Performance in visual tasks is limited by the low-level mechanisms that sample the visual field. It is well documented that contrast sensitivity and spatial resolution decrease as a function of eccentricity and that those factors impair performance in “higher level” tasks, such as visual search. Performance also varies consistently at isoeccentric locations in the visual field. Specifically, at a fixed eccentricity, performance is better along the horizontal meridian than the vertical meridian, and along the lower than the upper vertical meridian. Whether these asymmetries in visual performance fields are confined to the vertical meridian or extend across the whole upper versus lower visual hemifield has been a matter of debate. Here, we measure the extent of the upper versus lower asymmetry. Results reveal that this asymmetry is most pronounced at the vertical meridian and that it decreases gradually as the angular distance (polar angle) from the vertical meridian increases, with eccentricity held constant. Beyond 30° of polar angle from the vertical meridian, the upper to lower asymmetry is no longer reliable. Thus, the vertical meridian is uniquely asymmetric and uniquely insensitive. This pattern of results is consistent with early anatomical properties of the visual system and reflects constraints that are critical to our understanding of visual information processing.

Voluntary (endogenous, sustained) covert spatial attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. Here we ask whether voluntary attention affects the subjective appearance of contrast-a fundamental dimension of visual perception. We used a demanding rapid serial visual presentation (RSVP) task to direct endogenous attention to a given location and measured perceived contrast at the attended and unattended locations. Attention increased perceived contrast of supra-threshold stimuli and also improved performance on a concurrent orientation discrimination task at the cued location. We ruled out response bias as an alternative account. Thus, this study establishes that voluntary attention enhances perceived contrast. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.Our sensory systems have a limited information processing capacity. For example, the visual world normally contains much more information than we can process at a given time. Visual attention helps us to overcome this limitation by selecting certain aspects of the scene for prioritized processing. Attention improves behavioral performance in a variety of tasks (reviewed in ) and enhances the neural processing of sensory stimuli (reviewed in (Reynolds & Chelazzi, 2004)). Although these effects are well-established, the effect of voluntary attention on subjective appearance is still a matter of debate. In this study, we ask: When one pays attention to a location, does the object at that location look different?This question has been debated since the beginning of experimental psychology and physiology. Whereas Fechner believed that attention does not alter sensory impressions, James and Helmholtz claimed that attention intensifies sensory impressions (Helmholtz, 1866; James, 1890James, /1983. Empirical evidence regarding voluntary attention and appearance has been scarce and mixed: the finding that attention reduces perceived brightness contrast (Tsal, Shalev, Zakay, & Lubow, 1994) was disputed by a report that attention reduced response variability without changing brightness perception (Prinzmetal, Nwachuku, Bodanski, Blumenfeld, & Shimizu, 1997). Another study reported that attention increased brightness of overlapping transparent surfaces (Tse, 2005). However, it is not clear whether this effect is attributable to the perceptual grouping mechanism involving multiple surfaces or to a general mechanism of attention. Thus the question remains as to whether attention can change the appearance of a single, isolated stimulus; here, we investigated the effect of voluntary attention on perceived contrast.

A fundamental everyday visual task is to detect target objects within a background scene. Using relatively simple stimuli, vision science has identified several major factors that affect detection thresholds, including the luminance of the background, the contrast of the background, the spatial similarity of the background to the target, and uncertainty due to random variations in the properties of the background and in the amplitude of the target. Here we use an experimental approach based on constrained sampling from multidimensional histograms of natural stimuli, together with a theoretical analysis based on signal detection theory, to discover how these factors affect detection in natural scenes. We sorted a large collection of natural image backgrounds into multidimensional histograms, where each bin corresponds to a particular luminance, contrast, and similarity. Detection thresholds were measured for a subset of bins spanning the space, where a natural background was randomly sampled from a bin on each trial. In low-uncertainty conditions, both the background bin and the amplitude of the target were fixed, and, in high-uncertainty conditions, they varied randomly on each trial. We found that thresholds increase approximately linearly along all three dimensions and that detection accuracy is unaffected by background bin and target amplitude uncertainty. The results are predicted from first principles by a normalized matched-template detector, where the dynamic normalizing gain factor follows directly from the statistical properties of the natural backgrounds. The results provide an explanation for classic laws of psychophysics and their underlying neural mechanisms.natural scene statistics | detection | masking | normalization | Weber's law V isual systems are the result of evolution by natural selection, and, as a consequence, their design is strongly constrained by the properties of natural visual stimuli and by the specific visual tasks performed to survive and reproduce. Thus, to understand the human visual system, it is critical to characterize natural visual stimuli and performance in natural visual tasks (1, 2).Perhaps the most fundamental visual task is to identify target objects in the natural backgrounds that surround us. It is known that the specific properties of a background can have a strong influence on detectability (Fig. 1). For example, the detectability of a target pattern with given amplitude decreases with increases in background luminance (3, 4), background contrast (5-7), and the similarity of the spatial properties of the background to those of the target (8-12). In addition to the direct effects of such background properties, there are other factors that affect detection performance. Specifically, under natural conditions, the strength (amplitude) and location of the target often randomly vary on every occasion, and the target typically appears against a different background scene on every occasion. The uncertainty created by the random amplitude and location of the target ("target...

Covert attention not only improves performance in many visual tasks but also modulates the appearance of several visual features. Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment (e.g., M. Carrasco, S. Ling, & S. Read, 2004). Recently, K. A. Schneider and M. Komlos (2008) questioned the validity of those results because they did not find a significant effect of attention on contrast appearance using an equality task. They claim that such equality judgments are bias-free whereas comparative judgments are bias-prone and propose an alternative interpretation of the previous findings based on a decision bias. However, to date there is no empirical support for the superiority of the equality procedure. Here, we compare biases and sensitivity to shifts in perceived contrast of both paradigms. We measured contrast appearance using both a comparative and an equality judgment. Observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1 and 2) or exogenous attention was drawn to it (Experiments 3 and 4). We demonstrate several methodological limitations of the equality paradigm. Nevertheless, both paradigms capture shifts in PSE due to physical and perceived changes in contrast and show that attention enhances apparent contrast.

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