Visual Search Mechanisms Research Articles

Visual search and its disorders.

A greater understanding of the underlying component mechanisms of normal visual search provides explanations for disturbances seen in certain neurological conditions. This review focuses on recent advances in this field which bear on the neurology of visual search in health and disease. Foremost, visual search requires a normal apparatus for the application of attentional resources to the visual environment and, with that facility lost in hemispatial neglect, search becomes uselessly stuck within one portion of the field. New evidence suggests that loss of normal registration of where the eyes have been compounds the problem. Even if attention can be deployed flexibly, its parameters must be chosen strategically, in terms of saccade amplitude, size of attentional window at each fixation and search path taken. Evidence is growing that the prefrontal cortex plays a complex role in this strategic control. Rehabilitation strategies of the future may be tailored according to which component functions have been lost in different patient groups. Visual search is a dominant human activity and provides not only a window into how brain function is deranged after structural damage, but also offers the prospect of an ideal modality through which to deliver future behavioural therapies. New techniques have advanced our understanding of the physiology of visual search enormously in the past few decades. The time is now ripe in which to begin to integrate these findings into our understanding of the pathophysiology and treatment prospects of neurological disorders like hemispatial neglect, hemianopia and other deficits after stroke.

Serial attention mechanisms in visual search: a direct behavioral demonstration.

In visual search, inefficient performance of human observers is typically characterized by a steady increase in reaction time with the number of array elements-the so-called set-size effect. In general, set-size effects are taken to indicate that processing of the array elements depends on limited-capacity resources, that is, it involves attention. Contrasting theories have been proposed to account for this attentional involvement, however. While some theories have attributed set-size effects to the intervention of serial attention mechanisms, others have explained set-size effects in terms of parallel, competitive architectures. Conclusive evidence in favor of one or the other notion is still lacking. Especially in view of the wide use of visual search paradigms to explore the functional neuroanatomy of attentional mechanisms in the primate brain, it becomes essential that the nature of the attentional involvement in these paradigms be clearly defined at the behavioral level. Here we report a series of experiments showing that highly inefficient search indeed recruits serial attention deployment to the individual array elements. In addition, we describe a number of behavioral signatures of serial attention in visual search that can be used in future investigations to attest a similar involvement of serial attention in other search paradigms. We claim that only after having recognized these signatures can one be confident that truly serial mechanisms are engaged in a given visual search task, thus making it amenable for exploring the functional neuroanatomy underlying its performance.

Visual search: psychophysical models and practical applications

Abstract Extensive research into the role of saccadic eye movements in human visual perception has been carried out for many years. Although the search patterns of different observers while studying the same image bear some common characteristics, there are often variations in the temporal order in which fixation points are viewed. During visual search for a defined target, there is evidence for both parallel search, with which all objects are processed concurrently, and for sequential search, in which several fixation points are found leading to the target. In this article, we present a review of both theoretical and experimental research directed towards better understanding of the underlying mechanisms of visual search. We begin by looking at the basic dynamics of saccadic eye movements and some of the major psychophysical models that have been developed over the years. An overview of the practical applications and future trends of visual search is then provided. Visual search is a common task that people perform throughout their daily life, and the number of applications inspired by the human visual search mechanism is potentially large. The purpose of this paper is to highlight some of the key opportunities for the image and vision computing community and promote further interactions between biological and computational vision research.

Uncertainty, attentional capacity and chromatic mechanisms in visual search

Two general questions were investigated using a visual search task. First, we asked whether effects of target uncertainty on reaction time varied with the discriminability of the target and distractors. Second, a higher order chromatic mechanism model was tested against a flexible model in which the signals in cardinal color-opponent mechanisms are combined through an attentional process. The models were tested by measuring the effects of target uncertainty on search time. A regression analysis indicated that the magnitude of the uncertainty effect was approximately constant in logarithmic units as a function of the chromatic difference between the target and distractors. The constant magnitude of the uncertainty effect suggested that an attentional capacity limit was exceeded when observers were required to monitor several chromatic mechanisms at several locations. The results of experiments 3 and 4 suggested that search for chromatic targets among distractors was mediated by diagonally tuned higher order chromatic mechanisms, rather than by signals in cardinal color-opponent mechanisms that were combined through an attentional mechanism.

The integration of parallel and serial processing mechanisms in visual search: evidence from eye movement recording

We examined timing and scanning paths of eye movements during a visual search task, in which subjects had to detect, as quickly as possible, the presence or absence of a target among distractors [Q-like element among O stimuli (QvsO) and vice-versa (OvsQ)]. According to an influential theory [Treisman, A. & Gelade, G. (1980) Cognitive Psychol, 12, 97--136; Treisman, A. & Sato, S. (1990) J. Exp. Psychol. Hum. Percept. Perform., 16, 459--478], only tasks yielding nonflat search functions (OvsQ) involve focal attention. Alternative models propose that all kinds of visual search are resolved by a biased competitive process, working in parallel across the visual field. Data show that QvsO and OvsQ tasks are characterized by quantitative rather than by qualitative differences in search strategy. No differences between the two tasks were found regarding either the percentage of saccades foveating single stimulus items or the timing of the button response with respect to the onset of the last foveation saccade within a trial. Furthermore, the number of saccades made during search predicted very accurately the time required to accomplish the task and fixation times were independent of the number of stimulus items. On the basis of our results there is no reason to postulate the occurrence of shifts of visuospatial attention, other than those associated with the executions of saccadic eye movements, which are driven by a parallel feature analysis of the visual scene, in both types of search tasks. A time-limited competitive model for attentive target identification, in which both parallel (competitive) and serial (attentive) processing mechanisms are integrated, can account for these findings, providing a unified conceptual framework for all kinds of visual search.

The integration of parallel and serial processing mechanisms in visual search: evidence from eye movement recording

AbstractWe examined timing and scanning paths of eye movements during a visual search task, in which subjects had to detect, as quickly as possible, the presence or absence of a target among distractors [Q‐like element among O stimuli (QvsO) and vice‐versa (OvsQ)]. According to an influential theory [Treisman, A. & Gelade, G. (1980) Cognitive Psychol, 12, 97–136; Treisman, A. & Sato, S. (1990) J. Exp. Psychol. Hum. Percept. Perform., 16, 459–478], only tasks yielding nonflat search functions (OvsQ) involve focal attention. Alternative models propose that all kinds of visual search are resolved by a biased competitive process, working in parallel across the visual field. Data show that QvsO and OvsQ tasks are characterized by quantitative rather than by qualitative differences in search strategy. No differences between the two tasks were found regarding either the percentage of saccades foveating single stimulus items or the timing of the button response with respect to the onset of the last foveation saccade within a trial. Furthermore, the number of saccades made during search predicted very accurately the time required to accomplish the task and fixation times were independent of the number of stimulus items. On the basis of our results there is no reason to postulate the occurrence of shifts of visuospatial attention, other than those associated with the executions of saccadic eye movements, which are driven by a parallel feature analysis of the visual scene, in both types of search tasks. A time‐limited competitive model for attentive target identification, in which both parallel (competitive) and serial (attentive) processing mechanisms are integrated, can account for these findings, providing a unified conceptual framework for all kinds of visual search.

Refixation frequency and memory mechanisms in visual search.

Visual search-looking for a target object in the presence of a number of distractor items-is an everyday activity for humans (for example, finding the car in a busy car park) and animals (for example, foraging for food). Our understanding of visual search has been enriched by an interdisciplinary effort using a wide range of research techniques including behavioural studies in humans [1], single-cell electrophysiology [2], transcranial magnetic stimulation [3], event-related potentials [4] and studies of patients with focal brain injury [5]. A central question is what kind of information controls the search process. Visual search is typically accompanied by a series of eye movements, and investigating the nature and location of fixations helps to identify the kind of information that might control the search process. It has already been demonstrated that objects are fixated if they are visually similar to the target [6]. Also, if an item has been fixated, it is less likely to be returned to on the subsequent saccade. This automatic process is referred to as inhibition of return (IOR [7,8]). Here, we investigated the role of memory for which items had been fixated previously. We found that, during search, subjects often refixated items that had been previously fixated. Although there were fewer return saccades than would be expected by chance, the number of refixations indicated limited functional memory, indeed the memory effects that were present may primarily be a result of IOR.

Serial attention mechanisms in visual search: a critical look at the evidence.

Until a few years ago, visual search tasks were of exclusive pertinence to psychophysicists and cognitive psychologists trying to understand the operating principles and computational constraints of visual perception and visual selective attention. In recent years, cognitive neuroscience, with its powerful tools, has started to explore more directly the neuronal mechanisms underlying search performance in humans and macaques, aiming at the same general goals. New observations from a number of cognitive neuroscience approaches are promising a near future of great excitement in this field of research. This article offers a critical review of some of these recent contributions and highlights some of the interpretational problems that they pose.

Interactions between achromatic and chromatic mechanisms in visual search

The purpose of this study was to explore the interaction between achromatic information and chromatic information in a visual search task. It is widely accepted that signals in second stage color opponent mechanisms vary with both the luminance and chromaticity of a stimulus. However, detection experiments suggest a large degree of independence between chromatic thresholds and achromatic thresholds. The independence at threshold has led to the proposal of a third processing stage in which achromatic and chromatic information is separated. Experiments were designed to determine if variability in the luminance of distractor stimuli made it more difficult to search for a target that differed in chromaticity. When the chromaticity of the distractors was held constant variability in distractor luminance had little effect on search performance, but when signals in second stage color opponent mechanisms were held constant variability in distractor luminance resulted in poorer performance. The results suggest that search for chromatic targets is mediated by a processing stage that calculates the ratio of chromatic and achromatic signals so that the chromatic signal is independent of stimulus luminance.

Visual search mechanisms in schizophrenia

Visual search mechanisms in schizophrenia

Cognitive and neural mechanisms of visual search

The visual search paradigm is a useful tool for studying the perception of multiple-element stimulus arrays, and recent studies have identified a number of factors that influence search performance. Of special importance is the selection process, and recent neurophysiological experiments have shown that the suppression of information from irrelevant objects plays a crucial role in the selection of visual search targets.