The effectiveness of the discrimination of segment sizes by people with different cognitive style parameters - field dependence vs. field independence, analytical vs. synthetic, and flexible vs. rigid cognitive control - was studied. Discrimination effectiveness was assessed in terms of the magnitude of the Ponzo illusion. The magnitude of the Ponzo illusion was found to be significantly smaller in subjects with a field-independent cognitive style than in those with a field-dependent style. People with the flexible type of cognitive control demonstrated a tendency to more accurate perception of segment size in the Ponzo figure than those with rigid control. There was no relationship between the analytical-synthetic style of categorization and the magnitude of the Ponzo illusion.