Test of models of achromatic transparency

Consider an achromatic disk transparent on an achromatic background formed by two adjoining rectangles, with the common border of the rectangles dividing the disk in half. Current models of achromatic transparency contend that the perceived extent of transparency of the disk depends on the luminance contrast inside the disk and on the luminance contrast in the background outside the disk. Here, a model is proposed which contends that this perceived extent is determined only by the luminance contrasts inherent in the disk: inside the disk and between the disk and the background. Two experiments were designed to determine which luminance contrasts influence transparency. In the first experiment, subjects rated the perceived extent of transparency of the disk for different combinations of the luminances of the disk and of the background. The results strengthen the view that the perceived extent of transparency depends on the luminance contrasts inherent in the disk. In the second experiment, a test was made of the possibility that luminance contrasts between adjoining areas of the background outside the disk are nonessential for transparency. The results show that transparency occurred both when the areas of the background outside the transparent region adjoined one another and when they were separated, confirming that the perceived extent of transparency depended only on luminance contrasts between adjoining areas inherent in the disk.     

The luminance conditions of Fuchs's transparency in two-dimensional patterns

Fuchs's transparency occurs when the contour of a transparent surface encloses the contour of another surface located on an underlying homogeneous background. The luminance conditions of Fuchs's transparency have not yet been determined. Six experiments were designed to study this problem with achromatic two-dimensional patterns. An ellipse enclosing a coplanar square was briefly presented. It simulated the cast of an elliptical spotlight or shadow on the square. The duration of the ellipse, the luminance of the square before the ellipse appeared, and the luminance of two squares outside the ellipse did not substantially affect the probability of perceiving the ellipse as transparent. However, this probability varied largely with the single values of the stimulus luminance differences and with the order relations of the stimulus luminances. It is concluded that this local and global luminance information conditioned the occurrence of Fuchs's transparency in two-dimensional patterns.     

A weighted-average model of achromatic transparency

A model of achromatic transparency based on the idea that neural representations of transparency are activated by proximal contours is described. It is proposed that the weighted average of the magnitudes of the representations of transparency relative to a perceived continuous transparent surface corresponds to the judgement of the overall degree of transparency of the same surface. Tests of this weighted-average model were carried out with bistable patterns formed by two overlapping surfaces that appeared opaque where they were superimposed on the background and transparent where they were superimposed on each other (partial transparency). In agreement with predictions from the weighted-average model, the rated degrees of transparency of these two surfaces were noncomplementary and independent of background reflectance. Two experiments confirmed the contention of this model that the relevant proximal contours for the judgement of partial transparency of the two overlapping transparent surfaces in a bistable pattern correspond to the part where these surfaces are superimposed.     

Infants' perception of transparency

Four- and 7-month-old infants' perception of transparency was investigated with computer-generated achromatic or color displays depicting a semitransparent box occluding the center of a rod. Following habituation, infants viewed test displays consisting of either a two-color rod (corresponding to the habituation display's proximal characteristics) or a solid rod (corresponding to the distal characteristics of the event depicted by the habituation display). Looking-time results from 4-month-olds suggested perception of transparency in color displays but not in an achromatic display. An additional condition indicated that transparency perception may rely on the visibility of background texture through the transparent surface. Seven-month-olds, in contrast, provided some evidence of transparency perception in the achromatic display. Implications for the development of infants' responses to object properties and perceptual segregation are discussed.     

Information integration study of Metelli's and Morinaga's theories of achromatic transparency

Information integration methodology was used to test Metelli's and Morinaga's theories of achromatic transparency. Stimuli were transparent achromatic disks on a background formed by two adjacent horizontal rectangles. The common border of these rectangles divided each disk in two halves. Let P and Q be the luminances of the left and right halves of a disk and let A and B be those of the left and right rectangles, respectively. Transparency is given by the ratio (P – Q)/(A – B) in Metelli's theory and is given by a weighted average of the ratios (P – Q)/(A – Q) and (P – Q)/(P – B) in Morinaga's theory. Participants rated the transparency of disks with A and B fixed and P and Q combined factorially. Morinaga's theory closely predicted the resulting pattern of curves and Metelli's theory predicted it incorrectly. Morinaga's theory could also account well for individual differences in the ratings of transparency. The results support the general idea that transparency depends on the integration of photometric luminance information rather than on the integration of perceived lightness information.     

Checkerboard-specific color aftereffects: A failure to find effects of perceptual organization

Two experiments investigated the effects of differing perceptual organizations of reversible figures on McCollough aftereffects. Experiment 1 used colored checkerboard inducing stimuli and achromatic grating test stimuli. While some subjects tended to organize the checkerboards into rows and/or columns and others to organize them into obliques, these variations did not result in differences in aftereffect direction or magnitude. Experiment 2 induced an aftereffect with colored gratings and tested with checkerboards, gratings, and a reversible concentric octagon pattern. Perceptual organization had no effect on results for checkerboards, but was related to aftereffect strength for the octagon pattern. Indirect evidence suggests that, in the latter case, differences in aftereffect strength may have influenced the perceived organization, rather than vice versa. Finally, regardless of the specific organization perceived, spontaneous viewing of all test stimuli produced stronger aftereffects than were found when subjects reorganized the pattern. This may have resulted from a viewing strategy associated with reorganization, since similarly small aftereffects were found when subjects concentrated their attention on a single pattern element.     

The role of contrast in the perception of achromatic transparency: comment on Singh and Anderson (2002) and Anderson (2003)

M. Singh and B. L. Anderson proposed a perceptual theory of achromatic transparency in which the perceived transmittance of a perceived transparent filter is determined by the ratio of the Michelson contrast seen in the region of transparency to that of the background seen directly. Subsequently, B. L. Anderson, M. Singh, and J. Meng proposed that Michelson contrast should be replaced by perceived contrast in this theory. However, their experimental stimuli were nongeneric (i.e., special cases), and their observers had little choice in their matching strategy. Here, the author shows that both of their ratio-of-contrasts theories are incorrect and that problems with their theoretical analyses and experimental designs led them to conclude that mean luminance does not affect perceived transmittance when contrast is held constant. The author also shows that B. L. Anderson's (2003) transmittance anchoring principle is not valid when a transparent surface is perceived to penetrate rather than overlay another surface. Finally, the author shows that M. Singh's (2004) theoretical proof that the ratio-of-Michelson-contrasts theory accurately predicts lightness matching through transparency is incorrect.     

Transparent layer constancy

Perceived transparency was studied as a constancy problem. In the episcotister (E-) model of scission, luminances are partitioned into layer and background components; four luminances determine values of two layer parameters that specify constancy of a transparent layer on different backgrounds. The E-model was tested in an experiment in which 12 Ss matched 24 pairs of four-luminance patterns by adjusting two luminances of the comparison pattern. Both the standard and the comparison were perceived as a transparent layer on a checkerboard. The E-model predicts matches when layer values are identical in the two patterns. One parameter was constant, constraining the adjustment along the second dimension. Obtained values corresponded well with E-predictions. Alternative models based on local luminance or average contrast ratios accounted for less variability. Results indicate that transparency models should utilize luminance, not reflectance, as the independent variable.     

Newborns' discrimination of chromatic from achromatic stimuli

Two experiments assessed newborns' ability to discriminate chromatic from achromatic stimuli. In Experiment 1, newborns differentiated gray from green, from yellow, and from red: For each of these hues they preferred chromatic-and-gray checkerboards over gray squares matched in mean luminance, even though the luminance of the gray checks was varied systematically over a wide range so as to minimize nonchromatic cues. However, newborns showed no evidence of differentiating gray from blue: At some luminances they showed no preference for a blue-and-gray checkerboard over a gray square. In Experiment 2, newborns differentiated red from gray but appeared not to differentiate blue from gray: Following habituation to a series of gray squares of varying luminance, they looked longer at a red square than at a gray square of novel luminance but showed no such pattern of recovery to a blue square. The results imply that newborns have some, albeit limited, ability to discriminate chromatic from achromatic stimuli and hence, that they are at least dichromats.     

Spatial-frequency-contingent color aftereffects: Adaptation with two-dimensional stimulus patterns

The spatial-frequency theory of vision has been supported by adaptation studies using checkerboards in which contingent color aftereffects (CAEs) were produced at fundamental frequencies oriented at 45\dg to the edges. A replication of this study failed to produce CAEs at the orientation of either the edges or the fundamentals. Using a computer-generated display, no CAEs were produced by adaptation of a square or an oblique checkerboard. But when one type of checkerboard (4 cpd) was adapted alone, CAEs were produced on the adapted checkerboard and on sine-wave gratings aligned with the fundamental and third harmonics of the checkerboard spectrum. Adaptation of a coarser checkerboard (0.80 cpd) produced CAEs aligned with both the edges and the harmonic frequencies. With checkerboards of both frequencies, CAEs were also found on the other type of checkerboard that had not been adapted. This observation raises problems for any edge-detector theory of vision, because there was no adaptation to edges. It was concluded that spatial-frequency mechanisms are operating at both low- and high-spatial frequencies and that an edge mechanism is operative at lower frequencies. The implications of these results are assessed for other theories of spatial vision.     

Spatial-frequency-contingent color aftereffects: adaptation with two-dimensional stimulus patterns.

The spatial-frequency theory of vision has been supported by adaptation studies using checkerboards in which contingent color aftereffects (CAEs) were produced at fundamental frequencies oriented at 45 degrees to the edges. A replication of this study failed to produce CAEs at the orientation of either the edges or the fundamentals. Using a computer-generated display, no CAEs were produced by adaptation of a square or an oblique checkerboard. But when one type of checkerboard (4 cpd) was adapted alone, CAEs were produced on the adapted checkerboard and on sine-wave gratings aligned with the fundamental and third harmonics of the checkerboard spectrum. Adaptation of a coarser checkerboard (0.80 cpd) produced CAEs aligned with both the edges and the harmonic frequencies. With checkerboards of both frequencies, CAEs were also found on the other type of checkerboard that had not been adapted. This observation raises problems for any edge-detector theory of vision, because there was no adaptation to edges. It was concluded that spatial-frequency mechanisms are operating at both low- and high-spatial frequencies and that an edge mechanism is operative at lower frequencies. The implications of these results are assessed for other theories of spatial vision.     

Effect of image orientation on the eye direction aftereffect

After observing a face with the eyes looking to the left or right (adaptation stimulus), the perception of the eye direction of the subsequent face (test stimulus) is biased in the opposite direction of the adapted eye direction; this is called the eye direction aftereffect (EDAE). In the present study, the adaptation stimuli were rotated 90° (clockwise or counterclockwise) or 180° relative to the viewer. The EDAE was measured using upright test stimuli. For the 90° rotation, prior observation of the leftward and rightward eye directions biased the perceived eye directions of the upright test stimuli to the right and left, respectively. These results suggest that the adaptation was induced utilizing an object-based (or face-based) reference frame. For the 180° rotation, however, the results suggest that the adaptation was induced in a viewer-centered reference frame. The involvement of an object-based reference frame suggests that the EDAE reflected the adaptation of a relatively higher-level mechanism at least when the rotation angle from the upright position did not exceed 90°.     

Perceptual transparency in 3- to 4-month-old infants

We examined perceptual transparency in infants. In a previous study, Johnson and Aslin (2000 Developmental Psychology 36 808 - 816) found that 4-month-olds could perceive transparency in a moving chromatic display, but not in an achromatic display. In this study, we further examined perceptual transparency in infants using a static achromatic display. Considering the development of figural organisation and contrast sensitivity, we assumed that 3- to 4-month-olds would perceive transparency even in a static achromatic display. We created a transparency and a non-transparent display composed of a partially overlapping circle and square, by switching the colours. Infants aged 3 to 4 months (n = 24) were familiarised with the transparency display (experiment 1) or with the non-transparent display (experiment 2). Then, they were confronted with a uniform colour and a two-colour figure. Infants showed novelty preference for the two-colour figure after they had been familiarised with the transparency display (experiment 1), but not after they had been familiarised with the non-transparent display (experiment 2). These results suggest that 3- to 4-month-old infants can perceive transparency in a static achromatic display.     

Achromatic transparency and the role of local contours

In this paper we investigate the role of contours and junctions in the perception of single-plane achromatic transparency. In order to measure the accuracy with which observers encode transparency, a six-luminance stimulus was employed in which the figural properties could be easily manipulated. Accuracy was measured by requiring subjects to select (either by the method of adjustment or by using a forced-choice procedure) the luminance that best completed a simulated transparent filter. The X junctions in the stimulus were destroyed or perturbed in three experiments. Simple occlusion of the junction (experiment 1), and perturbation of the orientation of the contours of the filter as they pass through the junction (experiment 3) resulted in small but significant reductions in performance. On the other hand, a sudden change in orientation of the background (material) contours (experiment 2) resulted in a small but significant enhancement of overall performance compared with the control stimulus. In the forced-choice task, reversals in the polarity of contours (as defined by the brightness order of flanking regions) around the junction were shown to effect large changes in subjects' accuracy in processing transparency. The overall results show that X and Psi junctions are indeed salient properties of transparent stimuli. The findings suggest that jagged contours with sudden changes in direction are more likely to be attributed to reflectance (material) changes than to changes due to a transparent filter (or to illumination).     

The spatial signal for saccadic eye movements emphasizes visual boundaries

By investigating the visual processing involved when saccades are made to newly appearing targets, we show that this processing is significantly nonlinear and that texture boundary information predominates. We used the global, or center-of-gravity, effect whereby a saccadic eye movement directed to a target consisting of a pair of elements has an amplitude intermediate between that of saccades directed to the individual elements. We measured the effect using target elements with different visual characteristics, including phase-reversal checkerboard targets that had the same space-average luminance as the background. The contribution to the center-of-gravity calculation was used to measure relative salience. We found that positive and negative contrast elements contribute almost equal weightings. Thus, salience, assessed in this way, is a highly nonlinear function of luminance. The salience of checkerboard targets was found to decrease as check size was decreased and increase as the overall size of the target was increased. Checkerboards with an empty center were as effective as were full checkerboards, showing the importance of boundaries in the salience signal.     

Luminance determinants of perceived surface stratification in two-dimensional achromatic transparent patterns

Two overlapping transparent surfaces forming a two-dimensional pattern stand out in front of each other alternately. Let y denote the luminance of the region where these surfaces overlap and b the luminance of the background. In achromatic patterns, the probability that the lighter transparent surface appears to be in front of the other surface is known to increase with y and with b. The present results show that grouping by achromatic colour similarity cannot explain the effect of b. An alternative conjecture is that the luminance factors that control perceived surface segregation can explain the effects of y and b. Such an explanation predicts a new effect: the probability that one transparent surface appears to be in front increases with the absolute difference in luminance between the surface and the background. The present results confirm this prediction.     

Hemispheric specialization for language according to grapho-phonemic transformation and gender. A divided visual field experiment

This behavioral study aimed at assessing the effect of two variables on the degree of hemispheric specialization for language. One of them was the grapho-phonemic translation (transformation) (letter-sound mapping) and the other was the participants’ gender. The experiment was conducted with healthy volunteers. A divided visual field procedure has been used to perform a phoneme detection task implying either regular (transparent) grapho-phonemic translation (letter-sound mapping consistency) or irregular (non-transparent) grapho-phonemic translation (letter-sound mapping inconsistency). Our results reveal a significant effect of grapho-phonemic translation on the degree of hemispheric dominance for language. The phoneme detection on items with transparent translation (TT) was performed faster than phoneme detection on items with non-transparent translation (NTT). This effect seems to be due to faster identification of TT than NTT when the items were presented in the left visual field (LVF)-right hemisphere (RH). There was no difference between TT and NTT for stimuli presented in the right visual field (RVF)-left hemisphere (LH). This result suggests that grapho-phonemic translation or the degree of transparency can affect the degree of hemispheric specialization, by modulating the right hemisphere activity. With respect to gender, male participants were significantly more lateralized than female participants but no interaction was observed between gender and degree of transparency.     

Detection of counter-changing contrast: second-order apparent motion without postrectification motion-energy analysis or salience mapping/feature tracking

The perception of 2nd-order, texture-contrast-defined motion was studied for apparent-motion stimuli composed of a pair of spatially displaced, simultaneously visible checkerboards. It was found that background-relative, counter-changing contrast provided the informational basis for the perception of 2nd-order apparent motion; motion began where contrast changed toward the contrast value of the background checkerboard and ended where contrast changed away from the background value. The perceived apparent motion was not attributable to either postrectification motion-energy analysis or salience-mapping/feature-tracking mechanisms. Parallel results for 1st-order, luminance-defined motion (H. S. Hock, L. A. Gilroy, & G. Harnett, 2002) suggest that counter-changing activation provides a common basis for the perception of both luminance- and texture-contrast-defined apparent motion.