SPEED OF PROCESSING IN THE HUMAN VISUAL-SYSTEM

How long does it take for the human visual system to process a complex natural image? Subjectively, recognition of familiar objects and scen es appears to be virtually instantaneous, but measuring this processin g time experimentally has proved difficult. Behavioural measures such as reaction times can be used(1), but these include not only visual pr ocessing but also the time required for response execution. However, e vent-related potentials (ERPs) can sometimes reveal signs of neural pr ocessing well before the motor output(2). Here we use a go/no-go categ orization task in which subjects have to decide whether a previously u nseen photograph, flashed on for just 20 ms, contains an animal. ERP a nalysis revealed a frontal negativity specific to no-go trials that de velops roughly 150 ms after stimulus onset. We conclude that the visua l processing needed to perform this highly demanding task can be achie ved in under 150 ms.