Ultra-rapid categorisation of natural scenes does not rely on colour cues: a study in monkeys and humans.

In a rapid categorisation task, monkeys and humans had to detect a target (animal or food) in briefly flashed (32 ms) and previously unseen natural images. Removing colour cues had very little effect on average performance. Impairments were restricted to a mild accuracy drop (in some human subjects) and a small reaction time mean increase (10-15 ms) observed both in monkeys and humans but only in the detection of food targets. In both tasks, accuracy and latency of the fastest behavioural responses were unaffected, suggesting that such ultra-rapid categorizations could depend on feed-forward processing of early coarse achromatic magnocellular information.