Abstract
Examination of certain illusory conjunction (IC) errors may provide insight into the mechanisms of object recognition when multiple stimuli are attended. An IC error occurs when a subject reports a stimulus that was not present but that combines features of target and distractor stimuli. While ICs between nearby stimuli have been frequently studied and may be related to crowding (McClelland & Henderson, 2012), the properties of ICs are less clear in cases where target and distractor stimuli are distant from each other. A series of experiments replicated and extended a representative study (Cohen & Ivry, 1989). Subjects observed a display with two white digits and a colored target and distractor letter. Stimuli were interleaved and horizontally displaced from a central fixation mark. On each trial, subjects reported the identities of the digits and then the identity and color of the target letter. The frequency of ICs was critically related to both mnemonic demands and dual task aspects of the procedure. ICs were less frequent though not eliminated when the target letter was reported first, suggesting that some ICs were related to memory errors from reporting the digits first. However, ICs were eliminated when subjects could entirely ignore the digits, even controlling for accuracy. Interestingly, when digits were attended, the relative placement of digits and letters strongly influenced errors – for example, ICs were greatly reduced when the digits were vertically offset from fixation. When the digits and letters were interleaved horizontally, accuracy was reduced when a digit was present just outside rather than inside a target letter’s position relative to fixation (the distractor letter’s position had no similar effect). This effect did not occur when subjects ignored the digits. These results suggest that attending to and encoding additional stimuli has a particular and positionally-sensitive effect on errors reporting a target stimulus.
Meeting abstract presented at VSS 2013