Neural noise distorts perceived motion: the special case of the freezing illusion and the Pavard and Berthoz effect.

When a slowly moving pattern is presented on a monitor which itself is moved, the pattern appears to freeze on the screen (Mesland and Wertheim in Vis Res 36(20):3325-3328, 1996) even if we move our head with the monitor, as with a head mounted display (Pavard and Berthoz in Perception 6:529-540, 1977). We present a simple model of these phenomena, which states that the perceived relative velocity between two stimuli (the pattern and the moving monitor) is proportional to the difference between the perceived velocities of these stimuli in space, minus a noise factor. The latter reflects the intrinsic noise in the neural signals that encode retinal image velocities. With noise levels derived from the literature the model fits empirical data well and also predicts strong distortions of visually perceived motion during vestibular stimulation, thus explaining both illusions as resulting from the same mechanism.