Academic journal article Perception and Psychophysics

Is It Impossible to Inhibit Isoluminant Items, or Does It Simply Take Longer? Evidence from Preview Search

Academic journal article Perception and Psychophysics

Is It Impossible to Inhibit Isoluminant Items, or Does It Simply Take Longer? Evidence from Preview Search

Article excerpt

Visual search can be facilitated when participants receive a preview of half the distractors (the preview benefit in search; Watson & Humphreys, 1997). Donk and Theeuwes (2001) have argued that preview-based benefits are abolished if the display items are isoluminant to a background. This is consistent with the preview benefit being due to onset capture by the new stimuli. In contrast, the present experiments challenge this suggestion and show that preview benefits can occur under isoluminant conditions, providing that they are given enough time to occur. In Experiment 1, we showed that a preview benefit can occur even with isoluminant stimuli, provided that the old items are previewed for a sufficient time. In Experiment 2, we tested and rejected the idea that this advantage is due to low-level sensory fatigue for the preview stimuli. These findings indicate that the preview effect is not caused solely by onset capture.

Performance in visual search tasks can be greatly improved if observers are given a preview of half of the irrelevant distractors in advance of the other distractors and the target (Watson & Humphreys, 1997, 1998, 2000). Under these preview circumstances, search can be as efficient as it is when only the second set of items is presented and is much more efficient than it is with the full combined display of the items appearing together (a full-set search baseline). Originally, Watson and Humphreys (1997) suggested that for static stimuli, this preview benefit was the result of goal-directed inhibition directed toward the old, irrelevant stimuli-a process they termed visual marking (Watson & Humphreys, 1997, 1998, 2000, 2002). The inhibitory filtering of irrelevant information aided capture mechanisms and increased the signal-to-noise ratio for subsequently appearing important and relevant new information (see Watson, Humphreys, & Olivers, 2003). Further evidence for old items being inhibited has been provided by cases in which probe detection tasks are combined with visual search. For instance, probe dot detection is worse when the dots fall at the locations of old items, relative to when they fall at the locations of new stimuli (Olivers & Humphreys, 2002; Watson & Humphreys, 2000). Crucially, this deficit in probe detection at old locations is found only when participants are engaged in a search task in which new items must be prioritized, and it is not found when probe detection is the primary task. This is consistent with the suggestion that the preview benefit is influenced by the current goal state of the observer, which is to prioritize the new and to ignore the old stimuli.

Similar evidence for inhibitory effects in preview search, but linked to the features and not just to the locations of old items, has come from Braithwaite and colleagues (Braithwaite, 2002; Braithwaite & Humphreys, 2003; Braithwaite, Humphreys, & Hodsoll, 2003, 2004; Braithwaite, Humphreys, & Hulleman, 2005; see also Gibson & Jiang, 2001; Olivers & Humphreys, 2002). In these studies, target search was slowed if the target carried feature properties of the old items, and detection was slowed for probes that fell on new distractors that shared the same feature. Collectively, these studies suggest that old, irrelevant items can be filtered from search on the basis of inhibition directed to both their locations and their featural properties.

Noninhibitory Accounts for the Preview Benefit

Alternative accounts of the preview benefit can also be offered. For instance, Donk and Theeuwes (2001) suggested that the preview benefit simply reflects automatic attentional capture by the onsets from the new stimuli. Here, search is improved not because old items are deprioritized, reducing their chances of selection, but because the new items automatically capture attention, improving their chances of selection. This, too, would predict that search will be restricted to just the new items, but without recourse to any notion of inhibitory coding at the old locations. …

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