Sequential-surface-integration-process (SSIP) Hypothesis Of The Ground Surface Representation In Distance Perception

Every aspect of our lives is dependent upon our ability to perceive and interpret the space around us in a way that allows us to effectively direct our actions and behaviors. While there are many processes at work as we perform even very easy tasks, one skill that becomes particularly critical is our ability to estimate distance. Distance perception is the basic ability of our human being. To interact with the space, human beings must be able to effectively perceive the space around them.A number of researchers have found that, within the intermediate distance range (2-25m), human being can accurately judge the distance between a target and themselves, that is egocentric distance. An enduring problem is how our visual system determines the distance between ourselves and a target object? Recently some studies demonstrated that the ground surface that extends from one's feet to the horizon is very import for distance perception. On the basis of a lot of researches, He et al (2004) proposed the sequential-surface-integration-process (SSIP) hypothesis to elucidate how the visual system constructs a representation of the ground-surface in the intermediate distance range.In the present study, we created a visual-reality setting much like He et al(2008)'s studies to further test the sequential-surface-integration-process (SSIP) hypothesis, based on the improved He et al(2007)'s paradigm. In two experiments all subjects had to perform two tasks, that is egocentric distance-judgment task and the surface slant-judgment task. The egocentric distance-judgment task firstly requires subjects to view a target distance briefly in the near distance (a stimulus phase),the observer then response the distance by adjusting the distance of the matching target until it appeared equidistant to him/her as the test-target distance or by walking without vision a distance appeared equidistant to the test-target distance (a response phase). In the surface slant-judgment task, the observer was instructed to firstly judge if the surface near the occlusion (near surface) and the surface beyond the occlusion (far surface) were coplanar, if he/she felt the two surfaces did not appear coplanar, then adjust the slant of far surface until appeared coplanar with the near surface.In Exp 1, there are two tasks," egocentric distance-judgment task" and "surface slant-judgment task". A 2(surface:occlusion vs no occlusion) x 3(target distance:9,11 and 13m) x 2(occlusion distance:3 and 5m) within subject design was conducted in "egocentric distance-judgment task" and "visual matching task" was used to measure distance perception. As to "surface slant-judgment task", "occlusion distance" was manipulated, there was three level:3,4 and 5m. As to "egocentric distance-judgment task", the results indicated that the target distance was underestimated in the occlusion condition compared to the no occlusion condition(control) no matter whether the occlusion distance is 3 or 5m. As far as "surface slant-judgment task" the participants required surface beyond the occlusion to be slanted downward to perceive it as coplanar with the flat, near the occlusion and the effect of occlusion distance on the surface-slant judgment was not significant. Control experiment indicated that no systematic distance over-estimation was found in the far-wall condition compared to the no-occlusion condition. The result did not support the notion that the distance underestimation was due to the "attraction" of the target toward the brick wall in the occlusion condition. The calculated "predicted slant errors" by using the equation was not significant different from the "judged slant errors". The experiment was consistent with the SSIP hypothesis.In Exp 2 we further investigated the SSIP (Sequential-surface-integration-process) hypothesis, this Exp was similar with Exp 1 excepting that the blind walking task was used to measure the distance perception and target distance were:8,10 and 12m. The result of Exp 2 was consistent with Exp 1, the average judged distance were underestimated in the occlusion condition relative to the no-occlusion condition. The novel aspect of the result is that both for two surface conditions the judged distance were underestimated compared to the physical distance. As for the surface slant-judgment task the participants required surface beyond the occlusion to be slanted downward to perceive it as coplanar with the flat, near the occlusion and the effect of occlusion distance on the surface-slant judgment was not significant. The control experiment result indicated that although the target distance was underestimated both for the far-wall condition and no-occlusion condition, there were no significant difference between these two condition. The calculated "predicted slant errors" by using the equation was not significant different from the "judged slant errors". The experiment supported the SSIP hypothesis.The results of these two Exps both supported the sequential-surface-integration- process (SSIP) hypothesis that was proposed to elucidate how the visual system constructs the ground-surface representation in the intermediate distance range.