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

## TL;DR

A new illusion shows that sounds can make moving objects seem faster, and this effect doesn't scale proportionally with speed.

## Contribution

The beep-speed illusion is used to show non-proportional speed perception effects contradicting classical Weber law predictions.

## Key findings

- The beep-speed illusion occurs with circular motion and across different speeds.
- The illusion's size and speed estimation precision do not scale proportionally with object speed.
- Results contradict classical Weber law predictions for speed perception.

## Abstract

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.