Not proportional after all: Investigating speed perception with the beep-speed illusion

Recently, a new audiovisual illusion, the beep-speed illusion, was discovered in which a visually presented, linearly moving object is perceived to be faster if directional changes occur simultaneously with a short auditory burst compared to a purely visually presented moving object of the same speed. The present study uses this new illusion to test the representation of motion speed in human perception. Across two experiments (each N = 30), the beep-speed illusion was observed with circular motion of the objects and across different speeds of the audiovisual object. Interestingly, the size of the illusion, as well as the precision of the speed estimation as measured by the just notable difference, was not proportional in size to the audio-visual object speed across the different speeds. These results contradict predictions of proportionality in speed estimation derived from classical Weber law, and are discussed in light of recent theoretical developments in the field of motion/speed perception.