Determining the visibility of noise in motion signals.

A procedure is described for generating stimuli to study the detection of noise components in motion signals. By using random dots with intensities distributed according to a Gaussian probability function, a temporally and spatially continuous mixture of signal and noise components can be realized in random dot kinematograms. These stimuli were used in a noise detection task, a signal detection task and a direction discrimination task. Signal-to-noise ratio ('coherence') thresholds for the signal detection and direction discrimination tasks were consistent with previous research. Noise can be detected at levels of approximately 0.5-2.5%, depending on the size of the motion stimulus. We argue that the noise in the motion stimulus becomes detectable when it exceeds the noise intrinsic to the various stages of motion processing. Therefore,the method provides a simple procedure for obtaining measures of equivalent input noise and can be used for estimating internal noise levels of motion processing mechanisms.