If 2 targets are to be identified among distractors displayed in rapid sequence, correct identification of the 1st target hinders identification of the 2nd. To obtain this additional blink (AB), the 1st target must be masked with a simultaneous (integration) or a delayed (interruption) mask indifferently. In 3 experiments, it was shown that the 2nd target must also be masked, but that the precise form of masking is important: An AB occurs with interruption but not with integration masking. This nonequivalence of masking paradigms parallels that found in studies of masked priming, a phenomenon arguably related to the AB. The results are explained by a revised 2-stage model (M. M. Chun & M. C. Potter, 1995). Visual attention can be deployed over space or over time. Deployment over space has been studied extensively with several different paradigms (see reviews by Kinchla, 1992, and by LaBerge. 1990), Deployment over time has been studied less extensively, mainly with two related paradigms. In one paradigm, known as rapid serial visual presentation (RSVP), stimuli are presented sequentially in the same location, and observers are asked to identify one or more targets within the stream. When two targets are to be reported, the first is identified almost perfectly, but identification of the second is substantially impaired. The impairment is most evident when the second target is presented with a temporal lag of about 200-500 ms after the first (Raymond, Shapiro, & Arnell, 1992). At shorter or longer lags, performance is impaired less or not at all, thus yielding a characteristic U-shaped function over lags. This secondtarget deficit, also known as the attentiona! blink (AB), is said to occur because processing mechanisms required commonly by the two targets are unavailable (or are less available) for processing the second target until first-target processing has been completed (e.g., Chun & Potter, 1995). A very similar second-target deficit has been obtained in an elegantly simplified paradigm in which two targets, masked by trailing pattern-masks, are displayed at different screen locations, at various temporal lags from each other (Duncan, Ward, & Shapiro, 1994). The terms attentianal blink and
Accumulating evidence suggests that the brain can efficiently process both external and internal information. The processing of internal information is a distinct “offline” cognitive mode that requires not only spontaneously generated mental activity; it has also been hypothesized to require a decoupling of attention from perception in order to separate competing streams of internal and external information. This process of decoupling is potentially adaptive because it could prevent unimportant external events from disrupting an internal train of thought. Here, we use measurements of pupil diameter (PD) to provide concrete evidence for the role of decoupling during spontaneous cognitive activity. First, during periods conducive to offline thought but not during periods of task focus, PD exhibited spontaneous activity decoupled from task events. Second, periods requiring external task focus were characterized by large task evoked changes in PD; in contrast, encoding failures were preceded by episodes of high spontaneous baseline PD activity. Finally, high spontaneous PD activity also occurred prior to only the slowest 20% of correct responses, suggesting high baseline PD indexes a distinct mode of cognitive functioning. Together, these data are consistent with the decoupling hypothesis, which suggests that the capacity for spontaneous cognitive activity depends upon minimizing disruptions from the external world.
Access to visual awareness is often determined by covert, voluntary deployments of visual attention. Voluntary orienting without eye movements requires decoupling attention from the locus of fixation, a shift to the desired location, and maintenance of attention at that location. We used event-related functional magnetic resonance imaging to dissociate these components while observers shifted attention among 3 streams of letters and digits, one located at fixation and 2 in the periphery. Compared with holding attention at the current location, shifting attention between the peripheral locations was associated with transient increases in neural activity in the superior parietal lobule (SPL) and frontal eye fields (FEF), as in previous studies. The supplementary eye fields and separate portions of SPL and FEF were more active for decoupling attention from fixation than for shifting attention to a new location. Large segments of precentral sulcus (PreCS) and posterior parietal cortex (PPC) were more active when attention was maintained in the periphery than when it was maintained at fixation. We conclude that distinct subcomponents of the dorsal frontoparietal network initiate redeployments of covert attention to new locations and disengage attention from fixation, while sustained activity in lateral regions of PPC and PreCS represents sustained states of peripheral attention.
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