A classic question concerns whether humans can attend multiple locations or objects at once. Although it is generally agreed that the answer to this question is “yes,” the limits on this ability are subject to extensive debate. According to one view, attentional resources can be flexibly allocated to a variable number of locations, with an inverse relationship between the number of selected locations and the quality of information processing at each location. Alternatively, these resources might be quantized in a “discrete” fashion that enables concurrent access to a small number of locations. Here, we report a series of experiments comparing these alternatives. In each experiment, we cued participants to attend a variable number of spatial locations and asked them to report the orientation of a single, briefly presented target. In all experiments, participants’ orientation report errors were well-described by a model that assumes a fixed upper limit in the number of locations that can be attended. Conversely, report errors were poorly described by a flexible-resource model that assumes no fixed limit on the number of locations that can be attended. Critically, we showed that these discrete limits were predicted by cue-evoked neural activity elicited before the onset of the target array, suggesting that performance was limited by selection processes that began prior to subsequent encoding and memory storage. Together, these findings constitute novel evidence supporting the hypothesis that human observers can attend only a small number of discrete locations at an instant.
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Note that our cued locations were always arranged contiguously. One might wonder whether our results would generalize to a situation in which cued locations were randomly arranged or interleaved between uncued locations. The present results cannot speak directly to this question. However, in pilot experiments not reported here, we obtained similar findings (i.e., the discrete-resource model always outperformed the flexible-resource model) when we cued random locations in the left and right hemifields on each trial (with the constraint that each hemifield contained two cued positions).
The smooth curves overlaid in Fig. 2 were fit to the mean distributions of report errors across participants. Fits at the individual-participant level were substantially noisier.
The nr variable corresponds to the proportion of trials on which the participant failed to select the cued location and was forced to guess. Thus, we defined “capacity” as N(1 – nr N ), where N is the number of cued locations.
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Supported by Grant No. NIH R01-MH087214 to E.A. and E.K.V. E.F.E. and E.A. conceived and designed all experiments. E.F.E. and L.M.M. collected the data and analyzed them using software developed by K.F. Author E.K.V. provided guidance and assistance with electrophysiological recordings. E.F.E. and E.A. wrote the manuscript.
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Ester, E.F., Fukuda, K., May, L.M. et al. Evidence for a fixed capacity limit in attending multiple locations. Cogn Affect Behav Neurosci 14, 62–77 (2014). https://doi.org/10.3758/s13415-013-0222-2
- Working memory