Rapid processes (45-90 msec) for low-level visual categorization

Using a similar experimental paradigm, we had unexpectedly observed early differences over mid-parietal sites between a condition in which male and female stimuli were randomly intermixed (possibly inducing incidental categorization of gender) and a condition in which the stimuli of different gender were presented in separate blocks (preventing any gender categorization).

These early effects were found for faces and for hand stimuli (Mouchetant-Rostaing et al, 2000a), and were also observed for non-biological, geometrical figures of two categories separated by salient visual characteristics (Mouchetant-Rostaing et al, 2000b). In the present experiment, we found again very early differential ERP effects (45-90 msec) on a wide centro-parietal region in the three age or gender categorization conditions relative to the No-discrimination condition.

Similar early neural activities have been recently reported in human ERPs during visual categorization of complex natural images including human faces (George et al., 1997; Seeck et al., 1997; Debruille et al., 1998; VanRullen and Thorpe, 2001). While in some cases these early activities might have been explained by a repetition factor due to the fact that the same stimuli were used between different experimental conditions (George et al., 1997; Debruille et al., 1998), they were also interpreted as possibly reflecting early perceptual, task-independent processes for global extraction of basic visual differences (VanRullen and Thorpe, 2001). As suggested previously (Mouchetant-Rostaing et al, 2000a, 2000b), these very early effects might similarly reflect the existence of coarse and automatic, low-level categorization processes for rapid distinction between two wide stimulus categories, which are probably based, at this stage of sensory analysis, on feed-forward mechanisms.