Mary J. Bravo

The role of attention in different visual-search tasks

Observers viewed displays containing a variable number of distractors of one color and a target of another color. In some experiments, the target and distractors maintained their color from trial to trial; in others, they reversed unpredictably. Observers made a speeded two-choice judgment concerning either the presence, the color, or the shape of the odd-colored target. With only one exception, all of these conditions produced the same pattern of results: reaction times remained constant as the number of distractors increased. The exceptional result occurred when observers judged the shape of the odd-colored target and the color of the target and distractors reversed unpredictably. In this case, reaction times decreased as the number of distractors increased. These results are interpreted in terms of the attentional requirements of the different judgments and the mechanisms that guide attention.

Preattentive vision and perceptual groups.

Recent evidence suggests that preattentive processing may not be limited to the analysis of simple stimulus features as previously suggested. To explore this idea a visual search task was used to test whether the shapes of several perceptual groups can be processed in parallel. Textured displays that give rise to strong perceptual grouping were used to create figures on a background. Search times for a target figure distinguished by a unique shape were found to be independent of the number of distractor figures in the display. This result indicates that perceptual groups may be processed in parallel and suggests an expanded role for preattentive processing in vision.

Cats see subjective contours

Behavioural techniques were used to determine whether cats are able to see subjective contours. Through several stages of testing with increasingly complex displays, cats continued to respond to a figure defined by subjective contours. This result provides the first direct evidence that a nonhuman perceives subjective contours.

Stereo matching precedes dichoptic masking

Stereo matching can intervene to prevent dichoptic masking. In a dichoptic masking paradigm we measured the contrast threshold for a bar target, presented to one eye, as a function of the contrast of an identical masking bar, presented at retinal correspondence in the other eye. Confirming previous studies of dichoptic masking with sinusoidal gratings, the test bar thresholds rose proportionally with increasing masking contrast. This threshold elevation was almost nullified when an extra bar was presented to the eye seeing the test stimulus. Release from masking occurred when the disparity between the masking bar and extra bar was < 20 min arc over a range of contrast levels (8-45%), and for bars containing either broad spatial frequency spectra or bars with only high spatial frequencies (peak = 12 c/deg). The latter result rules out an explanation for the release from masking based on contrast discrimination in low spatial frequency channels. The extra bar was effective in releasing the test bar from masking as long as the extra bars contrast was greater than about one-fifth the contrast of the mask, a result that suggests that there is a contrast threshold for stereo matching. We interpret our findings to indicate that a stage of stereo matching occurs prior to the neural site limiting dichoptic contrast discrimination.

The specificity of the search template

When searching for a target object, observers use an internal representation of the targets appearance as a search template. This study used naturalistic stimuli to examine the specificity of this template. Observers first learned several name-image pairs; they then participated in a search experiment in which the names served as cues and the images served as targets. To test whether the observers searched for the targets using an exact image template, we included targets that were transformations of the studied image and targets that belonged to the same subordinate-level category as the studied image. The same stimuli were also used in a search experiment involving image cues. The name cue and image cue experiments produced different patterns of results. Unlike image cues, name cues produced similar benefits for transformations of the studied images as for the studied images themselves. Also unlike image cues, names cues produced no benefit for members of the same subordinate-level category as the studied image. These results suggest that when observers are trained on an image, they develop a search template that is relatively specific for the image but still tolerant to changes in scale and orientation.

A scale invariant measure of clutter.

We propose a measure of clutter for real images that can be used to predict search times. This measure uses an efficient segmentation algorithm (P. Felzenszwalb & D. Huttenlocher, 2004) to count the number of regions in an image. This number is not uniquely defined, however, because it varies with the scale of segmentation. The relationship between the number of regions and the scale of segmentation follows a power law, and the exponent of the power law is similar across images. We fit power law functions to the multiple scale segmentations of 160 images. The power law exponent was set to the average value for the set of images, and the constant of proportionality was used as a measure of image clutter. The same 160 images were also used as stimuli in a visual search experiment. This scale-invariant measure of clutter accounted for about 40% of the variance in the visual search times.

Search for a category target in clutter

An airport security worker searching a suitcase for a weapon is engaging in an especially difficult search task: the target is not well-specified, it is not salient, and it is not predicted by its context. Under these conditions, search may proceed item-by-item. In the experiment reported here we tested whether the items for this form of search are whole familiar objects. Our displays were composed of color photographs of ordinary objects, that were either uniform in color and texture (simple), or had two or more parts with different colors or textures (compound). The observers task was to detect the presence of a target belonging to a broad category (food). We found that when the objects were presented in a sparse array, search times to find the target were similar for displays composed of simple and compound objects. But when the same objects were presented as dense clutter, search functions were steeper for displays composed of compound objects. We attribute this difference to the difficulty of segmenting compound objects in clutter: compared with simple objects, compound objects are less likely to be organized into a single object by bottom-up grouping processes. Our results indicate that while search rates in a sparse display may be determined by the number of objects, search rates in clutter are also affected by the number of object parts.

Image forensic analyses that elude the human visual system

While historically we may have been overly trusting of photographs, in recent years there has been a backlash of sorts and the authenticity of photographs is now routinely questioned. Because these judgments are often made by eye, we wondered how reliable the human visual system is in detecting discrepancies that might arise from photo tampering. We show that the visual system is remarkably inept at detecting simple geometric inconsistencies in shadows, reflections, and perspective distortions. We also describe computational methods that can be applied to detect the inconsistencies that seem to elude the human visual system.

Perceptual discrimination of computer generated and photographic faces

Abstract Modern day computer graphics are capable of generating highly photorealistic images resulting in challenging legal situations. For example, as a result of a 2002 U.S. Supreme Court ruling, computer generated child pornography is protected speech, while pornographic photographs depicting an actual child remains illegal. The ability to distinguish between protected and illegal material assumes that law enforcement agents, attorneys, jurors, and judges can reliably distinguish between computer generated and photographic imagery. We describe a series of psychophysical experiments that used images of varying resolution, JPEG compression, and color to explore the ability of observers to distinguish computer generated from photographic images of people. The results allow us to assign a probability that an image that is judged to be a photograph is, in fact, a photograph.

Evidence for two speed signals: a coarse local signal for segregation and a precise global signal for discrimination

In calculating the precise speed of an object, the visual system must integrate motion measurements across time and space while keeping motion measurements from different objects separate. We examined whether an initial coarse estimate of local speed may be used to segregate the motions of different objects prior to a precise calculation of object speed. Our stimuli consisted of 256 dots that moved upward at two speeds. In Expt 1, each dot alternated between the two speeds every 133 msec. When the speed alternations were asynchronous across dots, subject saw two transparent surfaces moving at different speeds and their ability to discriminate changes in the slow speed were unaffected by the presence of the fast speed. This experiment suggests that before integration, motion measurements may be segregated according to speed. We sought more conclusive evidence for this claim in Expts 2 and 3. In Expt 2, dots with 33 msec lifetimes were used to generate the two speeds. Although individual dots permitted only crude speed discrimination, subjects perceived this stimulus as two surfaces moving at different speeds and they precisely judged the slower speed. Apparently, the coarse local signals generated by the slow dots were segregated from those of the fast dots and then separately integrated to produce a precise speed signal. In Expt 3, the dots again moved at two speeds, but each speed was generated by a range of spatial and temporal displacements. Once more, subjects saw two surfaces and precisely judged the speed of the slower surface, demonstrating that segregation may be based solely on differences in local speed.(ABSTRACT TRUNCATED AT 250 WORDS)