Subjective Audibility Modulates the Susceptibility to Sound-Induced Flash Illusion

When a brief flash is presented along with two brief sounds, the single flash is often perceived as two flashes. This phenomenon is called a sound-induced flash illusion, in which the auditory sense, with its relatively higher reliability in providing temporal information, modifies the visual perception. Decline of audibility due to hearing impairment is known to make subjects less susceptible to the flash illusion. However, the effect of decline of audibility on susceptibility to the illusion has not been directly investigated in subjects with normal hearing. The present study investigates the relationship between audibility and susceptibility to the illusion by varying the sound pressure level of the stimulus. In the task for reporting the number of auditory stimuli, the lowering of the sound pressure level caused the rate of perceiving two sounds to decrease on account of forward masking. The occurrence of the illusory flash was reduced as the intensity of the second auditory stimuli decreased, and was significantly correlated with the rate of perceiving the two auditory stimuli. These results suggest that the susceptibility to sound-induced flash illusion depends on the subjective audibility of each sound.