Background: Disruption of contour relatability in depth blocks unit formation (Kellman, Garrigan & Shipley, 2005), but recent work in 4D interpolation (combining 3D and spatiotemporal unit formation) has produced mixed findings (Erlikhman, Ghose & Kellman, VSS 2011). The latter studies used object fragments specified over time by occlusion of small background elements (c.f., Palmer, Kellman & Shipley, 2006). Question: One hypothesis about equivocal results in 4D is that subjects may not register stereoscopic depth veridically in these displays; depth perception may be affected by an interaction between contour completion and dynamic specification of object parts. Method: Display objects were pairs of black squares or circles that were vertically aligned. Subjects adjusted the disparity of one object until they felt it matched the depth of the other. In one stationary and one lateral movement condition, the black objects were fully visible against a red background. In a third condition, the black objects matched the background and were visible only via progressive occlusion of small, red, rectangular inducing elements. The objects were separated by three possible dividers: none, a green rectangular strip abutting each object, or a similar, black strip. For squares, when the divider was black, completion was possible. Results: We tested 10 subjects who passed a demanding stereoscopic depth test. Depth adjustments were highly precise for fully specified, stationary and moving shaped. Variability in adjustment error increased to 13-28 arcmin of disparity when objects were dynamically specified. There was an interaction with background condition. Either presence of a visible divider (interfering with completion) or contour relatability (squares) appeared to be responsible for inaccurate depth perception. Conclusion: Dynamically specified object fragments have unstable stereoscopic depth that may be easily affected by nearby contours and surfaces. These results place constraints on the depth sensitivity of 4D interpolation.
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