Effect of Attention on Early Cortical Processes Associated with the Sound-induced Extra Flash Illusion Jyoti Mishra 1 , Antigona Martínez 1,2 , and Steven A. Hillyard 1 Abstract ■ When a single flash of light is presented interposed between two brief auditory stimuli separated by 60–100 msec, subjects typically report perceiving two flashes [Shams, L., Kamitani, Y., & Shimojo, S. Visual illusion induced by sound. Brain Research, Cognitive Brain Research, 14, 147–152, 2002; Shams, L., Kamitani, Y., & Shimojo, S. Illusions. What you see is what you hear. Nature, 408, 788, 2000]. Using ERP recordings, we previously found that perception of the illusory extra flash was accompanied by a rapid dynamic interplay between auditory and visual cortical areas that was triggered by the second sound [Mishra, J., Martínez, A., Sejnowski, T. J., & Hillyard, S. A. Early cross-modal interactions in auditory and visual cortex underlie a sound-induced visual illu- sion. Journal of Neuroscience, 27, 4120–4131, 2007]. In the cur- rent study, we investigated the effect of attention on the ERP components associated with the illusory extra flash in 15 indi- viduals who perceived this cross-modal illusion frequently. All early ERP components in the cross-modal difference wave asso- ciated with the extra flash illusion were significantly enhanced by selective spatial attention. The earliest attention-related mod- ulation was an amplitude increase of the positive-going PD110/ PD120 component, which was previously shown to be correlated with an individualʼs propensity to perceive the illusory second flash [Mishra, J., Martínez, A., Sejnowski, T. J., & Hillyard, S. A. Early cross-modal interactions in auditory and visual cortex un- derlie a sound-induced visual illusion. Journal of Neuroscience, 27, 4120–4131, 2007]. The polarity of the early PD110/PD120 component did not differ as a function of the visual field (upper vs. lower) of stimulus presentation. This, along with the source localization of the component, suggested that its principal gen- erator lies in extrastriate visual cortex. These results indicate that neural processes previously shown to be associated with the extra flash illusion can be modulated by attention, and thus are not the result of a wholly automatic cross-modal integration process. ■ INTRODUCTION Events in the natural world are often multimodal, and intersensory interactions in the brain are critical to the generation of coherent percepts and the control of behav- ior (reviewed in Driver & Noesselt, 2008; Amedi, Von Kriegstein, Van Atteveldt, Beauchamp, & Naumer, 2005; Macaluso & Driver, 2005; Calvert, 2001; Stein & Meredith, 1993). Within the audiovisual domain, numerous behav- ioral studies have shown that simultaneous auditory and visual inputs interact such that visual perception can be altered by audition and vice versa. For example, the per- ceived location of sounds is robustly altered by concurrent visual stimuli at a nearby location, a phenomenon known as the ventriloquist illusion (Bonath et al., 2007; Vroomen & De Gelder, 2004; Hairston et al., 2003; Bertelson, 1999; Pick, Warren, & Hay, 1969). Conversely, the concurrent presentation of sounds can strikingly alter visual perception (McDonald, Teder-Sälejärvi, Di Russo, & Hillyard, 2003, 2005; Recanzone, 2003; Fendrich & Corballis, 2001; Sekuler, Sekuler, & Lau, 1997; Stein, London, Wilkinson, & Price, 1996). One of the most striking visual illusions induced by auditory stimulation is that introduced by Shams, Kamitani, and Shimojo (2000, 2002), wherein a single brief flash pre- sented interposed between two pulsed sounds separated by 60–100 msec generates the percept of two distinct flashes, of which the second is illusory. The neural basis of the illusory extra flash has been investigated in several physiological studies (Mishra, Martínez, Sejnowski, & Hillyard, 2007; Watkins, Shams, Tanaka, Haynes, & Rees, 2006; Shams, Iwaki, Chawla, & Bhattacharya, 2005; Arden, Wolf, & Messiter, 2003; Shams, Kamitani, Thompson, & Shimojo, 2001). Using ERP re- cordings, Mishra et al. (2007) found that the cross-modal interactions underlying the illusory flash phenomenon had a complex but distinct neural signature. An early pos- itive component peaking at 120 msec after the onset of the first sound was identified in the cross-modal inter- action waveforms associated with the illusion. This PD120 component was found to originate within extrastriate visual cortex, and its amplitude in individual subjects was predic- tive of the frequency with which they perceived the extra flash illusion. Indeed, the PD120 was completely absent in subjects who did not perceive the illusion. Interestingly, however, this component did not vary within individual subjects on trials when the illusion was seen versus not seen (Mishra et al., 2007). Accordingly, we suggested that 1 University of California, San Diego, La Jolla, CA, 2 Nathan S. Kline Institute for Psychiatric Research, Orangeburg, NY © 2009 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 22:8, pp. 1714–1729