The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Neural Mechanisms of Selective Visual Attention

Part I. Experimental Studies: 2. Experiment in psychology 3. Experiments on perceiving III Experiments on imaging 4-8. Experiments on remembering: (a) The method of description (b) The method of repeated reproduction (c) The method of picture writing (d) The method of serial reproduction (e) The method of serial reproduction picture material 9. Perceiving, recognizing, remembering 10. A theory of remembering 11. Images and their functions 12. Meaning Part II. Remembering as a Study in Social Psychology: 13. Social psychology 14. Social psychology and the matter of recall 15. Social psychology and the manner of recall 16. Conventionalism 17. The notion of a collective unconscious 18. The basis of social recall 19. A summary and some conclusions.

Remembering. A Study in Experimental and Social Psychology, Cambridge (University Press) 1964.

Five important trends have emerged from recent work on computational models of focal visual attention that emphasize the bottom-up, image-based control of attentional deployment. First, the perceptual saliency of stimuli critically depends on the surrounding context. Second, a unique 'saliency map' that topographically encodes for stimulus conspicuity over the visual scene has proved to be an efficient and plausible bottom-up control strategy. Third, inhibition of return, the process by which the currently attended location is prevented from being attended again, is a crucial element of attentional deployment. Fourth, attention and eye movements tightly interplay, posing computational challenges with respect to the coordinate system used to control attention. And last, scene understanding and object recognition strongly constrain the selection of attended locations. Insights from these five key areas provide a framework for a computational and neurobiological understanding of visual attention.

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Computational modelling of visual attention.

In 1974 an article appeared in Science magazine with the dry-sounding title “Judgment Under Uncertainty: Heuristics and Biases” by a pair of psychologists who were not well known outside their discipline of decision theory. In it Amos Tversky and Daniel Kahneman introduced the world to Prospect Theory, which mapped out how humans actually behave when faced with decisions about gains and losses, in contrast to how economists assumed that people behave. Prospect Theory turned Economics on its head by demonstrating through a series of ingenious experiments that people are much more concerned with losses than they are with gains, and that framing a choice from one perspective or the other will result in decisions that are exactly the opposite of each other, even if the outcomes are monetarily the same. Prospect Theory led cognitive psychology in a new direction that began to uncover other human biases in thinking that are probably not learned but are part of our brain’s wiring.

Thinking fast and slow.

Psychophysical and physiological evidence indicates that the visual system of primates and humans has evolved a specialized processing focus moving across the visual scene. This study addresses the question of how simple networks of neuron-like elements can account for a variety of phenomena associated with this shift of selective visual attention. Specifically, we propose the following: (1) A number of elementary features, such as color, orientation, direction of movement, disparity etc. are represented in parallel in different topographical maps, called the early representation. (2) There exists a selective mapping from the early topographic representation into a more central non-topographic representation, such that at any instant the central representation contains the properties of only a single location in the visual scene, the selected location. We suggest that this mapping is the principal expression of early selective visual attention. One function of selective attention is to fuse information from different maps into one coherent whole. (3) Certain selection rules determine which locations will be mapped into the central representation. The major rule, using the conspicuity of locations in the early representation, is implemented using a so-called Winner-Take-All network. Inhibiting the selected location in this network causes an automatic shift towards the next most conspicious location. Additional rules are proximity and similarity preferences. We discuss how these rules can be implemented in neuron-like networks and suggest a possible role for the extensive back-projection from the visual cortex to the LGN.

Shifts in selective visual attention: towards the underlying neural circuitry.

Tilt aftereffect due to adaptation to natural stimuli.

Visibility of an oriented stimulus may be enhanced by nearby stimuli that are co-aligned with the target. However, the underlying mechanism governing this facilitation is controversial. Here we measured the dependence of percent correct detection on the target's contrast (psychometric curve) with and without flankers, where flankers were either collinear with or orthogonal to the target. We find that the effect of collinear flankers can be described as a translation of the psychometric function along the linear contrast axis. This behavior is consistent, within experimental error, with two types of models: (1) non-linear transduction of target contrast with collinear flankers having additive effects on contrast, and (2) uncertainty reduction (Pelli, 1985) by collinear flankers. We discuss properties of collinear facilitation that can help deciding between these two models.

Psychometric curves of lateral facilitation.

—Textural gradients can be defined as differences across space in orientation and spatial frequency content, along with absolute luminance and contrast. In this study, stimuli were created with gradients of these types to see how changing the size and shape of the foreground region affects the psychophysical task. The foreground regions were designed as clusters of target texels alternating with interplaced background texels (of the same cluster size). This design gives rise to a texture square-wave, with texture frequency defined by the distance from the beginning of one target cluster to the next. It was found that for stimuli with vertical and horizontal Gabor patches, the relationship between the global and local orientation of the foreground region is a critical variable, indicating some global-local interaction. When the global orientation of the foreground region is orthogonal to local target texel orientation, visibility is optimal for high texture frequencies, while for parallel arrangements, low texture frequencies are most visible. The latter result was also found to a lesser degree for tasks involving contrast gradients as well as spatial-frequency gradients, but with no effect caused by varying the global orientation. The results indicate the existence of a second-stage filter that integrates (across space) responses of similar first-stage spatial filters, and then sums the resultant activities with those of orthogonal first-stage filters, whose spatial proximity are to the sides of the local orientation. The size of these integrating mechanisms may extend to more than 7 deg, with connections between smoothed activities of filters with orthogonal orientations spanning approximately 1-2 deg.

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Effects of foreground scale in texture discrimination tasks: performance is size, shape, and content specific.

Lateral interactions between spatial filters were explored with a lateral masking paradigm. Contrast sensitivity (two-alternative forced-choice) for a Gabor signal in the presence of two flanking high contrast Gabor signals (masks) was measured. When the target to mask distance was less than 2 target wavelengths the contrast sensitivity decreased up to a factor of two relative to a no mask condition. At larger separations, up to eight wavelengths, an increase in contrast sensitivity occurred. This increase was maximal at separation distances of 2-3 wavelengths, where sensitivity increased by a factor of two. However, the enhancement magnitude and range was dependent on tie offset between the Gabor signal orientation and the direction defined by the virtual line connecting the two masks (global orientation). Maximal effects occurred when this offset was zero (100% increase in sensitivity) and 90 deg (50% increase). A 45 deg offset yielded only a small enhancement (20%). The enhancement dependence on spatial arrangement was found to be invariant across different global orientations (meridian). This pattern of interactions may be involved in grouping collinear line segments into smooth curves.

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Dov Sagi

Papers

Tilt aftereffect due to adaptation to natural stimuli.

Psychometric curves of lateral facilitation.

Effects of foreground scale in texture discrimination tasks: performance is size, shape, and content specific.

The Architecture of Perceptual Spatial

Discriminability of suprathreshold compound spatial frequency gratings