Constrained sampling experiments reveal principles of detection in natural scenes.

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.

Retina-V1 model of detectability across the visual field.

A practical model is proposed for predicting the detectability of targets at arbitrary locations in the visual field, in arbitrary gray scale backgrounds, and under photopic viewing conditions. The major factors incorporated into the model include (a) the optical point spread function of the eye, (b) local luminance gain control (Weber's law), (c) the sampling array of retinal ganglion cells, (d) orientation and spatial frequency-dependent contrast masking, (e) broadband contrast masking, and (f) efficient response pooling. The model is tested against previously reported threshold measurements on uniform backgrounds (the ModelFest data set and data from Foley, Varadharajan, Koh, & Farias, 2007) and against new measurements reported here for several ModelFest targets presented on uniform, 1/f noise, and natural backgrounds at retinal eccentricities ranging from 0° to 10°.

Isoeccentric locations are not equivalent: the extent of the vertical meridian asymmetry.

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.

Equality judgments cannot distinguish between attention effects on appearance and criterion: a reply to Schneider (2011).

Whether attention modulates the appearance of stimulus features is debated. Whereas many previous studies using a comparative judgment have found evidence for such an effect, two recent studies using an equality judgment have not. Critically, these studies have relied on the assumption that the equality paradigm yields bias-free PSE estimates and is as sensitive as the comparative judgment, without testing these assumptions.

Trial-by-trial adjustments of top-down set modulate oculomotor capture.

The role of top-down control in visual search has been a subject of much debate. Recent research has focused on whether attentional and oculomotor capture by irrelevant salient distractors can be modulated through top-down control, and if so, whether top-down control can be rapidly initiated based on current task goals. In the present study, participants searched for a unique shape in an array containing otherwise homogeneous shapes.

Evaluating comparative and equality judgments in contrast perception: attention alters appearance.

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.

Voluntary attention increases perceived spatial frequency.

Voluntary covert 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. Involuntary attention increases perceived spatial frequency (Gobell & Carrasco, 2005), but involuntary attention can differ from voluntary attention in its effects on performance in tasks mediated by spatial resolution (Yeshurun, Montagna, & Carrasco, 2008).

Voluntary attention enhances contrast appearance.

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. We asked whether voluntary attention affects the subjective appearance of contrast--a fundamental dimension of visual perception.