Apparent contrast differs across the vertical meridian: Visual and attentional factors

Abstract

It is known that visual performance is better on the horizontal than the vertical meridian, and in the lower than the upper region of the vertical meridian (Vertical Meridian Asymmetry, "VMA"), and that exogenous spatial attention increases the apparent contrast of a stimulus. Here we investigate whether the VMA also leads to differences in the subjective appearance of contrast between the upper and lower vertical meridian, and how the effects of exogenous spatial attention on appearance interact with the VMA. Two Gabor stimuli were presented North and South of fixation at 4 degrees eccentricity along the vertical meridian. Observers were asked to report the orientation of the Gabor that was higher in contrast. By assessing which stimulus observers perceived to be higher in contrast, we obtained psychometric functions and their concomitant points of subjective equality (PSE). These functions were measured both when a neutral cue was presented in the middle of the display and transient attention was deployed via a peripheral cue to the location of one of the stimuli. Observers were told that the cues were uninformative as to the stimulus contrast or its orientation. We report two novel findings. First, apparent contrast is higher on the lower vertical meridian than on the upper. Second, the attentional enhancement of apparent contrast is asymmetrical with both low and high contrast stimuli; the effect of exogenous spatial attention is greater on the lower than the upper vertical meridian. As in prior studies, we find no corresponding asymmetry in orientation discrimination. Signal detection-based models explain the asymmetrical appearance effects as a function of differential multiplicative gain factors for the North and South locations, and predict a similar but much smaller asymmetry for orientation discrimination.

Cross-modal attention enhances perceived contrast

“…Crucially, the cueing of attention enhanced neural processing in the same ventral regions of the visual cortex, which, as the authors confirm, are responsive to physical differences in contrast. These results are consistent with the proposal that attention increases perceived contrast by boosting early sensory processing in visual cortex (9,(12)(13)(14), and they contradict the hypothesis that the effect of attention is due to a decisional bias (20).…”

“…By computing the contrast that is necessary for the attended stimulus to match the apparent contrast of the unattended stimulus, this paradigm allows one to measure the change that attention effects on the perceived contrast of a stimulus. This paradigm, coupled with control experiments, has ruled out alternative cue bias and response bias explanations (9,(12)(13)(14)(15)(16)(17)(18).…”

“…Using this paradigm, studies have shown that exogenous attention alters our subjective impression of many dimensions of spatial and temporal vision: e.g., contrast (9,(12)(13)(14), spatial resolution (15), color saturation (16), size of moving patterns (17), and speed (18). However, the issue continues to be debated (19,20).…”

Are the effects of attention on speed judgments genuinely perceptual?

“…Both the model proposed by Schneider and Komlos (2008) and the one proposed by Fuller, Rodriguez, and Carrasco (2008) are formulated in the signal detection theory framework. In both models, the signal intensities produced by the two stimuli are evaluated, leading to a difference signal.…”

Attending to illusory differences in object size

“…It is known that attentional resolution is greater in the lower visual field (He, Cavanaugh, & Intrilligator, 1996 and that attention increases apparent contrast more in the lower vertical meridian than in the upper vertical meridian (Fuller, Rodriguez & Carrasco, 2008). If RTs are faster to targets lower in the scene simply due to an enhancement of attentional resolution or of apparent contrast between the target and the rectangle, this could provide an alternative account of the results of Experiment 2a, because the perceptually smaller rectangle always appeared lower in the photograph than the perceptually larger rectangle.…”