Date Published: August 2, 2012
Publisher: Public Library of Science
Author(s): Karoline Spang, Barbara Gillam, Manfred Fahle, Markus Lappe. http://doi.org/10.1371/journal.pone.0040562
A small region of background presented to only one eye in an otherwise binocular display may, under certain conditions, be resolved in the visual system by interpreting the region as a small gap between two similar objects placed at different depths, with the gap hidden in one eye by parallax. This has been called monocular gap stereopsis. We investigated the electrophysiological correlate of this type of stereopsis by means of sum potential recordings in 12 observers, comparing VEP’s for this stimulus (“Gillam Stereo”, Author BG has strong reservations about this term) with those for similar stimuli containing disparity based depth and with no depth (flat). In addition we included several control stimuli. The results show a pronounced early negative potential at a latency of around 170 ms (N170) for all stimuli containing non- identical elements, be they a difference caused by binocular disparity or by completely unmatched monocular contours. A second negative potential with latency around 270 ms (N270), on the other hand, is present only with stimuli leading to fusion and the perception of depth. This second component is similar for disparity-based stereopsis and monocular gap, or “Gillam Stereo” although slightly more pronounced for the former. We conjecture that the first component is related to the detection of differences between the images of the two eyes that may then either be fused, leading to stereopsis and the corresponding second potential, or else to inhibition and rivalry without a later trace in the VEP. The finding that that “Gillam Stereo” leads to cortical responses at the same short latencies as disparity based stereopsis indicates that it may partly rely on quite early cortical mechanisms.
In recent years it has become clear that binocular depth perception is not restricted to locations in the perceived environment with features that can be explicitly matched in the two eyes. Depth can also be recovered for features that are imaged only monocularly if these are placed in an informative binocular context. We do not refer here to what are classically called monocular depth cues. The depth perception we refer to is entirely binocular in origin but incorporates monocular (unpaired) features. [For reviews see Harris & Wilcox , and Gillam , There are three major categories of such effects investigated so far. The first is the depth seen for an unpaired feature placed in the “occlusion zone” of a binocular surface (Fig. 1A, , . This is also known as “da Vinci stereopsis” since Leonardo da Vinci first pointed out that occluding surfaces may hide more distant surfaces differentially for the two eyes.
The statistical analysis produced significant main effects for all experiments but generally in line with the expected differences (see Tables 1–3 for details). The results for the contrast between stereo versus non- stereo conditions on the one hand and the confounds on the other hand will be discussed separately below.
We will first summarize the main results and then discuss each of them in more detail. The results for stimuli with nine rather than one stimulus element are quite similar to those for one stimulus (Figs. 3, 4) and hence, these conditions will generally be discussed together. The most important question was whether or not stimuli containing Gillam depth features evoke cortical potentials similar to stimuli producing a stereoscopic impression based on disparity differences. An earlier study compared the cortical potentials evoked by a depth-defined checkerboard with those evoked by a homogeneous, i.e. planar version of the same stimulus) . The difference between the cortical responses to these two stimuli hovered around zero for the first 100 ms after stimulus presentation and started to deviate towards more negative potentials for the checkerboard stimulus at around 170 ms. The amplitude of this negative potential increased over time, peaking at around 270 ms after stimulus onset. We therefore expected to find a similar potential also in the present study for those stimuli containing gradients in depth, and with a (negative) peak around 270 ms after stimulus presentation.