Bioactivation of biomorphous silicon carbide bone implants.

Wood-derived silicon carbide (SiC) offers a specific biomorphous microstructure similar to the cellular pore microstructure of bone. Compared with bioactive ceramics such as calcium phosphate, however, silicon carbide is considered not to induce spontaneous interface bonding to living bone. Bioactivation by chemical treatment of biomorphous silicon carbide was investigated in order to accelerate osseointegration and improve bone bonding ability. Biomorphous SiC was processed from sipo (Entrandrophragma utile) wood by heating in an inert atmosphere and infiltrating the resulting carbon replica with liquid silicon melt at 1450°C. After removing excess silicon by leaching in HF/HNO3 the biomorphous preform consisted of ß-SiC with a small amount (approximately 6wt.%) of unreacted carbon. The preform was again leached in HCl/HNO3 and finally exposed to CaCl2 solution. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared analyses proved that oxidation of the residual carbon at the surface induced formation of carboxyl [COO⁻] groups, which triggered adsorption of Ca(2+), as confirmed by XPS and inductively coupled plasma optical emission spectroscopy measurements. A local increase in Ca(2+) concentration stimulated in vitro precipitation of Ca5(PO4)3OH (HAP) on the silicon carbide preform surface during exposure to simulated body fluid, which indicates a significantly increased bone bonding activity compared with SiC.

The horizontal disparity direction vs. the stimulus disparity direction in the perception of the depth of two-dimensional patterns.

Horizontal binocular disparity has long been the conventional predictor of stereo depth. Surprisingly, an alternative predictor fairs just as well. This alternative predicts the relative depth of two stimuli from the relation between their disparity vectors, without regard to horizontal disparities. These predictions can differ; horizontal disparities accurately predict the perceived depth of a grating and a plaid only when the grating is vertical, while the vector calculation accurately predicts it at all except near-horizontal grating orientations. For spatially two-dimensional stimulus pairs, such as plaids, dots, and textures, the predictions cannot be distinguished when the stimuli have the same disparity direction or when the disparity direction of one of the stimuli is horizontal or has a magnitude of zero. These are the conditions that have prevailed in earlier studies. We tested whether the perceived depth of two-dimensional stimuli depends on relative horizontal disparity magnitudes or on relative disparity magnitudes along a disparity axis. On both measures tested-depth matches and depth-interval matches-the perceived depth of plaids varied with their horizontal disparities and not with disparity direction differences as observed for grating-plaid pairs. Differences in disparity directions as great as 120 degrees did not affect depth judgments. This result, though opposite the grating-plaid data, is consistent with them and provides a view into the construction of orientation-invariant disparity representations.

Projected disparity, not horizontal disparity, predicts stereo depth of 1-D patterns.

Binocular disparities have a straightforward geometric relation to object depth, but the computation that humans use to turn disparity signals into depth percepts is neither straightforward nor well understood. One seemingly solid result, which came out of Wheatstone's work in the 1830s, is that the sign and magnitude of horizontal disparity predict the perceived depth of an object: 'positive' horizontal disparities yield the perception of 'far' depth, 'negative' horizontal disparities yield the perception of 'near' depth, and variations in the magnitude of horizontal disparity monotonically increase or decrease the perceived extent of depth. Here we show that this classic link between horizontal disparity and the perception of 'near' versus 'far' breaks down when the stimuli are one-dimensional. For these stimuli, horizontal is not a privileged disparity direction. Instead of relying on horizontal disparities to determine their depth relative to that of two-dimensional stimuli, the visual system uses a disparity calculation that is non-veridical yet well suited to deal with the joint coding of disparity and orientation.

A new theory of structure-from-motion perception.

Humans can recover 3-D structure from the projected 2D motion field of a rotating object, a phenomenon called structure from motion (SFM). Current models of SFM perception are limited to the case in which objects rotate about a frontoparallel axis. However, as our recent psychophysical studies showed, frontoparallel axes of rotation are not representative of the general case. Here we present the first model to address the problem of SFM perception for the general case of rotations around an arbitrary axis. The SFM computation is cast as a two-stage process. The first stage computes the structure perpendicular to the axis of rotation. The second stage corrects for the slant of the axis of rotation. For cylinders, the computed object shape is invariant with respect to the observer's viewpoint (that is, perceived shape doesn't change with a change in the direction of the axis of rotation). The model uses template matching to estimate global parameters such as the angular speed of rotation, which are then used to compute the local depth structure. The model provides quantitative predictions that agree well with current psychophysical data for both frontoparallel and non-frontoparallel rotations.

Shape constancy and depth-order violations in structure from motion: a look at non-frontoparallel axes of rotation.

Humans can recover the structure of a 3D object from motion cues alone. Recovery of structure from motion (SFM) from the projected 2D motion field of a rotating object has been studied almost exclusively in one particular condition, that in which the axis of rotation lies in the frontoparallel plane. Here, we assess the ability of humans to recover SFM in the general case, where the axis of rotation may be slanted out of the frontoparallel plane. Using elliptical cylinders whose cross section was constant along the axis of rotation, we find that, across a range of parameters, subjects accurately matched the simulated shape of the cylinder regardless of how much the axis of rotation is inclined away from the frontoparallel plane. Yet, we also find that subjects do not perceive the inclination of the axis of rotation veridically. This combination of results violates a relationship between perceived angle of inclination and perceived shape that must hold if SFM is to be recovered from the instantaneous velocity field. The contradiction can be resolved if the angular speed of rotation is not consistently estimated from the instantaneous velocity field. This, in turn, predicts that variation in object size along the axis of rotation can cause depth-order violations along the line of sight. This prediction was verified using rotating circular cones as stimuli. Thus, as the axis of rotation changes its inclination, shape constancy is maintained through a trade-off. Humans perceive the structure of the object relative to a changing axis of rotation as unchanging by introducing an inconsistency between the perceived speed of rotation and the first-order optic flow. The observed depth-order violations are the cost of the trade-off.