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Lisa N. Jefferies, James T. Enns, Vincent Di Lollo; Temporal dynamics of dividing spatial attention. Journal of Vision 2009;9(8):119. doi: 10.1167/9.8.119.
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Mutually-exclusive theories posit that spatial attention is deployed either as a single focus or as divided foci. In fact, we have previously shown that whether the focus is unitary or divided depends on the observer's mental set. We now report that a unitary focus is the default mode and that it takes approximately 100 milliseconds to morph between unitary and divided modes. We employed an attentional-blink paradigm with two RSVP streams of distractors, one on either side of fixation, separated by a blank region. Two pairs of letter-targets (T1-pair/T2-pair) could appear either within the streams or in the central blank region. For one group, the T1-pair occurred predictably within the streams (encouraging a divided focus to optimize T1-pair identification); for the other group, it occurred unpredictably either within the streams or in the central region (encouraging a unitary focus). For both groups, the T2-pair appeared unpredictably either within the streams or in the central region. To assess the rate of morphing between unitary and divided modes, we varied the stimulus-onset-asynchrony (SOA) between successive items in the RSVP streams (70 or 100 ms). When the T1-pair location was unpredictable, T2-pairs in the central region were processed accurately at both SOAs, indicating a unitary focus. When the T1-pair location was predictable, however, T2-pair accuracy depended on the SOA: at an SOA of 70 ms, accuracy for a central T2-pair was relatively high, suggesting that there had been insufficient time to morph from the default (unitary) setting to the divided setting. At an SOA of 100 ms, however, accuracy for a central T2-pair was lower, indicating that there had been sufficient time for the focus to divide, leaving the central region unattended. We conclude the default mode is unitary, and that it takes a finite time period to morph to a divided focus.
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