Anticipation and Synchronization

In Nakatani et al.'s '05 paper in the Journal of Cognitive Neuroscience, they describe a peculiar EEG pattern in subjects experiencing a phenomenon known as "attentional blink." Attentional blink occurs when perceivers can only report one of two target images that are presented in quick succession. This is usually found in conditions of high processing load, such as those demanding of both early and higher visual processing.

Attentional blink is often studied in a paradigm where a sequence of images are presented in rapid succession, for about 100 msec each. Subjects have to detect the presence of two specific images within the sequence. When the target images are present and separated by more than 5 intervening distractor images, both targets are detected without much difficulty. When the two images are presented directly after one another (no intervening images) they can also be reported without much difficulty. However, when the targets are separated by between 1 and 4 intervening images, the second target is usually missed!

What happens during this time, when participants are essentially blind? No one really knows. One explanation is that this "attentional blink" is actually the switch cost associated with task switching between looking for target 1 and looking for target 2 (also known as switching the "attentional set"). Accordingly, several studies implicate the areas responsible for working memory: right posterior parietal, cingular, and left temporal/frontal regions. Further, when the second target was detected, subjects showed a large area of phase coherence in the gamma range (30-80 Hz) throughout the task, suggesting that this synchronized activity might reflect differences in attentional focus, which subsequently translate into improved target detection.

The frequency range of 30-80 Hz has many other relationships with visual attention: synchrony within this band has been implicated in object detection, memory retention, readiness, and consciousness. The authors propose that it may be involved in anticipatory processing, or as functioning something like a "procedural buffer" which can be used to alleviate some of the switch costs associated with changing attentional set.

Sure enough, the authors found increased baseline levels of synchrony in the experimental group (which had to find 2 targets) compared to the control group (which had to find only 1). Interestingly, global synchronization involving more than 188 different electrodes also appeared every 300-500 msec, even before the presentation of the first target, in those participants that were able to detect the second target. The authors suggest that this global, long-range synchrony may be a mechanism by which information is transmitted quickly from occipital to frontal areas, and that cycles of synchrony may correspond to processes such as visual "filter reconfiguration or, alternatively, a memory operation."

The authors continue to say that "the real challenge, rather than looking for traces of endogenous versus exogenous control mechanisms, might be to investigate how a system that is switching continuously between different intrinsic states of phase synchrony is able to adjust these rhythms in coordination with external events." Along those lines, one really interesting thing about this pattern of EEG activity is that it could be caused by a very slow oscillation (2-3 Hz) modulating a much faster rhythm (38-43 Hz). If these are actually two separable components, the slower oscillation could be a good candidate for synchronization with events in the external world (such as expectation of a target).

Of course, then we enter into the infinite regress often associated with assigning "agency" to brain functions: what then modulates the slower signal?

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