Perceived surface slant is systematically biased in the actively-generated optic flow

Humans make systematic errors in the 3D interpretation of the optic flow in both passive and active vision. These systematic distortions can be predicted by a biologically-inspired model which disregards self-motion information resulting from head movements (Caudek, Fantoni, & Domini 2011). Here, we tested two predictions of this model: (1) A plane that is stationary in an earth-fixed reference frame will be perceived as changing its slant if the movement of the observer's head causes a variation of the optic flow; (2) a surface that rotates in an earth-fixed reference frame will be perceived to be stationary, if the surface rotation is appropriately yoked to the head movement so as to generate a variation of the surface slant but not of the optic flow. Both predictions were corroborated by two experiments in which observers judged the perceived slant of a random-dot planar surface during egomotion. We found qualitatively similar biases for monocular and binocular viewing of the simulated surfaces, although, in principle, the simultaneous presence of disparity and motion cues allows for a veridical recovery of surface slant.

Bayesian modeling of perceived surface slant from actively-generated and passively-observed optic flow

We measured perceived depth from the optic flow (a) when showing a stationary physical or virtual object to observers who moved their head at a normal or slower speed, and (b) when simulating the same optic flow on a computer and presenting it to stationary observers. Our results show that perceived surface slant is systematically distorted, for both the active and the passive viewing of physical or virtual surfaces. These distortions are modulated by head translation speed, with perceived slant increasing directly with the local velocity gradient of the optic flow. This empirical result allows us to determine the relative merits of two alternative approaches aimed at explaining perceived surface slant in active vision: an "inverse optics" model that takes head motion information into account, and a probabilistic model that ignores extra-retinal signals. We compare these two approaches within the framework of the bayesian theory. The "inverse optics" bayesian model produces veridical slant estimates if the optic flow and the head translation velocity are measured with no error; because of the influence of a "prior" for flatness, the slant estimates become systematically biased as the measurement errors increase. The bayesian model, which ignores the observer's motion, always produces distorted estimates of surface slant. Interestingly, the predictions of this second model, not those of the first one, are consistent with our empirical findings. The present results suggest that (a) in active vision perceived surface slant may be the product of probabilistic processes which do not guarantee the correct solution, and (b) extra-retinal signals may be mainly used for a better measurement of retinal information.

Can IVUS-virtual histology improve outcomes of percutaneous carotid treatment?

Several previous studies focusing on comparison between outcomes of carotid artery stenting (CAS) and carotid endoarterectomy (CEA) have put forward conflicting results about the non-inferiority of CAS compared to CEA. Likely outcomes after CAS have been greatly limited by incomplete knowledge of atherosclerotic carotid pathology and probably inappropriate patient selection criteria. In the current practice, only the degree of lumen obstruction is indication to an invasive treatment (CEA or CAS) in symptomatic or asymptomatic patients, but it has been recently demonstrated that histology of carotid plaques also plays a major role. Indeed, plaque morphology and composition seem to influence more importantly outcomes of CAS than those of CEA. Angiography is a poor diagnostic tool to detect the severity and composition of atherosclerotic lesions. Virtual histology (VH) is a new technology incorporated in the latest intravascular ultrasound (IVUS) equipment that allows a validated histological characterization of plaques by performing a spectral, objective and highly-reproducible analysis of the radiofrequency and amplitude data of the ultrasound waves that cross different tissues. This manuscript reports authors' experience with the use of IVUS-VH during CAS. This new technology, by characterizing morphology, extension and histology of carotid plaque, seems to provide important information for confirming percentage of carotid stenosis and judging its embolic potential, tailoring the procedure and guiding the choice of stent and finally for checking stent apposition and complete covering of vulnerable plaques. According to authors' opinion IVUS-VH has the potential to optimize patients' and lesions' selection criteria for CAS in order to improve its outcomes.

Surface interpolation and 3D relatability

Although the role of surface-level processes has been demonstrated, visual interpolation models often emphasize contour relationships. We report two experiments on geometric constraints governing 3D interpolation between surface patches without visible edges. Observers were asked to classify pairs of planar patches specified by random dot disparities and visible through circular apertures (aligned or misaligned) in a frontoparallel occluder. On each trial, surfaces appeared in parallel or converging planes with vertical (in Experiment 1) or horizontal (in Experiment 2) tilt and variable amounts of slant. We expected the classification task to be facilitated when patches were perceived as connected. We found enhanced sensitivity and speed for 3D relatable vs. nonrelatable patches. Here 3D relatability does not involve oriented edges but rather inducing patches' orientations computed from stereoscopic information. Performance was markedly affected by slant anisotropy: both sensitivity and speed were worse for patches with horizontal tilt. We found nearly identical advantages of 3D relatability on performance, suggesting an isotropic unit formation process. Results are interpreted as evidence that inducing slant constrains surface interpolation in the absence of explicit edge information: 3D contour and surface interpolation processes share common geometric constraints as formalized by 3D relatability.

3D surface orientation based on a novel representation of the orientation disparity field

The orientation disparity field from two orthographic views of an inclined planar surface patch (covered by straight lines) is analyzed, and a new tool to extract the patch orientation is provided: the function coupling the average orientation of each pair of corresponding surface contours with their orientation disparity. This function allows identifying the tilt of the surface, and two indeterminacy functions describing the set of surface inclinations (around the vertical and horizontal axes) over convergence angle values compatible with the orientation disparity field. Results of simulations show that the selection of inclination values matching the difference between the areas below the indeterminacy functions are consistent with some surface orientation effects found in psychophysical and computational experiments, like: the unbiased tilt vs. biased slant estimates, the slant underestimation, the surface orientation anisotropy, and the slant/tilt covariation.