Intrusions into the shadow of attention: A new take on illusory conjunctions

We present new evidence about illusory conjunctions (ICs) suggesting that their current explanation requires revision. According to Feature Integration Theory (FIT; Treisman & Gelade Cognitive Psychology, 12, 97-136, 1980), focal attention to a single stimulus is required to bind its features into an integrated percept. FIT predicts that if attention is spread over multiple stimuli, features of these different stimuli can be combined into a single percept and produce ICs. Treisman and Schmidt (Cognitive Psychology, 14, 107-141, 1982) and Cohen & Ivry (Journal of Experimental Psychology: Human Perception and Performance, 15(4), 650-663, 1989) supported this prediction. In the latter study, participants viewed brief displays containing two digits and two colored letters. Digit locations were pre-cued, and participants were instructed to prioritize the digits and to spread their attention across the region encompassed by the digits. Cohen & Ivry found that reports of one letter (the 'target') produced ICs when both letters appeared between the digits. Expanding on Cohen & Ivry's paradigm, we find that both letters do not need to appear between the digits to produce ICs. While the target letter was highly susceptible to ICs if the target appeared inside the position of a nearby digit, the position of the other letter was largely irrelevant. Our experimental results also argue that these ICs were not due to mnemonic errors occurring while the digits are being reported. Based on our findings, we propose that attention to the digits casts an attentional 'shadow' projecting towards fixation, interfering with processing of target letters in that shadow and allowing color information from elsewhere in the display to be included in the resulting percept.