Short-term memory for the temporal order of events in monkeys.

Two monkeys (Macaca fascicularis) demonstrated short-term memory for the recency of unpredictable visual events in a probe recognition task. On each trial, from two to five 3-dimensional objects were randomly selected from a pool of 200 and presented sequentially. In the test probe that followed, the subjects saw two objects from the preceding list and chose the one that had appeared earlier. Overall accuracy was 73% correct. Like human judgments of recency, accuracy varied inversely with the lag and directly with the temporal separation of the objects in the probe. The data suggest that monkeys encode information about object order as well as object identity, their mnemonic representation of the ordered list contains information about all of its constituent elements, and recency judgments are made at the time the probe is presented and not before. Monkey processing of temporal order information is in these respects similar to human order memory.