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Julien Dubois, James Macdonald, Rufin VanRullen; Reevaluating the sustained division of the attentional spotlight at high temporal resolution. Journal of Vision 2010;10(7):148. doi: 10.1167/10.7.148.
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© ARVO (1962-2015); The Authors (2016-present)
Recent fMRI and EEG evidence suggests that attention can concurrently select multiple locations. Most studies, however, generally lack a sufficient temporal resolution to rule out a unique, rapidly switching spotlight sequentially sampling the target locations. For example, using a classic “frequency-tagging” technique Müller and colleagues demonstrated a concurrent EEG enhancement at the tagged frequencies of two disjoint target locations (Müller et al, Nature, 2003). The temporal resolution of this technique is, however, limited to a quarter of a second and often more: a few stimulus cycles are necessary to determine EEG amplitude at the tagged frequency, and higher frequencies (being more sensitive to measurement noise) require even longer analysis windows. So, can the spotlight really be divided in a sustained manner? Or does attention switch faster than can be resolved using existing techniques? The novel method we present here (“broadband frequency tagging”) aims at resolving this fundamental question. Ten subjects monitored two patches of randomly varying luminance, to the left and right of fixation, for the appearance of a subtle contrast decrement, over a period of 6.25 seconds in each of 480 trials. Both patches contained an equal amount of power in all frequency bands from 0 to 80Hz over the stimulus presentation period. But critically, at any given time the two stimuli had distinct (and independent) frequency signatures. Using time resolved EEG decomposition in the frequency domain, this broadband frequency tagging paradigm allows us to dynamically monitor the allocation of attention to the two targets. As all tagging frequencies (0-80Hz) can contribute to the decomposition and their respective contributions can be adjusted at will, this technique can be said to provide the best possible temporal resolution. Using this new method, we were able to reconsider the previous conclusion that multiple targets are monitored in parallel.
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