Derrick G. Watson

Visual marking : prioritizing selection for new objects by top-down attentional inhibition of old objects

The authors propose a new mechanism for prioritizing the selection of new events: visual marking. In a modified conjunction search task the authors presented one set of distractors before the remaining items, which contained the target if present. Search was as efficient as if only the second items were presented. This held when eye movements were prevented and required a gap of 400 ms between the old and new items. The effect was abolished by luminance changes at old distractor locations when the new items appeared, and it was reduced by the addition of an attention demanding load task. The authors propose that old items can be ignored by spatially parallel, top-down attentional inhibition applied to the locations of static stimuli. The authors discuss the relations between marking and other accounts of visual selection and potential neurophysiological mechanisms.

Visual marking: Evidence for inhibition using a probe-dot detection paradigm

Watson and Humphreys (1997,1998) have recently demonstrated that new objects can be prioritized for visual attentional processing by the top-down attentional inhibition of old objects already in the field, a mechanism they calledvisual marking. The experiments reported here show that the detection of a dim probe dot is impaired when it falls at the location of an old object (Experiments 1 and 3) but that this occurs only in conditions in which it is advantageous for subjects to mark (inhibit) old objects (Experiment 2). These results further support previous work showing that visual marking is based on the inhibition of the locations of old objects and that visual marking can be flexibly applied (or with-held), depending on the goals of current behavior.

Visual marking: using time in visual selection

Given human capacity limitations, to behave adaptively we need to prioritise the order of visual processing to ensure that the most relevant information is available to control action. One way to do this is to prioritise processing at a particular location in space. However, there are many situations where this strategy is not possible and recent studies have shown that, in such circumstances, observers can use time as well as space to prioritise selection. We propose that selection by time can be influenced by a process of visual marking, involving an active bias applied in parallel against old items in the field. Here we describe the properties of visual marking in relation to other mechanisms of visual selection.

Separating distractor rejection and target detection in posterior parietal cortex - An event-related fMRI study of visual marking

Successful survival in a competitive world requires the employment of efficient procedures for selecting new in preference to old information. Recent behavioral studies have shown that efficient selection is dependent not only on properties of new stimuli but also on an intentional bias that we can introduce against old stimuli. Event-related analysis of functional magnetic resonance imaging data from a task involving visual search across time as well as space indicates that the superior parietal lobule is specifically involved in processes leading to the efficient segmentation of old from new items, whereas the temporoparietal junction area and the ascending limb of the right intraparietal sulcus are involved in the detection of salient new items and in response preparation. The study provides evidence for the functional segregration of brain regions within the posterior parietal lobe.

Fractionating the preview benefit in search: dual-task decomposition of visual marking by timing and modality.

Providing participants with a preview of half the distractors in a visual search task facilitates performance. The present study examined the effects of secondary tasks on the preview benefit in search. Participants had to attend to a visual or an auditory stream of digits that began either (a) at the onset of the preview or (b) after the preview. Secondary tasks that onset with the preview disrupted the preview benefit irrespective of their modality. Only visual secondary tasks disrupted the benefit in the delayed condition. These selective interference effects suggest that the preview benefit can be fractionated into 2 components: an initialization component that involves modality-independent resources and a maintenance component that depends on visual resources. Results are discussed in relation to theoretical accounts of the preview benefit in search.

Top-down effects of semantic knowledge in visual search are modulated by cognitive but not perceptual load.

Moores, Laiti, and Chelazzi (2003) found semantic interference from associate competitors during visual object search, demonstrating the existence of top-down semantic influences on the deployment of attention to objects. We examined whether effects of semantically related competitors (same-category members or associates) interacted with the effects of perceptual or cognitive load. We failed to find any interaction between competitor effects and perceptual load. However, the competitor effects increased significantly when participants were asked to retain one or five digits in memory throughout the search task. Analyses of eye movements and viewing times showed that a cognitive load did not affect the initial allocation of attention but rather the time it took participants to accept or reject an object as the target. We discuss the implications of our findings for theories of conceptual short-term memory and visual attention.

Subitizing requires attention

While counting objects is typically a slow, serial process, enumerating about four or fewer objects has been considered to be a relatively effortless, parallel, and even preattentive process often referred to as subitizing. However, by combining a subitizing task with an attentional blink task, we show that subitizing is systematically affected by a closely preceding letter identification task. Vice versa, letter identification is also affected by a closely preceding subitizing task. Importantly, performance not only depended on the time between the two tasks, but also on the number of to-be-enumerated dots, even though this number fell within the subitizing range. The results imply that the processes underlying subitizing require attentional resources, suggesting that they are either serial in nature, or parallel, with capacity limited by the overall resources available.

Visual marking and visual change.

Five experiments investigated the types of changes that disrupt the preview effect--the benefit gained in difficult search tasks from presenting some distractors earlier in time. A shape change with or without an overall luminance change at the location of an old item was found to disrupt the preview effect, whereas an equivalent luminance change alone or an isoluminant color change was not disruptive. Results suggest that (a) relatively low-level visual changes may not be sufficient to abolish the benefit, (b) the benefit most likely occurs through inhibition applied to locations within a location master map, and (c) inhibition need not be applied to surface features of objects.

Object Recognition under Sequential Viewing Conditions: Evidence for Viewpoint-Specific Recognition Procedures:

In many computational approaches to vision it has been emphasised that object recognition involves the encoding of view-independent descriptions prior to matching to a stored object model, thus enabling objects to be identified across different retinal projections. In contrast, neurophysiological studies suggest that image descriptions are matched to less abstract, view-specific representations, resulting in more efficient access to stored object knowledge for objects presented from a view similar to a stored viewpoint. Evidence favouring a primary role for view-specific object descriptions in object recognition is reported. In a series of experiments employing line drawings of familiar objects, the effects of depth rotation upon the efficiency of object recognition were investigated. Subjects were required to identify an object from a sequence of very briefly presented pictures. The results suggested that object recognition is based upon the matching of image descriptions to view-specific stored representations, and that priming effects under sequential viewing conditions are strongly influenced by the visual similarity of different views of objects.

Input control processes in rapid serial visual presentations : Target selection and distractor inhibition

The attentional blink refers to the finding that the 2nd of 2 targets embedded in a stream of rapidly presented distractors is often missed. Whereas most theories of the attentional blink focus on limited-capacity processes that occur after target selection, the present work investigates the selection process itself. Identifying a target letter caused an attentional blink for the enumeration of subsequent dot patterns, but this blink was reduced when the dots shared their color with the target letter. In contrast, performance worsened when the color of the dots matched that of the remaining distractors in the stream. Similarity between the targets also affected competition between different sets of dots presented simultaneously within a single display. The authors conclude that the selection of targets from a rapid serial visual presentation stream is mediated by both excitatory and inhibitory attentional control mechanisms.