RESEARCH ARTICLE Illusory double ﬂashes can speed up responses like physical ones: evidence from the sound-induced ﬂash illusion Anja Fiedler • Julie L. O’Sullivan • Hannes Schro ¨ter • Jeff Miller • Rolf Ulrich Received: 7 June 2011 / Accepted: 21 July 2011 / Published online: 5 August 2011 Ó Springer-Verlag 2011 Abstract When a single brief ﬂash is accompanied by two auditory beeps, participants often report perceiving two ﬂashes. The present experiment examined whether the perception of illusory redundant ﬂashes can result in faster responses as compared to the perception of a single ﬂash, because previous research has shown such a redundancy gain for physical stimuli. To this end, participants were asked to respond as rapidly as possible to the onset of any ﬂash. Following their response, they additionally indicated whether they perceived a single ﬂash or a double ﬂash. Most importantly, we observed signiﬁcant shorter reaction times in response to redundant ﬂashes, irrespective of whether they were physically presented or illusorily per- ceived. Taken together, our results suggest that an illusory percept can affect simple reaction time in much the same manner as the corresponding physical stimulation. Keywords Sound-induced ﬂash illusion Á Percept Á Simple RT Á Redundancy gain Introduction The integration of visual and auditory information is an important capability of the cognitive system. It enables a coherent perception of the surrounding environment that in turn is a prerequisite for adaptive, goal-directed behavior required in daily life. Furthermore, when two modalities deliver conﬂicting information, the perception in one modality can be altered by the information in the other modality, thus leading to perceptual biases and illusions (e.g., the McGurk effect; McGurk and MacDonald 1976). Shams et al. (2000, 2002) reported an impressive example of the modulation of visual information by sound. The authors demonstrated that when a brief ﬂash is accompa- nied by two auditory beeps, the single ﬂash can be per- ceived as two successive ﬂashes. Hence, the physical ﬂash can be segregated into multiple percepts due to concurrent auditory stimulation. This audiovisual illusion is called the sound-induced ﬂash illusion (SIFI). To our knowledge, the SIFI has mainly been investi- gated in tasks that required participants to judge the num- ber of ﬂashes they perceived in a given trial (e.g., McCormick and Mamassian 2008; Shams et al. 2000, 2002, 2005). The SIFI occurs across a wide range of experimental parameters (cf., Shams et al. 2002) and is resistant to feedback training (Rosenthal et al. 2009). Recent imaging studies have shown that the SIFI is cor- related with a dynamic interplay between auditory and visual cortical areas (e.g., Mishra et al. 2007). Moreover, the number of perceived ﬂashes seems to determine the correlated activity pattern in V1 irrespective of whether the perceived ﬂashes are physically presented or illusorily perceived due to the SIFI (Watkins et al. 2006, 2007). Berger et al. (2003) showed that the illusory double ﬂash can have an inﬂuence on discrimination performance. Speciﬁcally, they observed that increasing the number of perceived ﬂashes resulted in an improvement of orientation discrimination regardless of whether the ﬂashes were physically presented or illusorily perceived. To our knowledge, however, no study has yet investigated whether the illusory double-ﬂash effect is restricted to perception A. Fiedler (&) Á J. L. O’Sullivan Á H. Schro ¨ter Á R. Ulrich Department of Cognitive and Biological Psychology, University of Tu ¨bingen, Friedrichstrasse 21, 72072 Tu ¨bingen, Germany e-mail: anja.ﬁedler@uni-tuebingen.de J. Miller Department of Psychology, University of Otago, Dunedin, New Zealand 123 Exp Brain Res (2011) 214:113–119 DOI 10.1007/s00221-011-2811-z