Perceived size and perceived direction: the interplay of the two descriptors of visual space

The stimulus in the outdoor psychophysical experiment was formed by two rods placed on the ground plane over a range of possible distances and orientations. Observers estimated its size and direction by positioning the third rod in the neighbouring space to form an evenly spaced collinear triple of rods. The data revealed interesting similarities between the profiles of the deviations in both judgments: for size judgments, the variability of the responses was least when the targets were at a frontal orientation and gradually increased as the orientation approached the medial plane. For direction judgments, on the other hand, the variability of the responses was least when the stimuli were aligned with the observer's line of sight and gradually increased as the orientation approached the frontoparallel plane. The finding of inverse relationship between the precision of size and direction judgments is interpreted as a consequence of the unequal precision in localisation between the frontal and in-depth dimensions of visual space. The question of the best parameterisation of the observers' responses is discussed.