THE HUMAN VISUAL-SYSTEM AVERAGES SPEED INFORMATION

It has been known for many years that human observers are unable to detect modest accelerations and decelerations in moving visual stimuli. We find that human observers can integrate speeds over many dots, moving at different speeds, producing a global speed percept analogous to the global direction percept first reported by Williams, D. W. and Sekuler, R. (1984, Vision Research, 24, 55-62). We measured speed discrimination for random dot stimuli which contained many different speeds. Our results show that observers always base their discrimination on the mean speed of the stimulus; changes in other stimulus characteristics (e.g. mode) are not detected. Moreover, discrimination thresholds for the global mean speed derived from many different speeds are comparable to those obtained with stimuli in which all dots move at the same speed suggesting that the internal noise associated with the encoding of speed information is quite high.