21st European Conference on Visual Perception
August 24-28 - 1998 - Oxford (England)
 
Ultra-rapid visual categorisation of natural scenes is color-blind.
 
A. Delorme, M. Fabre-Thorpe, G. Richard, D. Fize & S. J. Thorpe.
 

Centre de Recherche Cerveau et Cognition (UMR 5549) Faculté de Médecine de Rangueil, 133 route de Narbonne, 31062 Toulouse, France.

arno@cerco.ups-tlse.fr, mft@cerco.ups-tlse.fr, richard@cerco.ups-tlse.fr, thorpe@cerco.ups-tlse.fr

 
     Using a go/no-go categorisation task, we recently showed that both humans and monkeys are fast and efficient at detecting animals in natural images flashed for only 20-30ms. The visual processing can be done in under 150ms in humans and may be even faster in monkeys. Such rapid processing is likely to depend essentially on feed-forward processing and on the fastest visual information transmitted. Recent neurophysiological data (Nowak et al, Visual Neuroscience, 1995, 12, 371-384) demonstrate that the earliest responses in the visual cortex originate from the magnocellular stream, whereas chromatic parvocellular information is delayed by roughly 20ms.
     To test whether longer latency parvocellular chromatic information is used in ultra-rapid visual categorisation, we tested humans and monkeys with 400 -previously unseen- images, randomly presented half in color, half in black and white. The effect of removing color information was marginal both on accuracy (monkey: no effect, human: 2% decrease) and on median RT (10ms increase). In humans, the analysis of evoked potential showed that the strong differential response to targets and distractors started at 150ms in both conditions. Moreover, the advantage for coloured stimuli was directly related to RT, so that subjects with long RTs performed better with coloured images whereas subjects with the shortest RTs showed little or no advantage with chromatic stimuli.
     Such results support our hypothesis that ultra-rapid visual categorisation involves essentially feed-forward processing, since even information arriving in visual cortex with a 20ms lag appears to have no influence on behaviour.