Incongruence in number–luminance congruency effects

Congruency tasks have provided support for an amodal magnitude system for magnitudes that have a "spatial" character, but conflicting results have been obtained for magnitudes that do not (e.g., luminance). In this study, we extricated the factors that underlie these number-luminance congruency effects and tested alternative explanations: (unsigned) luminance contrast and saliency. When luminance had to be compared under specific task conditions, we revealed, for the first time, a true influence of number on luminance judgments: Darker stimuli were consistently associated with numerically larger stimuli. However, when number had to be compared, luminance contrast, not luminance, influenced number judgments. Apparently, associations exist between number and luminance, as well as luminance contrast, of which the latter is probably stronger. Therefore, similar tasks, comprising exactly the same stimuli, can lead to distinct interference effects.     

Object recognition and Random Image Structure Evolution

We present a technique called Random Image Structure Evolution (RISE) for use in experimental investigations of high-level visual perception. Potential applications of RISE include the quantitative measurement of perceptual hysteresis and priming, the study of the neural substrates of object perception, and the assessment and detection of subtle forms of agnosia. In simple terms, RISE involves the measurement of perceptual and/or neural responses as visual stimuli are systematically transformed—in particular, as recognizable objects evolve from, then dissolve into, randomness. Points along the sequences corresponding to the onset and offset of subjects’ percepts serve as markers for quantitatively and objectively characterizing several perceptual phenomena. Notably, these image sequences are created in a manner that strictly controls a number of important low-level image properties, such as luminance and frequency spectra, thus reducing confounds in the analysis of high-level visual processes. Here we describe the RISE paradigm, report the results of a few basic RISE experiments, and discuss a number of experimental and clinical applications of this approach.     

Resisting change: The influence of luminance changes on visual marking and the preview benefit

Visual search can benefit when one set of distractors is presented as a preview, prior to the appearance the second set of distractors plus the target (Watson & Humphreys, 1997). It has been shown that changing the shape of the old, previewed stimuli when the new items appear causes the old stimuli to recompete for selection with the new ones. In contrast, changing the luminance or color of the old stimuli has no detrimental effects. Here, we present five experiments that reassessed the effect of luminance changes in preview search. We show (1) that preview search is remarkably resistant to large changes in the absolute luminance of the old stimuli, even when those changes would ordinarily be sufficient to signal the appearance of a new object and draw attention (Experiments 1 and 2), and (2) that resistance to luminance changes can be bolstered by feature-based inhibitory processes (Experiments 3–5). These findings are discussed in terms of the possible ecological properties of time-based visual selection and possible mechanisms underlying the preview benefit.     

Isoluminant motion onset captures attention

In their 2003 article, Abrams and Christ found that the onset of motion captured attention more effectively than either the offset of motion or continuous motion. Abrams and Christ conceptualized the capture to be occurring at a level higher than does detection of luminance changes in the stimulus. To examine this claim, in the present experiments we replicated their critical experiment but used isoluminant stimuli, which do not produce the low-level luminance transients typically associated with motion. Under isoluminant conditions, we found a pattern of results very similar to that found previously with luminance-defined stimuli, indicating that attention can be prioritized on the basis of perceived motion onset by an object in the absence of low-level luminance transients. This may reflect an evolutionary adaptation to bias attention toward objects that exhibit characteristics of animacy, such as abruptly changing from a static to a dynamic state.     

The role of first- and second-order stimulus features for human overt attention

When processing complex visual input, human observers sequentially allocate their attention to different subsets of the stimulus. What are the mechanisms and strategies that guide this selection process? We investigated the influence of various stimulus features on human overt attention--that is, attention related to shifts of gaze with natural color images and modified versions thereof. Our experimental modifications, systematic changes of hue across the entire image, influenced only the global appearance of the stimuli, leaving the local features under investigation unaffected. We demonstrated that these modifications consistently reduce the subjective interpretation of a stimulus as "natural" across observers. By analyzing fixations, we found that first-order features, such as luminance contrast, saturation, and color contrast along either of the cardinal axes, correlated to overt attention in the modified images. In contrast, no such correlation was found in unmodified outdoor images. Second-order luminance contrast ("texture contrast") correlated to overt attention in all conditions. However, although none of the second-order color contrasts were correlated to overt attention in unmodified images, one of the second-order color contrasts did exhibit a significant correlation in the modified images. These findings imply, on the one hand, that higher-order bottom-up effects--namely, those of second-order luminance contrast--may partially account for human overt attention. On the other hand, these results also demonstrate that global image properties, which correlate to the subjective impression of a scene being "natural," affect the guidance of human overt attention.     

数量适应后效的皮层映射特征

本研究探讨了观察者与观察目标存在相对运动时视觉系统对目标数量特征的适应后效的皮层映射特征, 并与对比度适应后效的映射规律进行比较。包括两项实验。其中, 实验一要求被试在适应目标后转换注视点, 考察眼跳后相同和不同视网膜区域以及相同和不同空间区域的适应后效, 发现数量适应后效具有部分空间-皮层映射特性, 而对比度适应后效则表现出完全的视网膜-皮层映射特征。实验二采用固定的注视点, 考察目标运动后目标原位置和新位置区域的适应后效, 发现数量适应后效不完全依赖于视网膜-皮层映射, 它可以“追随”客体运动重新映射到新的位置, 表现出基于客体映射的特征, 而对比度适应后效则完全依赖于视网膜-皮层映射, 不能“追随”客体移动在目标新位置重新形成映射。两项实验结果提示, 相对于对比度等低级表面特征而言, 数量特征对目标的描述涉及更高的加工水平, 它可以与观察目标的相对运动信息进行整合, 且这种整合在眼跳和非眼跳的观察条件下都可发生。     

“Filling-in” colour in natural scenes

Our subjective experience of the world as being in full colour across the entire visual field is at odds with the highly fovea-biased distribution of cones in the retina. It is unclear how this percept of “pan-field colour” comes about. We use novel stimuli—“colour chimeras”—to demonstrate a related visual phenomenon in which observers perceive rich colour throughout images with large achromatic regions. This percept appears to critically depend on natural scene statistics. By separately manipulating chromatic and structural content in such images, we demonstrate that both the spatial distribution of colour and the presence of recognizable scene structure contribute to the experience of pan-field colour in these stimuli. Our results suggest that this percept is unlikely to be due to a low-level colour spreading process. Instead, we suggest that mechanisms dependent on natural scenes’ chromatic and luminance statistics provide the basis for the phenomenon.     

Learning efficient visual search for stimuli containing diagnostic spatial configurations and color-shape conjunctions

Visual search is often slow and difficult for complex stimuli such as feature conjunctions. Search efficiency, however, can improve with training. Search for stimuli that can be identified by the spatial configuration of two elements (e.g., the relative position of two colored shapes) improves dramatically within a few hundred trials of practice. Several recent imaging studies have identified neural correlates of this learning, but it remains unclear what stimulus properties participants learn to use to search efficiently. Influential models, such as reverse hierarchy theory, propose two major possibilities: learning to use information contained in low-level image statistics (e.g., single features at particular retinotopic locations) or in high-level characteristics (e.g., feature conjunctions) of the task-relevant stimuli. In a series of experiments, we tested these two hypotheses, which make different predictions about the effect of various stimulus manipulations after training. We find relatively small effects of manipulating low-level properties of the stimuli (e.g., changing their retinotopic location) and some conjunctive properties (e.g., color-position), whereas the effects of manipulating other conjunctive properties (e.g., color-shape) are larger. Overall, the findings suggest conjunction learning involving such stimuli might be an emergent phenomenon that reflects multiple different learning processes, each of which capitalizes on different types of information contained in the stimuli. We also show that both targets and distractors are learned, and that reversing learned target and distractor identities impairs performance. This suggests that participants do not merely learn to discriminate target and distractor stimuli, they also learn stimulus identity mappings that contribute to performance improvements.     

Spotting animals in natural scenes: efficiency of humans and monkeys at very low contrasts

The ability of monkeys to categorize objects in visual stimuli such as natural scenes might rely on sets of low-level visual cues without any underlying conceptual abilities. Using a go/no-go rapid animal/non-animal categorization task with briefly flashed achromatic natural scenes, we show that both human and monkey performance is very robust to large variations of stimulus luminance and contrast. When mean luminance was increased or decreased by 25–50%, accuracy and speed impairments were small. The largest impairment was found at the highest luminance value with monkeys being mainly impaired in accuracy (drop of 6% correct vs. <1.5% in humans), whereas humans were mainly impaired in reaction time (20 ms increase in median reaction time vs. 4 ms in monkeys). Contrast reductions induced a large deterioration of image definition, but performance was again remarkably robust. Subjects scored well above chance level, even when the contrast was only 12% of the original photographs (≈81% correct in monkeys; ≈79% correct in humans). Accuracy decreased with contrast reduction but only reached chance level -in both species- for the most extreme condition, when only 3% of the original contrast remained. A progressive reaction time increase was observed that reached 72 ms in monkeys and 66 ms in humans. These results demonstrate the remarkable robustness of the primate visual system in processing objects in natural scenes with large random variations in luminance and contrast. They illustrate the similarity with which performance is impaired in monkeys and humans with such stimulus manipulations. They finally show that in an animal categorization task, the performance of both monkeys and humans is largely independent of cues relying on global luminance or the fine definition of stimuli.     

Relational Learning in Pigeons?

Two experiments investigated whether discrimination learning and transposition by pigeons were facilitated by the opportunity to compare rectangles differing in luminance or stars differing in number of vertices. In Experiment 1, one group was trained with stimuli from the same dimension appearing simultaneously on each trial, but for a second group such stimuli appeared on separate trials. The opportunity to compare stimuli from the same dimension on a single trial facilitated the learning of the luminance discrimination but not of the stars discrimination. Such comparison also resulted in greater luminance but not greater stars transposition. Using a different training procedure, Experiment 2 confirmed that the opportunity for comparison facilitated a luminance discrimination. The results for the star discrimination are entirely consistent with 'absolute' theories of discrimination learning; but the results for the luminance discrimination suggest some kind of 'relational' learning. Given the difference between the star and luminance discriminations, a low-level, sensory theory of relational learning seems most consistent with the data.     

Adaptation of suprathreshold contrast and luminance stimuli

The temporal and spatial properties of the difference in perceived contrast and brightness of two suprathreshold stimuli presented successively in different retinal locations were determined. Stimulus onset asynchrony (SOA) was varied and the perceived contrast or brightness of the first stimulus (S1) was measured as a function of SOA by matching the contrast or luminance of the second stimulus (S2) to that of S1. The two stimuli overlapped in time for 200 ms to allow the comparison to be made. The adjusted values for S2 could well be fitted with an exponential decay function of SOA. For luminance increments and decrements the time constant for this function was 253 ms; for checkerboards with checks of size 16 min square the time constant was 164 ms. The difference in perceived contrast was dependent on initial contrast in a nonlinear fashion. It increased with increasing check size and was independent of the mean luminance and spatial proximity of the two stimuli. The phenomenon was observed with different pattern types and with dichoptic presentation, but could only be seen when direct comparison of the two stimuli was possible.     

Constructing isoluminant stimuli for word recognition research: A precautionary study

Isoluminant stimuli are used increasingly often to investigate processes underlying visual word recognition. However, construction of isoluminant stimuli is not straightforward, and inappropriate construction may have the result of misinforming theories that relate word recognition to neurological function. To inform the use of isoluminant stimuli in studies of word recognition, the present article details two experiments in which isoluminant stimuli were constructed using physical onscreen luminance matching and heterochromatic flicker photometry (HFP) with four different stimulus types: disks, squares, rectangles, and letter strings. The findings reveal (1) substantial differences between isoluminance determined by physical onscreen luminance matching and HFP, (2) substantial differences in HFP isoluminance across stimulus types, and (3) substantial differences in HFP isoluminance across participants. These findings indicate that, in contrast to common practice in word recognition research, HFP provides a better indication of isoluminance than physical onscreen matching; but HFP stimuli should match those used in the experiment proper and should be used to assess isoluminance individually for each participant.     

The effect of spatial attention on invisible stimuli

The influence of selective attention on visual processing is widespread. Recent studies have demonstrated that spatial attention can affect processing of invisible stimuli. However, it has been suggested that this effect is limited to low-level features, such as line orientations. The present experiments investigated whether spatial attention can influence both low-level (contrast threshold) and high-level (gender discrimination) adaptation, using the same method of attentional modulation for both types of stimuli. We found that spatial attention was able to increase the amount of adaptation to low- as well as to high-level invisible stimuli. These results suggest that attention can influence perceptual processes independent of visual awareness.     

A new method for calibrating perceptual salience across dimensions in infants: the case of color vs. luminance

We report a new method for calibrating differences in perceptual salience across feature dimensions, in infants. The problem of inter-dimensional salience arises in many areas of infant studies, but a general method for addressing the problem has not previously been described. Our method is based on a preferential looking paradigm, adapted to determine the relative salience of two stimuli. We report here on the case of stimuli differing in color and luminance, though the method has wider potential. We were able to determine on a psychophysical curve the point at which a color contrast was equally salient to infants as a given luminance contrast. We then used these calibrated, 'iso-salient' stimuli in an object memory study. Results showed that 6.5-month-old infants noticed a color, but not a luminance, change while tracking an occluded object. Our method should have numerous applications in the study of bottom-up effects on infant attention and visual working memory.     

Effects of absolute luminance and luminance contrast on visual discrimination in low mesopic environments

Recent research revealed considerable decline in visual perception under low luminance conditions. However, systematic studies on how visual performance is affected by absolute luminance and luminance contrast under low mesopic conditions (<0.5 cd/m²) is lacking. We examined performance in a simple visual discrimination task under low mesopic luminance conditions in three experiments in which we systematically varied base luminance and luminance contrast between stimulus and background. We further manipulated eccentricity of the stimuli because of known rods and cones gradients along the retina. We identified a “deficiency window” for performance as measured by d’ when luminance was < 0.06 cd/m² and luminance contrast as measured by the luminance ratio between stimulus and background was below < 1.7. We further calculated performance-based luminance as well as contrast efficiency functions for reaction times (RTs). These power functions demonstrate the contrast asymptote needed to decrease RTs and how such a decrease can be achieved given various combinations of absolute luminance and luminance contrast manipulations. Increased eccentricity resulted in slower RTs indicative of longer scan distances. Our data provide initial insights to performance-based efficiency functions in low mesopic environments that are currently lacking and to the physical mechanisms being utilized for visual perception in these extreme environments.     

Effective luminance contrast as a parameter in contingent aftereffects

Variation in the magnitude of contingent aftereffects was explored as a function of the luminance contrast ratio profile of inspection stimuli. Positive contrast stimuli (chromatic bars interspersed with achromatically brighter bars) produced substantially attenuated aftereffects compared with negative contrast stimuli (chromatic and dark bars). This attenuation was hypothesized to result from retinal image deterioration of positive contrast gratings due to retinal reflection/scatter. In a second experiment, subjects selected for their light retinal pigmentation manifested substantial enhancement of the asymmetry between the aftereffects generated by positive and negative contrast gratings. Those with dark fundus pigmentation showed symmetrical aftereffects.     

Transparency: relation to depth, subjective contours, luminance, and neon color spreading

The perception of transparency is highly dependent on luminance and perceived depth. An image region is seen as transparent if it is of intermediate luminance relative to adjacent image regions, and if it is perceived in front of another region and has a boundary which provides information that an object is visible through this region. Yet, transparency is not just the passive end-product of these required conditions. If perceived transparency is triggered, a number of seemingly more elemental perceptual primitives such as color, contour, and depth can be radically altered. Thus, with the perception of transparency, neon color spreading becomes apparent, depth changes, stereoscopic depth capture can be eliminated, and otherwise robust subjective contours can be abolished. In addition, we show that transparency is not coupled strongly to real-world chromatic constraints since combinations of luminance and color which would be unlikely to arise in real-world scenes still give rise to the perception of transparency. Rather than seeing transparency as a perceptual end-point, determined by seemingly more primitive processes, we interpret perceived transparency as much a 'cause', as an 'effect'. We speculate that the anatomical substrate for such mutual interaction may lie in cortical feed-forward connections which maintain modular segregation and cortical feedback connections which do not.     

The effects of spatial phase on reaction time to spatially filtered images

Summary Many studies of visual perception have used periodic stimuli such as sine-wave gratings and checker-board patterns. It is well known that reaction time (RT) to such stimuli increases with increasing spatial frequency and decreasing contrast. While this is the case with periodic stimuli it is not clear that these relationships obtain for aperiodic stimuli such as natural scenes. A digitized image of an object (a vase) was submitted to two-dimensional Fourier analysis. Four pairs of spatial frequency band-limited images were created for each image. Each pair consisted of a normal-phase (NP) and a scrambled-phase (SP) version, with the magnitude spectrum and space-averaged luminance the same within each pair. Filter band-widths were 1 octave wide. Manual RT was measured for onset and offset of each spatially filtered image. Mean RT for SP images increased significantly with increasing spatial frequency, while no other significant differences were found with the NP images. This suggests that the temporal processing of complex, aperiodic images is influenced by the spatial frequency and contrast of local regions within the image, rather than by the space-averaged contrast of the entire image, and cannot be predicted by global estimates of contrast and spatial frequency.     

Looking more masculine among females: Spatial context modulates gender perception of face and biological motion

Perception of visual information highly depends on spatial context. For instance, perception of a low-level visual feature, such as orientation, can be shifted away from its surrounding context, exhibiting a simultaneous contrast effect. Although previous studies have demonstrated the adaptation aftereffect of gender, a high-level visual feature, it remains largely unknown whether gender perception can also be shaped by a simultaneously presented context. In the present study, we found that the gender perception of a central face or a point-light walker was repelled away from the gender of its surrounding faces or walkers. A norm-based opponent model of lateral inhibition, which accounts for the adaptation aftereffect of high-level features, can also excellently fit the simultaneous contrast effect. But different from the reported contextual effect of low-level features, the simultaneous contrast effect of gender cannot be observed when the centre and the surrounding stimuli are from different categories, or when the surrounding stimuli are suppressed from awareness. These findings on one hand reveal a resemblance between the simultaneous contrast effect and the adaptation aftereffect of high-level features, on the other hand highlight different biological mechanisms underlying the contextual effects of low- and high-level visual features.     

Fixation durations in scene viewing: Modeling the effects of local image features,oculomotor parameters,and task

Scene perception requires the orchestration of image- and task-related processes with oculomotor constraints. The present study was designed to investigate how these factors influence how long the eyes remain fixated on a given location. Linear mixed models (LMMs) were used to test whether local image statistics (including luminance, luminance contrast, edge density, visual clutter, and the number of homogeneous segments), calculated for 1° circular regions around fixation locations, modulate fixation durations, and how these effects depend on task-related control. Fixation durations and locations were recorded from 72 participants, each viewing 135 scenes under three different viewing instructions (memorization, preference judgment, and search). Along with the image-related predictors, the LMMs simultaneously considered a number of oculomotor and spatiotemporal covariates, including the amplitudes of the previous and next saccades, and viewing time. As a key finding, the local image features around the current fixation predicted this fixation’s duration. For instance, greater luminance was associated with shorter fixation durations. Such immediacy effects were found for all three viewing tasks. Moreover, in the memorization and preference tasks, some evidence for successor effects emerged, such that some image characteristics of the upcoming location influenced how long the eyes stayed at the current location. In contrast, in the search task, scene processing was not distributed across fixation durations within the visual span. The LMM-based framework of analysis, applied to the control of fixation durations in scenes, suggests important constraints for models of scene perception and search, and for visual attention in general.