Color improves edge classification in human vision.

Despite the complexity of the visual world, humans rarely confuse variations in illumination, for example shadows, from variations in material properties, such as paint or stain. This ability to distinguish illumination from material edges is crucial for determining the spatial layout of objects and surfaces in natural scenes. In this study, we explore the role that color (chromatic) cues play in edge classification. We conducted a psychophysical experiment that required subjects to classify edges into illumination and material, in patches taken from images of natural scenes that either contained or did not contain color information. The edge images were of various sizes and were pre-classified into illumination and material, based on inspection of the edge in the context of the whole image from which the edge was extracted. Edge classification performance was found to be superior for the color compared to grayscale images, in keeping with color acting as a cue for edge classification. We defined machine observers sensitive to simple image properties and found that they too classified the edges better with color information, although they failed to capture the effect of image size observed in the psychophysical experiment. Our findings are consistent with previous work suggesting that color information facilitates the identification of material properties, transparency, shadows and the perception of shape-from-shading.

Unmasking the dichoptic mask.

Previously, it has been shown that dichoptic color-contrast masking can be dramatically reduced by the introduction of task-irrelevant binocular features. It is unclear, however, whether or not the task-irrelevant features need to be matched in the two eyes in order to reduce dichoptic masking. We measured dichoptic masking between target and mask luminance decrement patches and between target and mask isoluminant violet patches. The stimuli were surrounded by a task-irrelevant feature that consisted of a ring of various widths: either a luminance decrement, an isoluminant violet, or an isoluminant red. When the ring was presented to just the target eye-that is, the eye opposite to that of the mask-dichoptic masking was reduced just as much as when the ring was binocular-that is, presented to both eyes. A model that incorporated the combined influence of interocular inhibition from all stimulus components-that is, mask, target, and rings-was found to give a good account of the pattern of dichoptic masking across the full range of conditions.

Interocular difference thresholds are mediated by binocular differencing, not summing, channels.

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we attempt to determine the mechanisms for detecting interocular differences in luminance contrast, in particular in relation to the possible contributions of binocular differencing and binocular summing channels. Test patterns were horizontally oriented multi-spatial-frequency luminance-grating patterns subject to variable amounts of interocular difference in grating phase, resulting in varying degrees of local interocular contrast difference. Two types of experiment were conducted. In the first, subjects discriminated between a pedestal with an interocular difference that ranged upward from zero (i.e., binocularly correlated) and a test pattern that contained a bigger interocular difference. In the second type of experiment, subjects discriminated between a pedestal with an interocular difference that ranged downward from a maximum (i.e., binocularly anticorrelated) and a test pattern that contained smaller interocular difference. The two types of task could be mediated by a binocular differencing and a binocular summing channel, respectively. However, we found that the results from both experiments were well described by a simpler model in which a single, linear binocular differencing channel is followed by a standard nonlinear transducer that is expansive for small signals but strongly compressive for large ones. Possible reasons for the lack of involvement of a binocular summing channel are discussed in the context of a model that incorporates the responses of both monocular and binocular channels.

Adaptation to interocular difference.

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we ask whether the mechanism for detecting interocular differences (IDs) is adaptable. Our stimuli were horizontally oriented multispatial-frequency grating patterns that could be subject to varying degrees of ID through the introduction of interocular phase differences in the grating components. Subjects adapted to patterns that were either correlated, uncorrelated, monocular (one eye only), or anticorrelated. Following adaptation, thresholds for detecting IDs were measured using a staircase procedure. It was found that ID thresholds were elevated following adaptation to uncorrelated, monocular, and anticorrelated but not correlated patterns. Threshold elevation was found to be maximal when the orientations of the adaptor and test gratings were the same, and when their spatial frequencies were similar. The results support the existence of a specialized mechanism for detecting IDs, the most likely candidate being the binocular differencing channel proposed in previous studies.

Symmetry perception for patterns defined by color and luminance.

Perception of visual symmetry is fast and efficient and relies on both early low-level and late mid- and high-level neural mechanisms. To test for potential influences of early low-level mechanisms on symmetry perception, we used isoluminant, achromatic, and combined (color + luminance) patterns in a psychophysical and an event-related-potential (ERP) experiment. In the psychophysical experiment, pattern contrast was fixed at individual symmetry-discrimination threshold. Participants then judged whether a pattern was symmetric or random. Stimuli at isoluminance were associated with a large bias toward symmetry, achromatic stimuli introduced the opposite bias, and stimuli containing a balance of both color and luminance were perceived without bias. These findings are in line with distinct contrast sensitivity functions for color and luminance, with color providing low-frequency information useful for symmetry detection and luminance providing high-frequency information useful for detection of detail. The subsequent ERP experiment was run at high contrasts to assess processing of symmetry in suprathreshold conditions. Sustained posterior negativity, a symmetry-sensitive ERP component, was observed in all conditions and showed the expected dependence on symmetry. However, interactions between symmetry and contrast type were not observed. In conclusion, while our findings at threshold support models that propose an important contribution of low-level mechanisms to symmetry perception, at suprathreshold these low-level contributions do not persist. Therefore, under everyday viewing conditions, symmetry perception engages a relatively broad cortical network that is not constrained by low-level inputs.

Pupil dilation to illusory motion in peripheral drift images: Perception versus reality.

Peripheral drift is a specific type of illusory motion that causes observers to perceive motion in a static image. We aimed to determine whether pupil dilation occurs during the perception of illusory motion. In three experiments investigating pupil-size changes to peripheral drift, pupil response differences were observed between symmetric patterns (SPs) that elicited no impression of motion and repeated asymmetric patterns (RAPs) that did. All participants reported the perception of motion in the RAP condition and showed significantly greater pupil dilation to these stimuli as compared with viewing stimuli in the SP condition. As a follow-up, we manipulated the RAP stimuli to reduce and then remove the illusion to determine (a) whether it was the asymmetry per se that induced the pupil dilation and (b) whether the amount of pupil dilation was contingent on the amount of observed illusory motion. Although a reduction in perceived illusory motion did not produce a reduction in pupil dilation, removal of the illusory motion did. Despite previous evidence reporting pupil constriction to the perception of motion, and the positive valence associated with symmetry, these experiments show that pupil dilation occurs during the perception of illusory motion. This is in keeping with previous evidence that pupil dilation is influenced by perceptual factors and not simply light level, and, in particular, shows that illusory motion is physiologically arousing.

Detection of between-eye differences in color: Interactions with luminance.

Between-eye differences in color or luminance result in the appearance of luster, which provides a cue for detecting between-eye differences. We measured thresholds for detecting between-eye differences in both hue and chromatic contrast (saturation) in dichoptically superimposed color patches. Sensitivity was found to be highest at isoluminance and decreased with the addition of task-irrelevant, spatially coextensive, binocular (i.e., same in both eyes) luminance contrast. However, when the members of each dichoptic pair were presented side by side on the screen and viewed with the same eye, the added luminance contrast had no effect on the detection of their differences. If the effect of the luminance contrast was simply to dilute or desaturate the chromatic signals, we would expect thresholds to increase for the within-eye and not just the between-eye (dichoptic) conditions. We suggest that the presence of binocular luminance contrast reduces the interocular suppression between the dichoptic colors, causing the dichoptic color pairs to blend, thus rendering their differences harder to detect.

Detection of chromatic and luminance distortions in natural scenes.

A number of studies have measured visual thresholds for detecting spatial distortions applied to images of natural scenes. In one study, Bex [J. Vis.10(2), 1 (2010)10.1167/10.2.231534-7362] measured sensitivity to sinusoidal spatial modulations of image scale. Here, we measure sensitivity to sinusoidal scale distortions applied to the chromatic, luminance, or both layers of natural scene images. We first established that sensitivity does not depend on whether the undistorted comparison image was of the same or of a different scene. Next, we found that, when the luminance but not chromatic layer was distorted, performance was the same regardless of whether the chromatic layer was present, absent, or phase-scrambled; in other words, the chromatic layer, in whatever form, did not affect sensitivity to the luminance layer distortion. However, when the chromatic layer was distorted, sensitivity was higher when the luminance layer was intact compared to when absent or phase-scrambled. These detection threshold results complement the appearance of periodic distortions of the image scale: when the luminance layer is distorted visibly, the scene appears distorted, but when the chromatic layer is distorted visibly, there is little apparent scene distortion. We conclude that (a) observers have a built-in sense of how a normal image of a natural scene should appear, and (b) the detection of distortion in, as well as the apparent distortion of, natural scene images is mediated predominantly by the luminance layer and not chromatic layer.

Chromatic contrast in luminance-defined images affects performance and neural activity during a shape classification task.

Models of object recognition generally emphasize the importance of luminance-defined shape. However, it is still not fully understood how color signals combine with luminance signals to affect object-related form processing. This electroencephalographic study aimed to examine the contribution of chromatic contrast by assessing its effects on the time course of shape-related processing. Participants classified Gaborized images of object shapes, nonobject shapes, and patches of pseudorandomly scattered Gabors. Stimuli excited (a) the luminance (L+M) channel alone, (b) luminance and L-M channels, or (c) luminance, L-M, and S-(L+M) channels and were presented either at mean discrimination threshold or at twice this mean threshold. As expected, classification accuracy was comparable at threshold, as were the attributes of the early, perceptual first negative (N1) component of the event-related potential (ERP). Differences emerged at suprathreshold: Objects defined by the full combination of channels were associated with the poorest performance and the lowest N1 amplitude. Shape sensitivity was not consistently observed in the N1 but was more evident in the late positive potential (LPP), a cognitive ERP component. Both the N1 and the LPP were affected by the amount and type of contrast in the image. While the effects of luminance and L-M contrast were similar, affecting the ERP selectively during the N1 and LPP period, S-(L+M) contrast elicited a sustained shift in amplitude. Our results demonstrate, for the first time using a combination of behavioral as well as early and late electrophysiological effects, that shape classification is determined by both the chromatic and the luminance content of the image.

Probability, not linear summation, mediates the detection of concentric orientation-defined textures.

Previous studies investigating signal integration in circular Glass patterns have concluded that the information in these patterns is linearly summed across the entire display for detection. Here we test whether an alternative form of summation, probability summation (PS), modeled under the assumptions of Signal Detection Theory (SDT), can be rejected as a model of Glass pattern detection. PS under SDT alone predicts that the exponent ß of the Quick- (or Weibull-) fitted psychometric function should decrease with increasing signal area. We measured spatial integration in circular, radial, spiral, and parallel Glass patterns, as well as comparable patterns composed of Gabors instead of dot pairs. We measured the signal-to-noise ratio required for detection as a function of the size of the area containing signal, with the remaining area containing dot-pair or Gabor-orientation noise. Contrary to some previous studies, we found that the strength of summation never reached values close to linear summation for any stimuli. More importantly, the exponent ß systematically decreased with signal area, as predicted by PS under SDT. We applied a model for PS under SDT and found that it gave a good account of the data. We conclude that probability summation is the most likely basis for the detection of circular, radial, spiral, and parallel orientation-defined textures.

Luminance and color inputs to mid-level and high-level vision.

We investigated the interdependence of activity within the luminance (L + M) and opponent chromatic (L - M and S - [L + M]) postreceptoral mechanisms in mid-level and high-level vision. Mid-level processes extract contours and perform figure-background organization whereas high-level processes depend on additional semantic input, such as object knowledge. We collected mid-level (good/poor continuation) and high-level (object/nonobject) two-alternative forced-choice discrimination threshold data over a range of conditions that isolate mechanisms or simultaneously stimulate them. The L - M mechanism drove discrimination in the presence of very low luminance inputs. Contrast-dependent interactions between the luminance and L - M as well as combined L - M and S - (L + M) inputs were also found, but S - (L + M) signals, on their own, did not interact with luminance. Mean mid-level and high-level thresholds were related, with luminance providing inputs capable of sustaining performance over a broader, linearly corresponding range of contrasts when compared to L - M signals. The observed interactions are likely to be driven by L - M signals and relatively low luminance signals (approximately 0.05-0.09 L + M contrast) facilitating each other. The results are consistent with previous findings on low-level interactions between chromatic and luminance signals and demonstrate that functional interdependence between the geniculate mechanisms extends to the highest stages of the visual hierarchy.

The association between childhood trauma and emotion recognition is reduced or eliminated when controlling for alexithymia and psychopathy traits.

Emotion recognition shows large inter-individual variability, and is substantially affected by childhood trauma as well as modality, emotion portrayed, and intensity. While research suggests childhood trauma influences emotion recognition, it is unclear whether this effect is consistent when controlling for interrelated individual differences. Further, the universality of the effects has not been explored, most studies have not examined differing modalities or intensities. This study examined childhood trauma's association with accuracy, when controlling for alexithymia and psychopathy traits, and if this varied across modality, emotion portrayed, and intensity. An adult sample (N = 122) completed childhood trauma, alexithymia, and psychopathy questionnaires and three emotion tasks: faces, voices, audio-visual. When investigating childhood trauma alone, there was a significant association with poorer accuracy when exploring modality, emotion portrayed, and intensity. When controlling for alexithymia and psychopathy, childhood trauma remained significant when exploring emotion portrayed, however, it was no longer significant when exploring modality and intensity. In fact, alexithymia was significant when exploring intensity. The effect sizes overall were small. Our findings suggest the importance of controlling for interrelated individual differences. Future research should explore more sensitive measures of emotion recognition, such as intensity ratings and sensitivity to intensity, to see if these follow accuracy findings.

Different symmetries, different mechanisms.

Three common symmetries exist in the natural visual world: (i) mirror symmetry, i.e., reflections around a vertical axis, (ii) radial symmetry, i.e., rotations around a point, and (iii) translational symmetry, i.e., shifted repetitions. Are these processed by a common class of visual mechanism? Using stimuli comprising arrays of Gaussian blobs we examined this question using a visual search protocol in which observers located a single symmetric target patch among varying numbers of random-blob distractor patches. The testing protocol used a blocked present/absent task and both search times and accuracy were recorded. Search times for mirror and radial symmetry increased significantly with the number of distractors, as did translational-symmetry patterns containing few repetitions. However translational-symmetry patterns with four repeating sectors produced search slopes close to zero. Fourier analysis revealed that, as with images of natural scenes, the structural information in both mirror- and radial-symmetric patterns is carried by the phase spectrum. However, for translational patterns with four repeating sectors, the amplitude spectrum appears to capture the structure, consistent with previous analyses of texture regularity. Modeling revealed that while the mirror and radial patterns produced an approximately Gaussian-shaped energy response profile as a function of spatial frequency, the translational pattern profiles contained a distinctive spike, the magnitude of which increased with the number of repeating sectors. We propose distinct mechanisms for the detection of different symmetry types: a mechanism that encodes local positional information to detect mirror- and radial-symmetric patterns and a mechanism that computes energy in narrowband filters for the detection of translational symmetry containing many sectors.

The effects of face coverings, own-ethnicity biases, and attitudes on emotion recognition.

As a result of the COVID-19 pandemic, face coverings were introduced as a safety measure in certain environments in England and some research suggests that they can affect emotion recognition. Factors such as own-ethnicity bias (e.g. whether people perceiving and expressing emotions are of the same ethnicity) and social biases are also known to influence emotion recognition. However, it is unclear whether these factors interact with face coverings to affect emotion recognition. Therefore, this study examined the effects of face coverings, own-ethnicity biases, and attitudes on emotion recognition accuracy. In this study, 131 participants viewed masked and unmasked emotional faces varying in ethnicity and completed a questionnaire on their attitudes towards face masks. We found that emotion recognition was associated with masks and attitudes: accuracy was lower in masked than unmasked conditions and attitudes towards masks Inside and Outside were associated with emotion recognition. However, a match between perceiver and stimulus ethnicity did not have a significant effect on emotion recognition. Ultimately, our results suggest that masks, and negative attitudes towards them, were associated with poorer emotion recognition. Future research should explore different mask-wearing behaviours and possible in-group/out-group biases and their interaction with other social cues (e.g. in-group biases).

Negative afterimages facilitate the detection of real images.

Negative, or complementary afterimages are experienced following brief adaptation to chromatic or achromatic stimuli, and are believed to be formed in the post-receptoral layers of the retinae. Afterimages can be cancelled by the addition of real images, suggesting that afterimages and real images are processed by similar mechanisms. However given their retinal origin, afterimage signals represented at the cortical level might have different spatio-temporal properties from their real images counterparts. To test this we determined whether afterimages reduce the contrast threshold of added real images, i.e. produce the classic "dipper" function characteristic of contrast discrimination, a behavior believed to be cortically mediated. Stimuli were chromatic and achromatic disks on a grey background. Observers adapted for 1.0 s to two side-by-side disks of a particular color. Following stimulus offset, a test disk added to one side was ramped downwards for 1.5 s to approximately match the temporal characteristic of the afterimage, and the observer was required to indicate the side containing the test disk. The test hue/brightness was either the same as that of the afterimage or a different hue/brightness. The independent variable was the contrast of the adaptor. A dipper followed by masking was observed in most conditions in which the afterimage and test colors had the same hue or brightness. We conclude that afterimages are represented similarly to their real image counterparts at the cortical level.

The Pupillary Response to the Unknown: Novelty Versus Familiarity.

Object recognition is a type of perception that enables observers to recognize familiar shapes and categorize them into real-world identities. In this preregistered study, we aimed to determine whether pupil size changes occur during the perception and recognition of identifiable objects. We compared pupil size changes for familiar objects, nonobjects, and random noise. Nonobjects and noise produced greater pupil dilation than familiar objects. Contrary to previous evidence showing greater pupil dilation to stimuli with more perceptual and affective content, these results indicate a greater pupil dilation to stimuli that are unidentifiable. This is consistent with the relative salience of novelty compared to familiarity at the physiological level driving the pupil response.

Chromatic blur perception in the presence of luminance contrast.

Hel-Or showed that blurring the chromatic but not the luminance layer of an image of a natural scene failed to elicit any impression of blur. Subsequent studies have suggested that this effect is due either to chromatic blur being masked by spatially contiguous luminance edges in the scene (Journal of Vision 13 (2013) 14), or to a relatively compressed transducer function for chromatic blur (Journal of Vision 15 (2015) 6). To test between the two explanations we conducted experiments using as stimuli both images of natural scenes as well as simple edges. First, we found that in color-and-luminance images of natural scenes more chromatic blur was needed to perceptually match a given level of blur in an isoluminant, i.e. colour-only scene. However, when the luminance layer in the scene was rotated relative to the chromatic layer, thus removing the colour-luminance edge correlations, the matched blur levels were near equal. Both results are consistent with Sharman et al.'s explanation. Second, when observers matched the blurs of luminance-only with isoluminant scenes, the matched blurs were equal, against Kingdom et al.'s prediction. Third, we measured the perceived blur in a square-wave as a function of (i) contrast (ii) number of luminance edges and (iii) the relative spatial phase between the colour and luminance edges. We found that the perceived chromatic blur was dependent on both relative phase and the number of luminance edges, or dependent on the luminance contrast if only a single edge is present. We conclude that this Hel-Or effect is largely due to masking of chromatic blur by spatially contiguous luminance edges.

Searching for Radial Symmetry.

Symmetry is ubiquitous in the natural world. Numerous investigations, dating back over one hundred years, have explored the visual processing of symmetry. However, these studies have been concerned with mirror symmetry, overlooking radial (or rotational) symmetry, which is also prevalent in nature. Using a visual search paradigm, which approximates the everyday task of searching for an object embedded in background clutter, we have measured how quickly and how accurately human observers detect radially symmetric dot patterns. Performance was compared with mirror symmetry. We found that with orders of radial symmetry greater than 5, radial symmetry can be detected more easily than mirror symmetry, revealing for the first time that radial symmetry is a salient property of objects for human vision.

Common contextual influences in ambiguous and rivalrous figures.

Images that resist binocular fusion undergo alternating periods of dominance and suppression, similarly to ambiguous figures whose percepts alternate between two interpretations. It has been well documented that the perceptual interpretations of both rivalrous and ambiguous figures are influenced by their spatio-temporal context. Here we consider whether an identical spatial context similarly influences the interpretation of a similar rivalrous and ambiguous figure. We developed a binocularly rivalrous stimulus whose perceptual experience mirrors that of a Necker cube. We employed a paradigm similar to that of Ouhnana and Kingdom (2016) to correlate the magnitude of influence of context between the rivalrous and ambiguous target. Our results showed that the magnitude of contextual influence is significantly correlated within observers between both binocularly rivalrous and ambiguous target figures. This points to a similar contextual-influence mechanism operating on a common mechanism underlying the perceptual instability in both ambiguous and rivalrous figures.