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Martina Poletti, Marisa Carrasco, Michele Rucci; Covert attention within the foveola enhances fine discrimination . Journal of Vision 2016;16(12):1264. doi: https://doi.org/10.1167/16.12.1264.
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Background: Vision is not homogenous within the foveola, the high-acuity region of the fovea. Tiny saccades are finely controlled to compensate for this inhomogeneity by bringing the locus of highest visual acuity in the foveola on the object of interest (Poletti et al, 2013 Current Biology). We recently showed that such high level control is not limited to oculomotor activity, but it also extends to covert attention. Attention can be selectively allocated toward objects separated by only 20 arcminutes in the foveola, leading to faster detection of targets presented at the attended location. Here, we investigated whether microscopic shifts of covert attention can also improve sensitivity (d') in a visual discrimination task. Methods: Observers (n=5) reported the orientation of a tiny bar (7x2 arcminutes) that could appear at four different locations at 14 arcminutes from the center of gaze. The target was preceded by a cue that in most trials provided information about its future location. Measuring shifts of attention within the foveola is extremely challenging because fixational eye movements displace the retinal stimulus by an area as large as the foveola itself. We circumvented this problem by using a custom apparatus to stabilize the stimulus on the retina. Results: Performance was higher when the cue was informative about the target's location (valid trials: d'=2.2±0.3) than when it was not informative (neutral trials: d'=1.8±0.4) or provided wrong information (invalid trials: d'=1±0.5). There was no speed-accuracy trade-off: observers responded faster in the valid trials. Conclusion: Covert attention enhances visual discrimination not only at parafoveal and peripheral locations, but also within the foveola. These findings reveal that the resolution of attention is much finer than thus far assumed. This mechanism may enable serial processing or privileged parallel processing of crowded foveal stimuli during fixation.
Meeting abstract presented at VSS 2016
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