Detectability of velocity gradients in moving random-dot patterns

Abstract We present a subject with two moving spatial noise patterns at both sides of a common border. The patterns are masked with spatio-temporal white noise and we measure the threshold signal-to-noise ratio at which a subject can discriminate the case of two differently moving patterns from that of a single uniformly moving pattern over the whole field. At some experiments the patterns move in the same direction but the magnitude of the velocity of one of the patterns is varied. In other experiments both patterns move with the same velocity but in different directions. In no case do we find a dependence of the thresholds on the orientation of the common border with respect to the direction of movement of the patterns. In all cases we find a certain region of magnitudes of the variable velocity or of the orientation-difference between the two velocities in which the thresholds are very high, whereas for larger differences either in magnitude or in direction the thresholds are generally at values of less than one for the signal-to-noise ratio. The region of the raised thresholds for both magnitude and orientation differences can be described with a single simple expression: thresholds are high whenever the magnitude of the velocity difference of the two patterns is less than one half of the magnitude of the common component of the two velocities. This is a kind of Weber law for the velocity vectors. It is estimated that you may not pick many more than 150 differently moving patterns (both with respect to magnitude and direction) in such a way that any pair of them leads to easy discrimination (low threshold signal-to-noise ratio).