Academic journal article Perception and Psychophysics

Awareness of the Continuously Visible: Information Acquisition during Preview

Academic journal article Perception and Psychophysics

Awareness of the Continuously Visible: Information Acquisition during Preview

Article excerpt

What can we learn about a scene while we stare at it, but before we know what we will be looking for? Three experiments were performed to investigate whether previewing a search array prior to knowing the target allows search to operate more quickly (lower reaction time [RT]), more efficiently (reduced set size slope), and/or by consulting abstract mental representations. Experiment 1 compared RTs for previewed and nonpreviewed arrays, some of which were highly degraded with visual noise. Preview reduced RTs for the noisy displays but did not affect search efficiency. Limited interactions of visual quality and preview suggested that prior exposure allowed the extraction and maintenance of about three abstract identities. If the target was one of those items, the observer responded without searching; if not, the observer searched the remaining items as if there had been no preview. Experiment 2 replicated these findings with less extreme noise. In Experiment 3, subjects previewed 0-6 items of a 12-item display. RTs decreased linearly as the number of previewed items increased from 0 to 3 and then reached a plateau, confirming that the capacity of the representation was about 3 items. Implications for visual awareness are discussed.

Our subjective experience of visual awareness would seem to suggest that when we gaze at a complex scene, we simultaneously become aware of a great deal of information about the contents and layout of this scene. It is easy to envision important adaptive advantages to acquiring this kind of detailed information. Nonetheless, a number of writers have argued that this is an illusion (Dennett, 1991, 2002; O'Regan, 1992; but see O'Regan & Noë, 2001) and that the brain extracts far less detailed information about object identities in a scene than one would naturally suspect. It will be argued below that the kinds of data that have been used to argue for this conclusion have serious and unacknowledged limitations, leaving the question unsettled. The present article develops new techniques with which to ask about what kinds of information are extracted when viewers are allowed to inspect a display of familiar objects prior to searching for something in this display.

Change Detection and Change Blindness

Beginning in the 1980s, a number of studies examined people's ability to detect changes in displays that remain present for some period of time, disappear, and then reappear (sometimes with alterations). When the disappearance is extremely brief (under about 20 msec), people are usually very good at detecting such changes, apparently because the changes are detected by relatively peripheral visual mechanisms (Phillips & Singer, 1974; Stelmach, Bourassa, & Di Lollo, 1984). When the disappearance is long enough to generate a perception of flicker, however, change detection is quite poor, whether the display consists of an array of letters (Pashler, 1988) or a scene (Levin & Simons, 1997; O'Regan, Deubel, Clark, & Rensink, 2000; O'Regan, Rensink, & Clark, 1999; Rensink, O'Regan, & Clark, 1997; Simons & Levin, 1997). This surprisingly poor detection of change has sometimes been termed change blindness, although this term appears to overstate the phenomenon, given that observers sometimes retain several objects, probably the full complement of what they can store in visual short-term memory (VSTM; Pashler, 1988), and successfully detect changes that occur to those items. One might suppose that relatively poor change detection reflects the fact that observers have not expended the effort needed to identify those objects whose identities are subsequently changed, but studies that have required observers to demonstrate that they have exhaustively identified an entire array of characters nonetheless have shown that when the display subsequently flickers on and off, the usual deficit in change detection is present (Becker & Pashler, 2002).

These results from change detection studies are certainly consistent with the idea that people are aware only of a tiny subset of items at any instant of time and that the ability to maintain these representations over time and across saccades is fairly limited. …

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