Academic journal article Perception and Psychophysics

Evidence against Signal Enhancement as a Mechanism of Direct Selection by Color

Academic journal article Perception and Psychophysics

Evidence against Signal Enhancement as a Mechanism of Direct Selection by Color

Article excerpt

Two of the possible mechanisms that have been put forward to explain precuing effects are signal enhancement and uncertainty reduction. Signal enhancement leads to processing advantages for valid information because the signal at the known input channel is enhanced, whereas uncertainty reduction allows observers to ignore confusing distractor items in a display. Both mechanisms have been reported to be involved in location precuing, but it is still unclear which of these two mechanisms is responsible for color cuing effects. Two experiments are reported in which expectancy for a certain color in a single-item display was created. Targets were presented briefly and were masked. If color cues produce signal enhancement, then a color cuing effect should result. If color cues only allow uncertainty reduction, however, there should be no color cuing effect because there are no distractors in single-item displays. The results of both experiments favor uncertainty reduction as the mechanism behind color cuing, because no signs of signal enhancement-based cuing effects were observed.

Providing advance location information in multiple-item displays has frequently been shown to lead to processing advantages (see, e.g., Posner, Snyder, & Davidson, 1980; Theeuwes, 1989). These benefits are reported for various tasks, including detection (Posner et al., 1980), discrimination (Theeuwes, 1989), and identification (Cheal & Gregory, 1997). Typically, the effects of advance information are studied in precuing tasks. As an example of such a task, McCann, Folk, and Johnston (1992) cued one of two locations in a lexical decision task. Cues could be either valid or invalid. The authors found that responses were faster for both words and nonwords displayed at the cued location than for those at the uncued location. In addition, accuracy was higher in these valid-cue trials than in invalid trials. This and many analogous results clearly demonstrate an advantage of advance knowledge of stimulus location.

Within theories of attention, two mechanisms that can explain this benefit have been put forward. The first one is often referred to as signal enhancement and is based on the assumption of capacity limitation within the human information processing system (e.g., Bashinski & Bacharach, 1980). Because there are only limited resources available, it is necessary to have a mechanism allocating resources where they are needed to ensure fast and effective processing of relevant information. Attention is regarded as the process responsible for such allocation. According to this view of attention, a location precue allows participants to allocate a larger share of attention to the cued location in a top-down manner, with the exact share determined by cue validity. Consequently, more processing resources are assigned to this known input channel, enhancing or prioritizing the processing of its signal and ultimately leading to faster and more accurate processing. Invalid cues direct participants to allocate the majority of attentional resources to the wrong channel, leading, therefore, to processing costs. In the neutral cue condition, in which no advance location information is available, processing resources are simply distributed evenly over the whole display.

One other mechanism that can account for the results of location precuing experiments with multiple items is uncertainty reduction (sometimes also referred to as noise reduction; Shiu & Pashler, 1994). According to this view, precuing benefits can be conceived of as a statistical phenomenon (e.g., Kinchla, Chen, & Evert, 1995). When distractor items are present in the field, participants may incorrectly confuse one of these with a target, because of perceptual noise. The response to a distractor item will necessarily be at chance accuracy, so this perceptual noise degrades performance. A location precue allows the observer to give extra weight to the perceptual information from the location likely to contain the target (cf. …

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