Academic journal article Perception and Psychophysics

Ignorance Is Bliss: The Role of Observer Expectation in Dynamic Spatial Tuning of the Attentional Focus

Academic journal article Perception and Psychophysics

Ignorance Is Bliss: The Role of Observer Expectation in Dynamic Spatial Tuning of the Attentional Focus

Article excerpt

When two sequential targets (T1, T2) are inserted in an RSVP stream of abstractors, perception of T2 is impaired at intertarget lags shorter than 700 msec. Paradoxically, this deficit disappears when T2 is presented directly after T1 (lag-1 sparing). Visser, Bischof, and Di Lollo (1999) found that lag-1 sparing occurs only when T1 and T2 are presented in the same stream. In contrast, Shih (2000) obtained lag-1 sparing with targets in separate streams. Four experiments addressed this inconsistency and revealed lag-1 sparing with targets in different streams, but only when observers had no foreknowledge of T1's location. We hypothesized that when T1 location is known, attention is focused narrowly on that stream; if T2 then appears in the other stream it is missed, and lag-1 sparing does not occur. When T1 location is not known, attention is focused broadly, encompassing both streams, and lag-1 sparing ensues.

Brief stimuli presented in rapid sequence may exceed the processing capability of the visual system. This gives rise to a deficit known as the attentional blink (AS), which is typically obtained when observers are required to identify two targets inserted in a stream of distractors presented in rapid serial visual presentation (RSVP). Identification accuracy for the second target is most impaired when the temporal lag between the two targets is short, and it improves as the intertarget lag is increased (Raymond, Shapiro, & Arnell, 1992).

Theoretical accounts of the AB have relied on some form of limited attentional resource that is allocated to the leading target to the detriment of the trailing target. For example, in the interference model (Shapiro, Raymond, & Arnell, 1994) resources are said to be allocated in large part to the first target, and in diminishing amounts to the ensuing items in the RSVP stream. Similar assumptions underlie the bottleneck models of Chun and Potter (1995) and Jolicoeur and DelPAcqua (1998) in which the AB deficit is said to occur when the second target arrives while the resources at a high-level stage are preempted by the first target On all these accounts, the AB deficit is expected to be most pronounced when the second target is presented directly after the first, in the ordinal position known as lag 1.

Contrary to this expectation, a meta-analysis by Visser, Bischof, and Di Lollo (1999) revealed that in about half of the studies the AB deficit was much reduced or failed to occur when the second target was presented directly after the first. This yielded the characteristic U-shaped function of performance over lags often found in AB studies. Potter, Chun, Banks, and Muckenhoupt (1998) referred to this highly accurate performance at lag 1 as lag-1 sparing.

Lag-1 sparing has been ascribed to the sluggish closing of an attentional gate that serves the selective function of rejecting distractors and passing target items (Chun & Potter, 1995; Shapiro & Raymond, 1994). The gate is said to remain closed while the distractors prior to the first target are on display. Upon presentation of the first target, the gate is said to open rapidly, but to close sluggishly, thus allowing the next item in the stream (i.e., the lag-1 item) to gain access to processing resources along with the first target. If the trailing item happens to be die second target, both targets are processed together, and lag-1 sparing ensues. An important detail that emerged from Visser, Bischof, and Di Lollo's (1999) analysis was that lag-1 sparing occurs only when the two targets are presented at the same spatial location; it never occurs when they are presented at different locations. Such location specificity is exceptionally robust: Of the 41 relevant studies in which the two targets were presented at different locations, none revealed lag-1 sparing. This finding has been confirmed by Visser, Zuvic, Bischof, and Di Lollo (1999) in a study designed explicitly to examine the effect of the relative location of the two targets on lag-1 sparing. …

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