The Judd illusion: evidence for two visual streams or two experimental conditions?

In the Judd illusion, observers inaccurately bisect the shaft located betwe en two arrowheads pointing in the same direction. The magnitude of error is greater when verbal judgements are compared to action based responses (rea ching out and grasping the centre of the bar). This difference has been att ributed to the presence of two visual streams within cortical processing. I n contrast, we provide evidence that the improved accuracy in the reaching condition may be due to occlusion of the illusory background during the tra nsport phase of the movement. We suggest that caution is required when inte rpreting performance differences between two conditions that are not strict ly equivalent.