Book excerpt: "As part of the engineering psychology research program on tracking, experiments are being conducted to determine systematically how man discriminates velocity and acceleration. Results of these experiments provide for a fuller understanding of the visual processes of the tracker in predicting the motion of a target. The method of experimentation used involves an unequivocal response to visual movement. The observer, presented with a spot traveling at a controlled speed, reports its direction. In this experiment four observers discriminated velocity at eight speeds for each of eight exposure times. The minimal luminance required for the discrimination was determined for each condition. Analysis of the results indicated that the visual discrimination of velocity under the experimental conditions is a single discriminatory event controlled by photochemical activity in the retina. For exposure times less than 0.1 second the discrimination is determined by the product of stimulus luminance and duration. For longer intervals the discrimination tends to be dependent on luminance alone. The results further showed that duration and speed interact with each other as determinants of the discrimination. This finding suggests that distance, the product of duration and speed, is a relevant variable."--Abstract