Stimulus specificity and temporal dynamics of working memory for visual motion.

When asked to compare two moving stimuli separated by a delay, observers must not only identify stimulus direction but also store it in memory. We examined the properties of this storage mechanism in two macaque monkeys by sequentially presenting two random-dot stimuli, sample and test, in opposite hemifields and introducing a random-motion mask during the delay. The mask interfered with performance only at the precise location of the test, 100-200 ms after the start of the delay, and when its size and speed matched those of the remembered sample. This selective interference suggests that the representation of the motion stimulus in memory preserves its direction, speed, and size and is most fragile shortly after the completion of the encoding phase of the task. This precise preservation of sensory attributes of the motion stimulus suggests that the neural mechanisms involved in the processing of visual motion may also be involved in its storage.