Dissociable factors affect speed perception and discrimination

Factors affecting our judgement of the speed of visual motion were investig ated. Two types of judgement were made: perceived speed relative to a stand ard comparison stimulus, and discrimination between the speeds of similar s timuli. The factors affecting these two judgements were found to be doubly dissociable, suggesting that they may be constrained by processing at diffe rent levels of the visual hierarchy. The results are discussed in terms of the 3-D interpretation of visual image motion, and related to possible neur al substrates.