Errors in interception can be predicted from errors in perception

It has been hypothesised that our actions are less susceptible to visual illusions than our perceptual judgements because similar information is processed for perception and action in separate pathways. We test this hypothesis for subjects intercepting a moving object that appears to move at a different speed than its true speed due to an illusion. The object was a moving Gabor patch: a sinusoidal grating of which the luminance contrast is modulated by a two-dimensional Gaussian. We manipulated the patch's apparent speed by moving the grating relative to the Gaussian. We used separate two-interval forced choice discrimination tasks to determine how moving the grating influenced ten people's judgements of the object's position and velocity while they were fixating. Based on their perceptual judgements, and knowing that our ability to correct for errors that arise from relying on incorrect judgements are limited by a sensorimotor delay of about 100 msec, we predicted the extent to which subjects would tap ahead of or behind similar targets when trying to intercept them at the fixation location. The predicted errors closely matched the actual errors that subjects made when trying to intercept the targets. This finding does not support the two visual streams hypothesis. The results are consistent with the idea that the extent to which an illusion influences an action tells us something about the extent to which the action relies on the percept in question.