data are consistent with Choplin and Medin's 1999 claim that figural properties rather than semantic similarity influences size contrast and further show that the visual processes underlying size contrast include interactions of contours.     

The perception of melodies distorted by splitting into several octaves: Effects of increasing proximity and melodic contour

Splitting up the familiar tune “Yankee Doodle” by placing successive notes in different octaves makes it very difficult to recognize. The present experiments generalize this finding to more than one familiar tune and to presentation by actual musical instruments. Tunes differ in the degree to which their recognizability is affected by scrambling into different octaves, but recognition of all 10 tunes investigated was substantially diminished by the distortion. Increasing pitch proximity of temporally adjacent notes by leaving pairs or triples of successive notes intact within the same octave increases recognizability of at least some scrambled tunes. Leaving the melodic contour (the pattern of ups and downs) intact while splitting up the melody increases recognizability for subjects informed of the preservation of contour, though recognition is still worse than for undistorted versions.     

Real and subjective contour Poggendorff illusions: no differences based on eye color

Measures of illusion magnitude for a real contour and two subjective contour Poggendorff figures were obtained from a large number of dark-eyed and light-eyed undergraduates. Illusion magnitude varied as a function of figure type. However, eye color neither had a significant overall effect on illusion magnitude, nor did it interact with the figure-type variable. These data indicate that the presence of intersecting lines is not the basis of the iris-pigmentation effect reported by Coren and Porac.     

The proximity structure of achromatic surface colors and the impossibility of asymmetric lightness matching

In asymmetric lightness matching tasks, observers sometimes report that they cannot achieve satisfactory matches between achromatic surfaces under different neutral illuminants. The surfaces appear different, yet no further adjustment of either surface improves the match. There are evident difficulties in interpreting data from a task that the observer cannot always do, and these difficulties likely affect the interpretation of a large number of previous studies. We investigated, as an alternative to asymmetric matching, the direct use of proximity judgments in the study of surface lightness perception. We asked observers to rate the perceived dissimilarity of pairs of achromatic surfaces that were placed in identical scenes and viewed under different neutral illuminants. We develop a parametric model that accurately predicts perceived dissimilarity in terms of physical light intensities and surface albedos. The parameters of this model are readily interpretable. In particular, the ratio of the influence of changes in illuminant intensity and changes in surface albedo is a measure of the extent to which the observer discounts the illuminant. Asymmetric lightness matching can be interpreted as an unachievable limiting case of proximity judgment.     

Determinants of subjective contour: Bourdon illusions and "unbending" effects

Wenderoth and O'Connor (1987b) reported that, although matches to the straight edge of two triangles placed apex to apex revealed an apparent bending in the direction of the chevron formed by the hypotenuse pair (the Bourdon effect), no perceptual unbending of the bent chevron occurred. Using subjective contour figures, Walker and Shank (1988b) found large and approximately equal bending and unbending effects, consistent with two theories that they proposed. In Experiment 1, using adjustable chevron matching and subjective contours, we found that Bourdon effects, equivalent in magnitude to those reported by Walker and Shank, were 4-5 times larger than unbending effects. In Experiment 2, we used a variation of Walker and Shank's measurement technique, in which subjects selected a matching angle from a graded series. We obtained Bourdon effects similar to those in Experiment 1, but much larger unbending effects. Nevertheless, Bourdon effects were significantly larger than unbending effects in one set of data; and in another, Bourdon test means were larger than unbending test means. In both data sets, there was a large and significant pretest bending effect, which enhanced the magnitude of unbending test minus pretest scores. These results were consistent with our theory but not the theories of Walker and Shank. The variance of unbending test matches, 3-4 times that of Bourdon test matches, reflected the task difficulty. We propose that subjective obtuse angle contraction that exceeds real obtuse angle contraction explains the fact that unbending effects are larger in subjective than in real contours.     

Effects of visual illusions on grasping

In 2 experiments, the Muller-Lyer illusion (F. C. Muller-Lyer, 1889; N = 16) and the parallel-lines illusion (W. Wundt, 1898; N = 26) clearly affected maximum preshape aperture in grasping (both ps < .001). The grasping effects were similar but not perfectly equal to the perceptual effects. Control experiments show that these differences can be attributed to problems in matching the perceptual task and the grasping task. A model is described stating the assumptions that are needed to compare the grasping effects and the perceptual effects of visual illusions. Further studies on the relationship between perception and grasping are reviewed. These studies provide no clear evidence for a dissociation between perception and grasping and therefore do not support the action versus perception hypothesis (A. D. Milner & M. A. Goodale, 1995).     

Effects of luminance oscillations on simulated lightness discriminations

The speed of processes underlying lightness constancy was studied by having observers discriminate small differences in simulated lightness under an oscillating illumination. The period of oscillation varied from 0.25 to 120 sec. The target was a 1° square which appeared for 150 msec at random intervals either directly against a uniform background or separated from the background by a 1° dark gap. When the target and background were adjacent to each other, discrimination accuracy approached control levels (fixed illumination) at all but the shortest periods of oscillation. When the gap was introduced, accuracy increased as the period of oscillation increased, but never approached control levels. The results suggest that a fast local contrast mechanism is the primary mediator of lightness constancy for this task, but that there is also a slower mechanism that may be related to adaptation.     

Effects of luminance oscillations on simulated lightness discriminations

The speed of processes underlying lightness constancy was studied by having observers discriminate small differences in simulated lightness under an oscillating illumination. The period of oscillation varied from 0.25 to 120 sec. The target was a 1 degrees square which appeared for 150 msec at random intervals either directly against a uniform background or separated from the background by a 1 degrees dark gap. When the target and background were adjacent to each other, discrimination accuracy approached control levels (fixed illumination) at all but the shortest periods of oscillation. When the gap was introduced, accuracy increased as the period of oscillation increased, but never approached control levels. The results suggest that a fast local contrast mechanism is the primary mediator of lightness constancy for this task, but that there is also a slower mechanism that may be related to adaptation.     

In search of closure: subjective contour illusions, Gestalt completion tests, and implications

This study investigates the construct validity of perceptual closure tests (CTs), and isolates a common processing demand from the right-hemisphere. Sixty-seven patients with focal unilateral lesions (34 right side, 33 left side), and 80 control subjects participated. Multivariate analyses indicated that there was substantial age-related variance in all CTs, while sex was variably significant; CTs are not uniform in their discriminating ability, and hence factorially complex; and their specifically right hemisphere-sensitive dimension was subjective contour illusions, and interestingly unrelated to facial discrimination ability. The methodological and theoretical implications are discussed.     

Effects of social proximity on multiple aggressive behaviors

We systematically manipulated social proximity to examine its influence on multiple topographies of aggression. Aggression occurred at high levels during close-proximity sessions and at low levels during distant-proximity sessions even though social contact was presented continuously during both conditions. Topographies of aggression emerged sequentially across the close-proximity sessions, and all topographies were reduced to zero following extended sessions of this condition.     

Metacognitive illusions for auditory information: Effects on monitoring and control

Prior work has demonstrated that the perceptual features of visually presented stimuli can have a strong influence on predictions of memory performance, even when those features are unrelated to recall (Rhodes & Castel, 2008). The present study examined whether this finding would hold in an auditory domain and influence study-choice allocation. Participants listened to words that varied in volume, made judgments of learning (JOLs) for each item, and were then administered a test of free recall. In Experiment 1, we showed that JOLs were influenced by volume, with loud words given higher JOLs than quiet words, and that volume had no influence on recall, illustrating a metacognitive illusion based on auditory information. In Experiment 2, we extended these findings to control processes and showed that participants were more likely to choose to restudy quiet words than loud words. These findings indicate that highly accessible auditory information is integrated into JOLs and restudy choices, even when this information does not influence actual memory performance.     

Effects of tactual and phonological similarity on the recall of Braille letters by blind children.

Forty-eight severly blind children, able to cope with set-sizes of two and three, four, five or six serial items were tested on probed recall of Braille letters they could either name or discriminate, when the letters were (1) dissimilar in tactual characteristics and name sounds, (2) similar in feel, but different in name sound, and (3) dissimilar in feel but similar in name sound. Results showed that tactual similarity produced recall decrements by subjects able to be tested under set-sizes of up to five items, while phonological similarity produced recall decrements by subjects testable under set-sizes of five and six items. It was argued that tactual information survives perception and affects recall, and that blind children can encode tactual inputs either tactually or verbally, but that this probably depends upon different processes.     

Illusory illusions: the significance of fixation on the perception of geometrical illusions

Some well-known geometrical illusions disappear when the eyes are fixating and saccades are suppressed for a period of time. This disappearance is not accompanied by fading due to stabilisation of the retinal image. Any saccade made on purpose restores the illusion immediately. The fixation time after which some illusions disappeared was measured for four illusions and four subjects each. Effects of practice have been observed after measurements were repeated on successive days. Present theories of vision cannot readily explain the effect.     

Simultaneous lightness contrast on plain and articulated surrounds

Simultaneous lightness contrast is stronger when the dark and light backgrounds of the classic display (where one of the targets is an increment and the other is a decrement) are replaced by articulated fields of equivalent average luminances. Although routinely attributed to articulation per se, this effect may simply result from the increase in highest luminance in the light articulated, vs plain, background; by locally darkening the decremental target, such an increase would amplify the difference between the targets. We disentangled the effects of highest luminance and articulation by measuring, separately, the magnitude of lightness contrast on dark and light plain and articulated backgrounds. We found that highest luminance and articulation contribute separately to the final illusion.     

Relative influences of lightness and facial morphology on perceived race

In a recent study (Brooks and Gwinn, 2010 Perception 39 1142-1145), the lightness contrast illusion was employed to study the influences of skin tone and facial morphology on race perception. The findings were rather counterintuitive: they suggested that skin tone does not play a major role in racial categorisation. To investigate this further, we used a parametric paradigm including five lightness levels, five morphing levels, and two face orientations. In accordance with Brooks and Gwinn, we found that race categorisation of African-American and Caucasian faces by Caucasian participants relied mainly on morphological cues. However, the relative influence of lightness increased when morphological information was ambiguous and when the faces were upside down. Overall, the results point to a flexible multicue-based mechanism underlying race perception.