On the speed of natural scene categorisation in human and non-human primates

Two rhesus monkeys were tested in a visual categorisation task in which they had to respond to the presence of a given target (food for one monkey, animals for the other). All stimuli were natural images taken from a vast CD ROM data bank that were flashed for only 80 ms (to avoid eye movements). Every day, among the images that they had already experienced, the macaques were shown 5 new targets and 5 new distracters; their responses to the first presentation of each new image were analysed. Monkeys categorised with a high rate of success (90.5% correct for the food task and 84% for the animal task), and with very fast reaction times (median RTs were 344 ms and 245 ms, respectively). A group of 10 humans that categorised the same images in similar conditions responded somewhat more accurately (95% correct in both tasks) but much more slowly (median RT on the food and animal tasks: 446 ms and 422 ms). Moreover, the images incorrectly classified by humans and monkeys overlapped considerably. A last experimental study demonstrated that the macaques' performance was equally good when stimuli were proposed in black and white. These results show that monkeys can make extremely fast and reliable classifications of previously unseen stimuli, even without color, probably on the basis of abstract representations that overlap with those of humans. Moreover, the fast responses recorded show that such rapid categorisation cannot rely on time consuming visual processing and may well involve a single feed-forward pass through the visual system.