Spatial attention: Visual selection and deployment over space The attentional spotlight and spatial cueing Attentional shifts, splits, and resolution Object-based Selection The visual search paradigm Top-down and bottom-up control of attention Inhibitory mechanisms of attention Invalid cueing Negative priming Inhibition of return Temporal attention: Visual selection and deployment over time Single target search Attentional blink and attentional dwell time Repetition blindness NEURAL MECHANISMS OF SELECTION Single-cell physiological method Event-related potentials Functional imaging: PET and fMRI

Experiments are presented showing that visual search for Mueller-Lyer (ML) stimuli is based on complete configurations, rather than component segments. Segments easily detected in isolation were difficult to detect when embedded in a configuration, indicating preemption by low-level groups. This preemption—which caused stimulus components to become inaccessible to rapid search—was an all-ornothing effect, and so could serve as a powerful test of grouping. It is shown that these effects are unlikely to be due to blurring by simple spatial filters at early visual levels. It is proposed instead that they are due to more sophisticated processes that rapidly bind contour fragments into spatially-extended assemblies. These results support the view that rapid visual search cannot access the primitives formed at the earliest stages of visual processing; rather, it can access only higher-level, more ecologically-relevant structures. The processes that underlie human vision are often divided into two fundamentally different classes: operations that are carried out in parallel over space, and operations that are not (e.g., Neisser, 1967; von Helmholtz, 1867/1962). For the most part, parallel processes are rapid (i.e., they occur within a few hundred milliseconds), effortless, and automatic (i.e., they cannot be affected by immediate changes in higher-level goals), whereas nonparallel processes are slower, more effortful, and nonautomatic. In its current embodiment, this dichotomy divides vision into an early preattentive and a subsequent attentive stage (e.g.,

One of the classical topics in neural networks is winner-take-all (WTA), which has been widely used in unsupervised (competitive) learning, cortical processing, and attentional control. Because of global connectivity, WTA networks, however, do not encode spatial relations in the input, and thus cannot support sensory and perceptual processing where spatial relations are important. We propose a new architecture that maintains spatial relations between input features. This selection network builds on LEGION (Locally Excitatory Globally Inhibitory Oscillator Networks) dynamics and slow inhibition. In an input scene with many objects (patterns), the network selects the largest object. This system can be easily adjusted to select several largest objects, which then alternate in time. We further show that a two-stage selection network gains efficiency by combining selection with parallel removal of noisy regions. The network is applied to select the most salient object in real images. As a s...

Two pairs of experiments studied the effects of attention and of unilateral neglect on auditory streaming. The first pair showed that the build up of auditory streaming in normal participants is gready reduced or absent when they attend to a competing task in the contralateral ear. It was concluded that the effective build up of streaming depends on attention. The second pair showed that patients with an attentional deficit toward the left side of space (unilateral neglect) show less stream segregation of tone sequences presented to their left than to their right ears. Streaming in their right ears was similar to that for stimuli presented to either ear of healthy and of brain-damaged controls, who showed no across-ear asymmetry. This result is consistent with an effect of attention on streaming, constrains the neural sites involved, and reveals a qualitative difference between the perception of left- and right-sided sounds by neglect patients. Auditory streaming is an example of the grouping or binding processes that have been extensively studied both in the auditory (e.g., Bregman, 1990; Darwin & Carlyon, 1995) and visual (e.g., Treisman & Gormican, 1988) domains. It is well-illustrated by the stimulus shown in Figure 1 (van Noorden, 1975), in which a pair

Perceptual grouping is the process by which elements in the visual image are aggregated into larger and more complex structures, i.e., “objects. ” This paper reports a study of the spatial factors and time-course of the development of objects over the course of the first few hundred milliseconds of visual processing. The methodology uses the now well-established idea of an “object benefit ” for certain kinds of tasks (here, faster within-object than between-objects probe comparisons) to test what the visual system in fact treats as an object at each point during processing. The study tested line segment pairs in a wide variety of spatial configurations at a range of exposure times, in each case measuring the strength of perceptual grouping as reflected in the magnitude of the object benefit. Factors tested included nonaccidental properties such as collinearity, cotermination, and parallelism; contour relatability; Gestalt factors such as symmetry and skew symmetry, and several others, all tested at fine (25 msec) time-slices over the course of processing. The data provide detailed information about the comparative strength of these factors in inducing grouping at each point in processing. The result is a vivid picture of the chronology of object formation, as objects progressively coalesce, with fully bound visual objects completed by about 200 msec of processing. The organization of the initially inchoate visual field into coherent units or “objects, ” called perceptual grouping or binding, is of fundamental importance in visual perception, influencing the perception of lightness (Adelson, 1993; Gilchrist, 1977), motion (Shimojo, Silverman, & Nakayama, 1988), and recognition of objects (Biederman, 1987). Much has been learned about the grouping factors originally identified by the Gestaltists, such as spatial

Object-based models of visual attention purport to explain why it is easier to process information within one object or perceptual group than across two or more groups. Perceptual groups are generally defined in terms of Gestalt grouping principles. These models of attention have been used to explain the phenomenon of cognitive tunneling within Heads-Up Displays (HUDs), on the assumption that the symbology of a Heads-Up Display (HUD) in a cockpit forms a single perceptual group and the outside scene forms another. Despite extensive empirical support, object-based models have various shortcomings. In particular, the use of Gestalt grouping principles to define the notion of objects does not allow for an operational measure of what an object is to the visual system. Also, the Gestalt principles do not allow for a systematic distinction between spatial and object-based mechanisms of attention. Finally, it is generally assumed that Gestalt grouping occurs preattentively, whereas there is evidence that perceptual grouping requires attentional resources. The proposed line of research aims to develop an account of object-based attention that does not rely on these premises. Rather, it is assumed that the cost of dividing attention between

A principle of perceptual organization, called uniform connectedness (13C), is described, and a theoretical approach to perceptual organization is proposed inwhich this principle playsafundamental role. The principle ofUC states that closed regions ofhomogeneous properties-such as lightness,chromatic color, texture,and so forth--tend to be perceived initially as single units. We demonstrate its effects and show that they occur even when opposed bypowerful grouping principles such as proximityand similarity.Weargue that UGcannotbereduced to such grouping principles, because it is not a form ofgrouping at all.We thenproposeatheoreticalframework within whichUC accounts for the initial (or entry level) organization ofthe visual field into primitive units. Classical principles ofgrouping operate after UC, creating superordinate units consisting oftwo ormore basic-level units. Parsing processes also operate afterUC, dividing basic-level units into subordinate parts.UG in the retinal image is proposed to be a necessary, but not a sufficient, condition far unit formation, since connected elements on the retina that are perceived to lie in separatedepthplanes fail tobe perceived as units. This fact, together with otherevidence thatthe Gestalt principles ofgrouping arebased on perceived (rather than retinal) relations, suggeststhat the organization of visual stimulation intoUC objects is ultimately

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