The long-term retention of events in monkey memory.

Nine monkeys (Macaca fascicularis) demonstrated long-term memory for objects in a recognition task based on the non-matching-to-sample (NMTS) paradigm. In this task, the subjects were required to choose a novel object when it was paired with an alternative that had become familiar in previous NMTS training. When the familiar objects had been experienced an average of 3.4 times 4-9 months previously, 5 monkeys made 79% correct choices of the novel object. Three other monkeys exposed to the objects a mean of 12.8 times were 65% accurate at retention intervals of 20 months. A ninth subject achieved an accuracy of 68% after a retention interval of 34 months based on an exposure frequency of 10.6. These levels of performance indicate that in monkey event memory the mnemonic representation of an object is quite durable and a proportion of visual information may last for at least 3 years.