Maria Pereverzeva

Attention to Bright Surfaces Enhances the Pupillary Light Reflex

One longstanding question is how early in the visual system attention exerts its influence. Here we show that an effect of attention can be measured at the earliest possible stage of visual information processing, as a change in the optics of the eye. We tested human subjects and found that covertly attending to bright surfaces results in an enhanced pupillary light reflex (PLR)—the pupillary constriction that occurs in response to light increments. The PLR optimizes the optical quality of the retinal image across illumination conditions, increasing sensitivity by modulating retinal illumination, and improving acuity by reducing spherical aberrations. The attentional modulation of the PLR that we describe constitutes a new mechanism through which vision is affected by attention; we discuss three alternatives for the neural substrates of this effect, including the possibility that attention might act indirectly, via its well established effects in early visual cortex.

Pupil constrictions to photographs of the sun

The pupil constricts in response to light increments and dilates with light decrements. Here we show that a picture of the sun, introducing a small overall decrease in light level across the field of view, results in a pupillary constriction. Thus, the pictorial representation of a high-luminance object (the sun) can override the normal pupillary dilation elicited by a light decrement. In a series of experiments that control for a variety of factors known to modulate pupil size, we show that the effect (a) does not depend on the retinal position of the images and (b) is modulated by attention. It has long been known that cognitive factors can affect pupil diameter by producing pupillary dilations. Our results indicate that high-level visual analysis (beyond the simple subcortical system mediating the pupillary response to light) can also induce pupillary constriction, with an effect size of about 0.1 mm.

Pupil size reflects the focus of feature-based attention

We measured pupil size in adult human subjects while they selectively attended to one of two surfaces, bright and dark, defined by coherently moving dots. The two surfaces were presented at the same location; therefore, subjects could select the cued surface only on the basis of its features. With no luminance change in the stimulus, we find that pupil size was smaller when the bright surface was attended and larger when the dark surface was attended: an effect of feature-based (or surface-based) attention. With the same surfaces at nonoverlapping locations, we find a similar effect of spatial attention. The pupil size modulation cannot be accounted for by differences in eye position and by other variables known to affect pupil size such as task difficulty, accommodation, or the mere anticipation (imagery) of bright/dark stimuli. We conclude that pupil size reflects not just luminance or cognitive state, but the interaction between the two: it reflects which luminance level in the visual scene is relevant for the task at hand.

Infant photometry: are mean adult isoluminance values a sufficient approximation to individual infant values?

Individual differences in isoluminance values were studied in infants and adults using a motion nulling paradigm. Two luminance-modulated sinusoidal grating components (spatial frequency=0.25 cpd, temporal frequency=5.6 Hz, speed=22.4 deg/s) were superimposed and moved in opposite directions across a color video screen. The contrasts of the two components were traded off to determine motion nulls. Two conditions were used: red/black vs. green/black, and red/black vs. blue/black grating components. An eye movement based response measure was used for infant subjects, and an average of 308 trials per infant were obtained. As observed in earlier studies, the mean motion null values for infants and adults were highly similar in each condition. The standard errors of motion null values for individual subjects were very small. Individual differences among infants were also small, and were clearly measurable only in the red/black vs. blue/black condition. The close similarity of mean null values, combined with the small individual differences among infants, supports the idea that under the right circumstances mean adult isoluminance values can be used as a sufficient approximation to individual infant isoluminance values in studies of infant color vision. These circumstances are discussed and evaluated in detail.

Neural activity in human V1 correlates with dynamic lightness induction.

Two circles of the same luminance will appear to have different lightness if one is embedded in a dark and another in a light surround. Known as simultaneous lightness contrast, this phenomenon demonstrates that our perceptions are not simply a reflection of the input from the retina but instead an inference about surface properties. Using functional magnetic resonance imaging (fMRI), we investigated whether the response in primary visual cortex (V1) more closely follows retinal information or perception. We induced illusory lightness changes of a disk by temporally modulating the surround luminance. In addition, we varied the luminance of the disk in order to disambiguate the fMRI response to perceived lightness modulation from the response to luminance contrast at the border of the disk. Perceptually, the disk with the lowest luminance (and the highest border contrast) had little or no induced lightness change while the disk with luminance equal to the time-averaged luminance of the surround (and the lowest border contrast) had the strongest induced lightness change. We found that neural activity in V1 strongly correlates with perceived lightness changes of the disk, suggesting significant involvement of early visual areas in processing surface lightness information.

Infant color vision: Influence of surround chromaticity on spontaneous looking preferences

When infants are tested with stimuli of various chromaticities embedded in a dark or achromatic (white) surround, they show maximal preference for stimuli of maximal colorimetric purity, and minimal preference for achromatic stimuli. We investigated how this pattern of preferences changes with changes of surround chromaticity. Sixteen-week-old infants were tested in two experimental conditions. The surrounds in the first condition were red and white; and in the second condition green and white. The three test stimuli varied in colorimetric purity from white to red in the first condition, and from white to green in the second condition. A test stimulus that appeared achromatic to adults when viewed in the chromatic surround was included. Infant spontaneous looking preferences changed with changes of surround chromaticity. The changes were consistent with the conclusion that infant looking behavior is governed by a preference for the stimuli that differ maximally in purity from the surround. The implications of this pattern of results are discussed.

Distant background information strongly affects lightness perception in dynamic displays

Lightness perception is strongly dependent on context, including the relative luminance of the adjacent surfaces, spatial configuration, and luminance contrast. The latter, local luminance contrast, is thought to be processed in relatively early stages of visual processing and has been shown to play a crucial role in lightness perception. However, more global processing, such as perceptual grouping of surfaces, can also have an effect on lightness perception. An unresolved question, which we will address in this paper, is how global and local processes interact. We used a static gray disk embedded in a temporally modulated in luminance ring, which gives rise to a lightness effect dependent on local luminance contrast. We manipulated global image information by presenting the stimulus on backgrounds of different luminances. Surprisingly, the induction effect was greatly attenuated at a background luminance equal to that of the disk. We show that this finding cannot be explained by common lightness induction models. However, it is consistent with an effect of grouping on lightness perception and demonstrates how processes that are dependent on local edge information can be overridden by global image information.

Simultaneous Color Contrast in 4-Month-Old Infants:

The present paper addresses the question of simultaneous color contrast in 4-month-old human infants. A temporal modulation paradigm was employed for infant testing. In this paradigm, infants viewed two test disks presented side-by-side: one of unchanging chromaticity (static) and another of the chromaticity varied in time (temporally modulated). The test stimuli were embedded in a surround that was either static or temporally modulated in phase with the modulated test stimulus. The temporally modulated test stimuli were chosen in such a way as to appear static to adults when viewed in the temporally modulated surround. On the basis of the observation that infants prefer to look more at flickering stimuli, the prediction is that, if infants have adult-like simultaneous color contrast, their preference for the temporally modulated stimulus should decrease and their preference for the static stimulus should increase when the surround is also temporally modulated as described. In concordance with this prediction, a significant increase in preference for the temporally static stimuli was observed with the introduction of temporal modulation in the surround. The data are consistent with the conclusion that infants as young as 4 months of age have simultaneous color contrast.

Luminance gradient configuration determines perceived lightness in a simple geometric illusion.

Accurate perception of surface reflectance poses a significant computational problem for the visual system. The amount of light reflected by a surface is affected by a combination of factors including the surface’s reflectance properties and illumination conditions. The latter are not limited by the strength of the illuminant but also include the relative placement of the light illuminating the surface, the orientation of the surface and its 3d shape, all of which result in a pattern of luminance gradients across the surface. In this study we explore how luminance gradients contribute to lightness perception. We introduce a novel, simple lightness illusion. It consists of six separate checks, organized in rows of two. Each check has a negative luminance gradient across it. The top and the bottom rows are the same: with the darker check on the left, and the lighter check on the right. Two checks in the middle row are identical; however, the check on the right appears darker than the check on the left. As there are no shared borders between the checks, simultaneous contrast cannot explain the effect. However, there are multiple possible explanations including spatial filtering (Blakeslee and McCourt, 2004) or some higher-order mechanism such as perceptual grouping or amodal completion. Here, we explore these possibilities by manipulating the luminance configurations and the gradient slopes of the checks.

Spatial nonuniformities and velocity of filling-in in dynamic brightness induction.

There is converging evidence that brightness induction is neither spatially uniform nor instantaneous. In the temporal domain, multiple studies have shown a relatively low (2-3 Hz) temporal frequency cutoff in brightness induction from temporally modulated surrounds (De Valois, Webster, De Valois, & Lingelbach, 1986; Paradiso & Nakayama, 1991; Rossi & Paradiso, 1996). It was also demonstrated that the temporal frequency cutoff depends on the spatial frequency of the stimulus (Rossi & Paradiso, 1996; see also Robinson & de Sa, 2008). These findings are consistent with the idea that brightness induction is mediated by a filling-in process that begins at the borders and travels at a relatively low velocity. However, in contradiction to observed low temporal cutoff in brightness induction, all earlier attempts to directly estimate the induction velocity resulted in very high estimates (e.g., 100°/s-200°/s). In the present study, using a novel paradigm, we obtained velocity estimates that are in line with predictions based on temporal frequency cutoff. In addition, the use of the same stimulus set to measure spatial nonuniformities, temporal frequency cutoff, and velocity allowed direct comparison of different velocity estimates.