The occurrence of illusory conjunctions correlates with the spatial noise in peripheral vision
Illusory conjunctions (IC) refer to errors in which an observer correctly reports features present in the display, but incorrectly pairs features or parts from multiple objects. There is a long-standing debate in the literature about the nature of ICs; for example, whether they arise from the lack of focused attention (Treisman & Schmidt, 1982) or from lossy peripheral representations (Rosenholtz et al., 2012). Here, we test the hypothesis that the occurrence of ICs relates to spatial uncertainty of features falling within the same noisy “window”. According to this idea, ICs occur when the spatial uncertainty is large compared to the distance between items, causing confusion over which features belong to which item. In Experiment 1, we directly measured the spatial noise at 3°, 6°, 9°, 12° from fixation. A compact “crowd” of four dots briefly appeared, followed by the presentation of a probe circle at various distances from the “crowd”. Observers had to respond whether any dot had fallen within the probed region. The probability of perceiving the dots as outside the probe as a function of distance provides a measure of spatial noise as a function of eccentricity. In Experiment 2, we presented four differently colored and oriented bars, located on an invisible circle with a diameter varying from 1° to 3.5° (the “separation”), and centered at one of three eccentricities (4°, 8°, 12°). Participants had to report the color, orientation, and location of any of the bars. The number of correct answers, guesses (reporting non-presented features), and ICs were estimated. The number of IC increased with eccentricity and decreased with separation. There is good resemblance between the spatial noise and the IC pattern. We conclude that there can be an overlap between the mechanisms of spatial localization and IC in peripheral vision.