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Revealing the dynamics of visual masking using a speeded saccadic choice task

56.422, Tuesday, 20-May, 2:45 pm - 6:45 pm, Banyan Breezeway
Session: Visual search: Eye movements

Sébastien M. Crouzet1, Simon Hviid Del Pin2, Morten Overgaard2,3, Niko A. Busch1,4; 1Institute of Medical Psychology, Charité University Medicine, Berlin, Germany, 2CCN, Dept. of Communication and Psychology, Aalborg University, Denmark, 3CNRU, CFIN, MindLab, Aarhus University, Denmark, 4Berlin School of Mind and Brain, Humboldt-University, Germany

Object substitution masking (OSM) occurs when a briefly presented target in a search array is surrounded by small dots that remain visible after the target disappears. The reduction of target visibility occurring after OSM has been suggested to result from a specific interference with reentrant visual processing. Here, we tested a prediction derived from this hypothesis: responses fast enough to have been triggered before the beginning of reentrant processing should escape this interference, and thus not be affected by masking. To this aim, we combined an OSM paradigm with a saccadic choice task, in which the fastest saccades occur as early as 120 ms after target onset, allowing to study the effect of masking at an early phase of visual processing. In the first experiment, we manipulated target visibility using either OSM, backward masking, or low stimulus contrast and compared their effect on accuracy over time. A general reduction of performance was observed in all three conditions. The analysis of accuracy as a function of response time revealed that the fastest saccades (120-150 ms time-window) were virtually unaffected by both OSM and backward masking, while performance was strongly reduced for saccades slower than 150 ms. This RT-dependent performance impairment was specific to the masking procedures, since the control condition in which performance was impaired through a contrast decrease showed no such difference between fast and slow saccades. The second experiment with additional EEG recordings revealed that pre-stimulus alpha band activity influences the accuracy of the saccade to come, especially for the fastest responses. These findings provide further evidence that masking interferes mostly with reentrant processing, while leaving early feedforward processing largely intact, and bring new insights into the trial-to-trial variability of reaction times and accuracy.

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