Abstract The perceived size of a fixated object is known to be a function of the perceived fixation distance. The size-distance paradox has been posited as evidence that the perceived distance of a fixated object is, in turn, influenced by the object’s perceived size. If this is correct then it challenges a widely accepted account (modified weak fusion) of how the nervous system combines multiple sources of information. We hypothesised that the influence of perceived size on the perception of distance is likely to be restricted to conscious perceptual judgements. If our hypothesis is correct then the size-distance paradox should not be observed when observers make action-based distance judgements. In line with this expectation we observed the size-distance paradox when participants made verbal reports on target distance but found no paradoxical judgements in a group who were asked to point at the target. We therefore suggest that the size-distance paradox should not be taken as evidence that perceived size feeds back into distance perception.

Monocular perception of egocentric distance via optic flow generated by head movement toward a target was investigated with a helmet-mounted video camera and display. Ability to perceive target distance was assessed with 2 response measures: verbal reports and reaches. Systematic and random errors differed as a function of the response measure. Verbal estimates of targets within and beyond reach were obtained before and after the performance of reaches to targets within reach. Systematic errors of verbal estimates changed but did not decrease overall. Random error decreased. Verbal estimates and reaches were performed concurrently to targets within reach. Verbal and reaching errors were uncorrelated. Verbal judgments appear to have been anchored using the range of distances experienced while reaching rather than being calibrated to the perceptual information itself. Discussion focuses on the advantages of action response measures. Reaching to bring the hand to a specific location in space is a usual component of everyday manual activities, such as reaching for a doorknob or a cup. The accurate execution of such activity requires information about both target distance and direction. We present research investigating the possibility that information about distance is revealed in optic flow generated by voluntary head motion. This possibility is underscored by several studies that confirm that reaching is more accurate when the head is free to move (Biguer,