Academic journal article Perception and Psychophysics

Task- and Location-Switching Effects on Visual Attention

Academic journal article Perception and Psychophysics

Task- and Location-Switching Effects on Visual Attention

Article excerpt

In two experiments, we examined the effects of task and location switching on the accuracy of reporting target characters in an attentional blink (AB) paradigm. Single-character streams were presented at a rate of 100 msec per character in Experiment 1, and successive pairs of characters on either side of fixation were presented in Experiment 2. On each trial, two targets appeared that were either white letters or black digits embedded in a stream of black letter distractors, and they were separated by between zero and five items in the stream (lags 1-6). Experiment 1 showed that report of the first target was least accurate if it immediately preceded the second target and if the two targets were either both letters or both digits (task repetition cost). Report of the second target was least accurate if one or two distractors intervened between the two targets (the U-shaped AB lag effect) and if one target was a letter and the other a digit (task switch cost). Experiment 2 added location uncertainty as a factor and showed similar effects as Experiment 1, with one exception. Lag 1 sparing (the preserved accuracy in reporting the second of two targets if the second immediately follows the first) was completely eliminated when the task required attention switching across locations. Two-way additive effects were found between task switching and location switching in the AB paradigm. These results suggests separate loci for their attentional effects. It is likely that the AB deficit is due mainly to central memory limitations, whereas location-switching costs occur at early visual levels. Task-switching costs occur at an intermediate visual level, since the present task switch involved encoding differences without changes in stimulus-response mapping rules (i.e., the task was character identification for both letters and digits).

The distribution of visual attention over natural environments has both spatial and temporal components. Attention helps us to identify objects against a background, as well as to follow these objects as they enter into events with causes and effects that unfold over time. Much research has shown that the distribution of spatial attention is determined by both endogenous processes (internal strategies related to task goals or instructions) and exogenous processes (reactions to external stimulus properties, such as contrast, motion, or sudden onsets; Briand & Klein, 1987; Juola, Koshino, & Warner, 1995; Müller & Rabbitt, 1989; Posner, 1980; Wolfe, 1994). Similarly, when a series of events is experienced, either through rapid changes, such as those that occur in sporting events, or through a succession of eye movements and fixations, some events appear to be selected for further processing to the relative exclusion of others, again due to the operation of both endogenous and exogenous processes (e.g., Norman & Shallice, 1986).

In the laboratory, the temporal distribution of visual attention has been studied most intensively using procedures such as visual masking and the rapid serial visual presentation (RSVP) technique. In these procedures, two or more individual stimuli are presented to a single central locus on a display. If they occur in close succession (e.g., with a stimulus onset asynchrony [SOA] of less than about 60 msec), backward masking by visual integration or interruption processes can result in poor recognition of earlier items (e.g., Breitmeyer, 1984; Turvey, 1973). Slightly slower rates (e.g., an SOA of about 100 msec) ensure recognition of most items, but if the series contains more than about five items, memory consolidation and detailed report is possible for only a few of them. In order to study the temporal distribution of visual attention, RSVP tasks commonly define one or more items in a series as targets, by differentiating them in some way from the distractors, and the accuracy of the target report is the dependent measure. Although there is some evidence that identification of the first target (T1) can be negatively affected by the presence of a second target (T2) that immediately follows it (e. …

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