The beep-speed illusion: Non-spatial tones increase perceived speed of visual objects in a forced-choice paradigm

We introduce a new audio-visual illusion revealing the interplay between audio-visual integration and selective visual attention. This illusion involves two simultaneously moving objects that change their motion trajectory occasionally, but only the direction changes of one object are accompanied by spatially uninformative tones. We observed a selective increase in perceived object speed of the audio-visually synchronized object by measuring the point of subjective equality in a forced-choice paradigm. The illusory increase in perceived speed of the audio-visually synchronized object persisted when preventing eye movements. Using temporally matched color changes of the synchronized object also increased the perceived speed. Yet, using color changes of a surrounding frame instead of tones had no effect on perceived speed ruling out simple alertness explanations. Thus, in contrast to coinciding tones, visual coincidences only elicit illusory increases in perceived speed when the coincidence provided spatial information. Taken together, our pattern of results suggests that audio-visual synchrony attracts visual attention toward the coinciding visual object, leading to an increase in speed-perception and thus shedding new light on the interplay between attention and multisensory feature integration. We discuss potential limitations such as the choice of paradigm and outline prospective research question to further investigate the effect of audio-visual integration on perceived object speed.