Consciously monitored grasping is vulnerable to perceptual intrusions

The idea of functional differentiation between vision-for-action and vision-for-perception has been supported by evidence from different domains. According to this account, perception is based on consciously accessible, relative representations, whereas vision-for-action is performed in an analytic, automatic manner. Support for this idea comes from studies that showed that unlike perception, grasping movements are refractory to illusions and to Weber's law. Yet, interactions between the systems may occur when an action is performed in a less automated fashion. To test this idea, we asked participants to monitor their fingers apertures in flight and to halt their movement for a short duration when they felt that their aperture reached a maximum amount. The results showed that movements in the monitored condition were biased by the Ponzo illusion and showed atypical adherence to Weber's law. These results show that action and perception are more likely to interact when movements are performed in a controlled manner.