Assimilation of achromatic color cannot explain the brightness effects in the achromatic neon effect

If the mouths of the pacmen of a Kanizsa square are colored, for example red, then an illusory red transparent square is seen. In many visual theories such 'neon color spreading' is explained by assimilation of chromatic and achromatic color. In this paper the achromatic case was investigated. In a two-alternative forced-choice task thirty observers judged the brightness of achromatic neon figures. The results suggest that assimilation of achromatic color inside and/or outside of the illusory figures cannot explain the brightness effects seen in achromatic neon color spreading. Although these displays may produce assimilation, it appears that contrast (perhaps acting nonlocally) is a stronger influence on their perceived brightness.     

The Sander illusion as a function of relative space and component lines

Magnitude of the Sander illusion was measured as a function of two variables: (1) the orientation of the line normally separating the two smaller parallelograms and (2) the presence or absence of the horizontal lines and of the diagonal test lines. The results showed that, as the angle of the dividing line varied so as to shift the relative sizes of the two parts of the figure toward equality, the illusion effect decreased, approaching zero when the two areas were equal. The illusory effect was enhanced by the removal of the two test lines. Results were discussed in relation to the problem of assimilation vs contrast effects.     

Illusory contours: On the efficacy of their need for expression

When an illusory-figure pattern is presented against a physical lightness gradient, the illusory figure’s lightness relative to the surround is often the reverse of the usual effect, a fact that suggests that, rather than the figure’s depending upon illusory lightness, real lightness differences can be utilized in its expression. A second demonstration suggests that the apparent strength of the lightness effect may be greater where there is relatively less physical expression of the contour. Again, the establishment of the contour seems to involve more than merely “capturing” illusory lightness.     

Illusory form with inducers of opposite contrast polarity: Evidence for multistage integration

The perception of brightness differences in Ehrenstein figures and of illusory contours in phaseshifted line gratings was investigated as a function of the contrast polarity of the inducing elements. We presented either continuous lines or line-like arrangements composed of aligned dashes or dots whose spacing was varied. Ayes/no procedure was used in which naive observers had to decide whether or not they perceived a brightness difference in a given Ehrenstein figure or an illusory contour in a phase-shifted line grating. The results show that brightness differences are perceived to some extent in Ehrenstein figures with inducers of opposite polarity of contrast; however, the percentage ofyes responses was systematically lower and response times were longer than for figures with inducers of the same polarity. Phase-shifted line gratings with lines of opposite polarity of contrast yielded stronger illusory contours and shorter response times than those with lines of the same polarity. When the sign of contrast was not the same within a given line of induction, neither differences in brightness nor illusory contours were perceived. The results suggest that the mechanisms that lead to apparent differences in brightness are more sensitive to input of the same contrast polarity, the mechanisms generating illusory contours more sensitive to input of opposite polarity. The data are discussed in the light of a multistage approach to illusory form perception and some implications for cortical models of illusory contour integration are discussed.     

The perceived strength of illusory contours.

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

Illusory figures: Individual differences apparent depth and lightness

When the luminance of a pattern that induces a sharp-edged illusory circle was decreased, both illusory contrast with the surround and the illusory depth difference between that circular area and the inducing elements reliably increased. Thus, changes in both of these effects apparently participate in (contribute to) the increased overall salience that had previously been found with decreased luminance. However, among individuals, the correlation between this improvement in contrast and this increase in depth was exceedingly small.     

The perceived strength of illusory contours

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength.. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.     

Illusory-figure lightness: evidence for a two-component theory

Physical darkness gradients were introduced into an illusory-figure pattern which were sufficient to eliminate the illusory lightness effect otherwise seen there. Next, such a gradient was left in place along one edge of that figure while the other edges were returned to physical homogeneity with the surround. Rather surprisingly, the physical gradient still present along the unaltered edge was now overwhelmed by illusory lightness. It is argued that this anomaly suggests that illusory-figure lightness may be composed of two additive components, one of which is figure-ground contrast enhancement and the other of which is contingent upon that first component.     

The assimilation theory applied to a modification of the Müller-Lyer illusion

The assimilation theory of illusions, which utilizes the principle that large magnitudes in a series are underestimated and small magnitudes are overestimated, was applied to a modification of the Müller-Lyer illusion. A close link was shown to exist between the Müller-Lyer illusion, a modification of the Müller-Lyer illusion, and the Sander parallelogram. In addition, a new illusory target was predicted.     

THE CONSISTENCY OF NIGERIANS' PERFORMANCE ON THREE TASKS MEASURING FIELD DEPENDENCE

The study attempts to examine the consistency of Nigerians' performance on three measures of field dependence. The measures used were the Embedded Figures Test (EFT), Block Design (BD) and the Rod and Frame Box (RFB), and samples were drawn from a university (N = 48) and two local schools (N = 38) located in east-central Nigeria. In line with previous studies, the most significant correlation was between EFT and BD. Interpretation of the partial correlations suggested that EFT and BD may share at least two common factors but that each appears to share only one of these with RFB. These factors were discussed in terms of the hemispheric processing requirements of these measures. It was suggested that EFT and BD, which involve cognitive restructuring, may require an ability to integrate the processing abilities of the two hemispheres, whereas RFB may only involve left hemisphere processing. This interpretation may also help to explain some of the sex and age effects found in the present study. More research is needed to help isolate the various processing requirements of the different measures of field dependence.     

Children with spina bifida perceive visual illusions but not multistable figures

We compared 32 children with spina bifida and 32 age-matched controls on two classes of illusory perception, one involving visual illusions and the other, multistable figures. Children with spina bifida were as adept as age peers in the perception of visual illusions concerned with size, length, and area, but were impaired in the perception of multistable figures that involved figure-ground reversals, illusory contours, perspective reversing, and paradoxical figures. That children with spina bifida reliably perceive illusions that rely on inappropriate constancy scaling of size, length, and area suggests that their brain dysmorphologies do not prevent the acquisition of basic perceptual operations that enhance the local coherence of object perception. That they do not perceive multistable figures suggests that their visual perception impairments may involve not object processing so much as poor top-down control from higher association areas to representations in the visual cortex.     

Amodal completion versus induced inhomogeneities in the organization of illusory figures.

An analysis is presented of a phenomenological model of illusory contours. The model is based on amodal completion as the primary factor giving rise to the illusory figure. In the experiment, conducted by the method of paired comparisons, the same parameter was manipulated in two series of equivalent configurations. The first series yielded examples of amodal completion, the second examples of illusory figures. Three groups of subjects evaluated the magnitude of completion, the brightness contrast of the illusory figure, and the contour clarity of the illusory figure. A control experiment was conducted, which demonstrated that in these configurations amodal completion and amodal continuation behave in the same way. Line displacement did not influence the brightness or the contour clarity of the illusory figures, though it influenced the magnitude of amodal completion. These results are in agreement with the energetic model developed by Sambin.     

Illusions of size change in dynamic displays

A line of constant width viewed against an expanding or contracting grating appears to become narrower or wider, respectively. This effect was studied using a computer-controlled video system with a motorized zoom lens. The magnitude of the illusory size (width) change with a vertical line did not differ when viewed against a horizontal or an oblique background, and the size change was not due to static size contrast between the line and the background. The illusory narrowing observed with an expanding background was equivalent to the widening observed with a contracting background. The apparent change in the width of lines that actually expanded was consistently underestimated, either on a similarly changing background or when viewed against a gray field. Although the transformation of the background and line produced changes in their perceived distance, this did not occur for a constant line against a transforming background, even though this condition induced the perceived size change.     

Apparent motion of subjective surfaces

Apparent motion of an illusory surface was produced by presenting two spatially separated illusory squares in an appropriately timed sequence. Control experiments showed that the effect arose from the illusory contours themselves and not from motion of the cut sectors on the discs. When a template of this movie was superimposed on 'wallpaper' composed of a regular matrix of spots, the spots appeared to move with the illusory surface even though they were physically stationary. This effect ('motion capture') suggests that the motion of certain salient features in the visual field gets spontaneously attributed to even static elements in the vicinity.     

Structure-from-motion: dissociating perception, neural persistence, and sensory memory of illusory depth and illusory rotation

In the structure-from-motion paradigm, physical motion on a screen produces the vivid illusion of an object rotating in depth. Here, we show how to dissociate illusory depth and illusory rotation in a structure-from-motion stimulus using a rotationally asymmetric shape and reversals of physical motion. Reversals of physical motion create a conflict between the original illusory states and the new physical motion: Either illusory depth remains constant and illusory rotation reverses, or illusory rotation stays the same and illusory depth reverses. When physical motion reverses after the interruption in presentation, we find that illusory rotation tends to remain constant for long blank durations (T _blank ≥ 0.5 s), but illusory depth is stabilized if interruptions are short (T _blank ≤ 0.1 s). The stability of illusory depth over brief interruptions is consistent with the effect of neural persistence. When this is curtailed using a mask, stability of ambiguous vision (for either illusory depth or illusory rotation) is disrupted. We also examined the selectivity of the neural persistence of illusory depth. We found that it relies on a static representation of an interpolated illusory object, since changes to low-level display properties had little detrimental effect. We discuss our findings with respect to other types of history dependence in multistable displays (sensory stabilization memory, neural fatigue, etc.). Our results suggest that when brief interruptions are used during the presentation of multistable displays, switches in perception are likely to rely on the same neural mechanisms as spontaneous switches, rather than switches due to the initial percept choice at the stimulus onset.     

A double neon colour illusion

A new visual illusion is presented. When two neon-coloured illusory bands overlap each other, the filling-in at the region of overlap is perceived as different from those of the overlapping bands, resulting in an additional illusory shape at the region of overlap. An experiment with ten na?ve participants was performed to measure the illusory percept. The results show that illusory surfaces may interact with each other, and, to a certain extent, create new illusory percepts.     

Subjective rarefaction in illusory figures: the inadequacy of apparent lightness as an explanation

Apparent rarefaction in subjective figures seems to violate the principle of size scaling. It has been claimed that this anomaly is due to a difference in illusory lightness which counteracts the expected effect of illusory depth. Qualitative evidence is presented that neither illusory lightness nor illusory depth have a relevant part in the phenomenon. An alternative account, in which contrast and size are shown to play major roles, is presented and discussed.     

Subjective organization and the visibility of illusory contours

The effects of the differential organization of a Necker cube on the perceived salience of an embedded illusory triangle were examined. Overall figural salience was greater when the triangle appeared to be localized on the front rather than the back cube face. Illusory contour salience also increased with increasing inducing area contrast and was greater when the figure was oriented on cardinal as compared to oblique coordinates. However, the latter effects were independent of perceived location within the cube. The finding that subjective organizational and structural factors influenced the perceived salience of the illusory figure but did not interact is consistent with van Tuijl and Leeuwenberg's (1982) suggestion that top-down and bottom-up determinants can operate independently in illusory contour perception.     

The perception of moving subjective contours by 4-month-old infants

We investigated whether 4-month-old infants are capable of perceiving illusory contours produced by the Kanizsa-square display, first introduced by Prazdny (1983, Perception & Psychophysics 34 403-404), which tests whether a viewer perceives the illusory contour in the absence of brightness contrast (illusory brightness). Because the illusory square appears to move across the computer screen and infants are attracted to motion, this display holds their interest. In experiment 1, 4-month-old infants were tested for their ability to distinguish between a continuously moving illusory square and a continuously moving control display in which the pacman elements were rotated so that the perception of subjective contours did not occur. Data analysis revealed a significant preference for the subjective contour display. In experiment 2, habituation-dishabituation was used with 4-month-old infants. They were tested for their ability to discriminate between the illusory Kanizsa square that continuously moved back and forth and an illusory square which changed positions randomly. Although the infants did not show differences in dishabituation as a function of the habituation display, they looked significantly longer at the continuously moving display.     

Perceptual dimorphism in visual motion from stationary patterns

Fraser and Wilcox [1979 Nature (London) 281 565-566] devised a series of complex stationary patterns that provoked episodes of compelling illusory motion, but only in about two-thirds of people tested. Using simplified versions of their stimuli, we have confirmed their claim of perceptual dimorphism. We show that the strength of the illusory motion depends upon stimulus duration, eccentricity, and contrast. The illusory motion does not require fluctuations in accommodation, as has been suggested for some other forms of illusory motion. Finally, we consider the relation of Fraser-type motion to other forms of illusory motion.