It is obvious that animals remember. When food is abundant, birds and mammals hoard it in caches and can retrieve it days, weeks, even months later when it has become scarce. But because they cannot speak, it is difficult to test whether animals are aware of their own memories.
An important property of human conscious memory is the ability to discern the presence and absence of memories. For instance, we can judge whether a telephone number has been remembered properly without having to dial the number.
This awareness had appeared to be a uniquely human capacity. But Robert Hampton, a neuroscientist at the National Institute of Mental Health in the United States, has reported a series of elegant experiments that appears to prove rhesus monkeys know when they remember. This vital finding suggests another example of a mind function that was once thought uniquely human but is also possessed by animals.
The monkeys were briefly shown images and then allowed to choose whether or not their recollections were tested. If the animal ducked out at this point, they were guaranteed a small reward. If they took the test and gave an accurate performance, they would get a favourite reward. If they failed, they would get nothing.
In theory, the monkey that knows whether it remembers the recently seen image should choose to take tests when it remembers and should decline tests when it has forgotten. The experimenters then altered the delay between the image and the test. As memory decays over time, monkeys should decline tests less frequently with shorter delays.
The study, published in the Proceedings of the National Academy of Sciences , revealed that monkeys had the ability to appropriately decline memory tests when they were unlikely to choose the correct image. The results strongly suggest that monkeys know when they remember. They can assess at least some of their own knowledge states, just like us.
Marc Hauser, an expert on animal intelligence at Harvard University, agreed that Hampton's finding is an important advance in proving that monkeys are more self-aware about mental functions than previously realised.
But he cautioned that the precise experiential nature of the memory is still unclear and hence it is difficult to say how similar the monkeys' awareness of memories was to that of humans'.
Raj Persaud, consultant psychiatrist, Maudsley Hospital, London.