Audition influences color processing in the sound-induced visual flash illusion

- The sound induced extra flash illusion is feature specific.
- Auditory induction of the illusory flash occurs subsequently to color encoding.
- Two rapid sounds influence the integrated color percept of two successive flashes.
- Concurrent auditory inputs modulate temporal and featural properties of visual illusions.

Multisensory interactions can lead to illusory percepts, as exemplified by the sound-induced extra flash illusion (SIFI: Shams, Kamitani, & Shimojo, 2000, 2002). In this illusion, an audio-visual stimulus sequence consisting of two pulsed sounds and a light flash presented within a 100 ms time window generates the visual percept of two flashes. Here, we used colored visual stimuli to investigate whether concurrent auditory stimuli can affect the perceived features of the illusory flash. Zero, one or two pulsed sounds were presented concurrently with either a red or green flash or with two flashes of different colors (red followed by green) in rapid sequence. By querying both the number and color of the participants' visual percepts, we found that the double flash illusion is stimulus specific: i.e., two sounds paired with one red or one green flash generated the percept of two red or two green flashes, respectively. This implies that the illusory second flash is induced at a level of visual processing after perceived color has been encoded. In addition, we found that the presence of two sounds influenced the integration of color information from two successive flashes. In the absence of any sounds, a red and a green flash presented in rapid succession fused to form a single orange percept, but when accompanied by two sounds, this integrated orange percept was perceived to flash twice on a significant proportion of trials. In addition, the number of concurrent auditory stimuli modified the degree to which the successive flashes were integrated to an orange percept vs. maintained as separate red-green percepts. Overall, these findings show that concurrent auditory input can affect both the temporal and featural properties of visual percepts.