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David McCormick, Pascal Mamassian; Response biases in the illusory-flash effect. Journal of Vision 2005;5(8):878. doi: 10.1167/5.8.878.
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In the illusory-flash effect (Shams et al., 2000, Nature, 408,788), two flashes presented with three tones have a tendency to be perceived as three flashes. The origin of this illusory percept is uncertain. In particular, it is not clear whether this illusory percept results from an increased willingness to report seeing three flashes (criterion shift) or a decrease in accuracy (sensitivity change). We address this issue by measuring the ability to detect a low-contrast target presented simultaneously with an auditory tone. Four possible combinations of audio-visual events were presented, with 2 or 3 flashed Gabors and 2 or 3 auditory tones. When present, the central flash had a modulated contrast. A tone was always presented simultaneously with both the first and the last flash. In half of the trials, a third tone was presented equidistant in time between the others, synchronously with the central flash if present. Observers reported whether they perceived 2 or 3 flashes. A comparison of the psychometric functions for contrast detection showed a lateral shift of the 3-tone relative to the 2-tone condition, while the slopes of the psychometric functions remained approximately constant. These results suggest that the additional tone produced a criterion shift in the observer's decision, not a change in sensitivity. Our results contrast with previous reports of an increased sensitivity in orientation discrimination when a rapid series of tones are presented simultaneously with visual flickering stimuli (Berger et al., 2003, J. Vision, 3, 406–412). We discuss potential reasons for this discrepancy and implications for models of cross-modal interaction.
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