Journal: Journal of vision
Integration of local elements into a coherent global form is a fundamental aspect of visual object recognition. How the different hierarchically organized stages of visual analysis develop in order to support object representation in infants remains unknown. The aim of this study was to investigate structural encoding of natural images in 4- to 6-month-old infants and adults. We used the steady-state visual evoked potential (ssVEP) technique to measure cortical responses specific to the global structure present in object and face images, and assessed whether differential responses were present for these image categories. This study is the first to apply the ssVEP method to high-level vision in infants. Infants and adults responded to the structural relations present in both image categories, and topographies of the responses differed based on image category. However, while adult responses to face and object structure were localized over occipitotemporal scalp areas, only infant face responses were distributed over temporal regions. Therefore, both infants and adults show object category specificity in their neural responses. The topography of the infant response distributions indicates that between 4 and 6 months of age, structure encoding of faces occurs at a higher level of processing than that of objects.
Unlike frozen snapshots of facial expressions that we often see in photographs, natural facial expressions are dynamic events that unfold in a particular fashion over time. But how important are the temporal properties of expressions for our ability to reliably extract information about a person’s emotional state? We addressed this question experimentally by gauging human performance in recognizing facial expressions with varying temporal properties relative to that of a statistically optimal (“ideal”) observer. We found that people recognized emotions just as efficiently when viewing them as naturally evolving dynamic events, temporally reversed events, temporally randomized events, or single images frozen in time. Our results suggest that the dynamic properties of human facial movements may play a surprisingly small role in people’s ability to infer the emotional states of others from their facial expressions.
When a plaid object is presented, the visual system decomposes it into its constituting orientation primitives and integrates them at later processing stages. The present study reveals the time course of this process by applying meta- and paracontrast masking to both simple oriented and plaid gratings. With various stimulus onset asynchronies (SOA) between target gratings and surrounding mask annuli, subjects were asked to identify whether targets were simple gratings collinear to the masks, orthogonal to the masks, plaid, or whether no target was presented. The resulting time courses for each type of stimulus confusion showed that metacontrast peaked when orientation primitives had already begun to be integrated into one object, indicated by a dominance of “no target” responses given to plaid stimuli at SOAs around 70 ms. At SOAs around 10 to 30 ms masking also had a significant impact but acted on separable components, indicated by a dominance of “orthogonal” responses given plaid stimuli. Probability summation of “no target” responses given simple gratings revealed that only at shorter SOAs performance for plaid stimuli could be predicted assuming independent features but not at SOAs of at 50-70 ms. We discuss in how far these results could also be explained by the dynamics of cross-orientation suppression (COS) and how they might relate to the process of feature integration in plaids.
Barn owls are effective nocturnal predators. We tested their visual performance at low light levels and determined visual acuity and contrast sensitivity of three barn owls by their behavior at stimulus luminances ranging from photopic to fully scotopic levels (23.5 to 1.5 × 10(-6)). Contrast sensitivity and visual acuity decreased only slightly from photopic to scotopic conditions. Peak grating acuity was at mesopic (4 × 10(-2) cd/m(2)) conditions. Barn owls retained a quarter of their maximal acuity when luminance decreased by 5.5 log units. We argue that the visual system of barn owls is designed to yield as much visual acuity under low light conditions as possible, thereby sacrificing resolution at photopic conditions.
The link between arousal and pupil dilation is well studied, but it is less known that other cognitive processes can trigger pupil responses. Here we present evidence that pupil responses can be induced by high-level scene processing, independent of changes in low-level features or arousal. In Experiment 1, we recorded changes in pupil diameter of observers while they viewed a variety of natural scenes with or without a sun that were presented either upright or inverted. Image inversion had the strongest effect on the pupil responses. The pupil constricted more to the onset of upright images as compared to inverted images. Furthermore, the amplitudes of pupil constrictions to viewing images containing a sun were larger relative to control images. In Experiment 2, we presented cartoon versions of upright and inverted pictures that included either a sun or a moon. The image backgrounds were kept identical across conditions. Similar to Experiment 1, upright images triggered pupil constrictions with larger amplitudes than inverted images and images of the sun evoked greater pupil contraction than images of the moon. We suggest that the modulations of pupil responses were due to higher-level interpretations of image content.
Symmetry is a biologically relevant, mathematically involving, and aesthetically compelling visual phenomenon. Mirror symmetry detection is considered particularly rapid and efficient, based on experiments with random noise. Symmetry detection in natural settings, however, is often accomplished against structured backgrounds. To measure salience of symmetry in diverse contexts, we assembled mirror symmetric patterns from 101 natural textures. Temporal thresholds for detecting the symmetry axis ranged from 28 to 568 ms indicating a wide range of salience (1/Threshold). We built a model for estimating symmetry-energy by connecting pairs of mirror-symmetric filters that simulated cortical receptive fields. The model easily identified the axis of symmetry for all patterns. However, symmetry-energy quantified at this axis correlated weakly with salience. To examine context effects on symmetry detection, we used the same model to estimate approximate symmetry resulting from the underlying texture throughout the image. Magnitudes of approximate symmetry at flanking and orthogonal axes showed strong negative correlations with salience, revealing context interference with symmetry detection. A regression model that included the context-based measures explained the salience results, and revealed why perceptual symmetry can differ from mathematical characterizations. Using natural patterns thus produces new insights into symmetry perception and its possible neural circuits.
A stereoscope displays 2-D images with binocular disparities (stereograms), which fuse to form a 3-D stereoscopic object. But a stereoscopic object creates a conflict between vergence and accommodation. Also, motion in depth of a stereoscopic object simulated solely from change in target vergence produces anomalous motion parallax and anomalous changes in perspective. We describe a new instrument, which overcomes these problems. We call it the dichoptiscope. It resembles a mirror stereoscope, but instead of stereograms, it displays identical 2-D or 3-D physical objects to each eye. When a pair of the physical, monocular objects is fused, they create a dichoptic object that is visually identical to a real object. There is no conflict between vergence and accommodation, and motion parallax is normal. When the monocular objects move in real depth, the dichoptic object also moves in depth. The instrument allows the experimenter to control independently each of several cues to motion in depth. These cues include changes in the size of the images, changes in the vergence of the eyes, changes in binocular disparity within the moving object, and changes in the relative disparity between the moving object and a stationary object.
A foundational issue in the study of unconscious processing concerns whether the stimuli of interest are truly out of awareness. Objective methods employing forced choice are typically championed as the gold standard and widely thought to be conservative. Here, however, as a case study, we demonstrate an underestimation of awareness in a collection of studies on unconscious cognitive control. Specifically, we found that (a) in addition to genuine unawareness, chance performance could be due to a failure to perform the task; (b) visual awareness for low-visibility trials was elevated when mixed with high-visibility trials compared with when presented alone as demonstrated in both objective awareness (forced-choice performance) and subjective awareness (rating based on a perceptual awareness scale); and © the elevation effect was partly due to a shape-specific template enhancement at both the block and intertrial levels. We term the awareness elevation effect priming of awareness: Visual priming fundamentally alters awareness, boosting otherwise invisible objects into consciousness. These results implicate two key requirements for measuring awareness: (a) verify that participants are truly performing the awareness task and (b) use all types of trials in the awareness test as in the main experiment. Priming of awareness is consistent with an expanded model of awareness and top-down attention in which awareness is determined by (a) retinal stimulus strength and (b) both goal-dependent and goal-independent extra-retinal modulation.
There has been considerable interest in a stimulus (“the dress”) that yields starkly divergent subjective color percepts between observers. It has been proposed that individual differences in the subjective interpretation of this stimulus are due to the different assumptions that individuals make about how the dress was illuminated. In this study, we address this possible explanation empirically by reporting on data from ∼13,000 observers who were surveyed online. We show that assumptions about the illumination of the dress-i.e., whether the stimulus was illuminated by natural or artificial light or whether it was in a shadow-strongly affects the subjective interpretation of observers, compared to demographic factors, such as age or gender, which have a relatively smaller influence. We interpret these findings in a Bayesian framework by also showing that prior exposure to long- or short-wavelength lights due to circadian type shapes the subjective experience of the dress stimulus in theoretically expected ways.
Binocular rivalry (BR) and motion-induced blindness (MIB) are two phenomena of visual awareness where perception alternates between multiple states despite constant retinal input. Both phenomena have been extensively studied, but the underlying processing remains unclear. It has been suggested that BR and MIB involve the same neural mechanism, but how the two phenomena compete for visual awareness in the same stimulus has not been systematically investigated. Here we introduce BR in a dichoptic stimulus display that can also elicit MIB and examine fluctuations of visual awareness over the course of each trial. Exploiting this paradigm we manipulated stimulus characteristics that are known to influence MIB and BR. In two experiments we found that effects on multistable percepts were incompatible with the idea of a common oscillator. The results suggest instead that local and global stimulus attributes can affect the dynamics of each percept differently. We conclude that the two phenomena of visual awareness share basic temporal characteristics but are most likely influenced by processing at different stages within the visual system.