Perceived relative distance on the ground affected by the selection of depth information

Our visual space does not appear to change when we scan or shift attention between locations. This appearance of stability implies that the depth information selection process is not crucial for constructing visual space. But we present evidence to the contrary. We focused on space perception in the intermediate distance, which depends on the integration of depth information on the ground. We propose a selection hypothesis that states that the integration process is influenced by where the depth information is selected. Specifically, the integration process inaccurately represents the ground when one samples depth information only from the far ground surface, instead of sequentially from the near to the far ground. To test this, observers matched the depth/length of a sagittal bar (test) to the width of a laterally oriented bar (reference) in three conditions in a full-cue environment that compelled the visual system to sample from different parts of the ground. These conditions had the lateral reference bar placed (1) adjacent to the test bar, (2) at the far ground, and (3) at the near ground. We found that the sagittal bar was perceived as shorter in conditions (1) and (2) than in Condition 3. This finding supports the selection hypothesis, since only Condition 3 led to more accurate ground surface integration/representation and less error in relative distance/depth perception. Also, we found that performances in all three conditions were similar in the dark, which has no depth information on the ground, indicating that the results cannot be attributed to asymmetric visual scanning but, rather, to differential information selection.