Perceptual centering effects in body orientation.

This study mathematically characterizes the results of DiZio and Lackner (Percept Psychphys 39(1): 39-46) on the perception of self-orientation during circular vection induced by an optokinetic stimulus. Using the hypothesis of perceptual centering, it is shown that five basic centering transformations can logically account for the full range of illusions reported by the subjects. All five of these transformations center the perceived orientations of body components, the rotating disk, and gravity : two align the perceived visual and inertial rotation axes, one centers the perceived axis of visual rotation in front of the head, and two straighten the perceived neck angle. These transformations generate a mathematical semigroup. Application of the semigroup to an actual stimulus condition generates an orbit of predicted illusions. The semigroup analysis of perceptual centering predicts all of the illusions observed in the experiments of DiZio and Lackner (Percept Psychphys 39(1): 39-46). Moreover, the structure of perceptual centering (1) provides a logical explanation for the occurrence of those misperceptions; and (2) predicts the complete set of perceptions that are expected to occur in a larger sample. In addition, our analysis predicts illusions in experimental conditions not yet investigated.

Calcium and oxalic acid kinetics differ in rats.

Small molecular weight calcium salts, if absorbed intact, could provide a nutritional source of calcium in subjects with impaired absorption of calcium by the saturable pathway. An understanding of the mechanism of absorption of calcium oxalate (as a representative salt) may be important nutritionally and therapeutically. The aim of the present study was to develop models to study absorption, distribution and retention of calcium and oxalate in rats as a basis for studying calcium oxalate absorption. Labeled compounds (45Ca and [14C]-oxalic acid) were administered to separate groups of rats orally (n = 8-11) or intravenously (n = 3-5) and blood was sampled for up to 240 min. Data were analyzed using SAAM/CONSAM. Calcium kinetics were fitted by a model with three compartments in the body and one absorption pathway from the intestine. By contrast, oxalic acid kinetics were fitted by two pools in the body and two absorption pathways from the intestine. Calcium and oxalic acid, therefore, demonstrate different absorption and distribution kinetics in rats.

Absorption of calcium oxalate does not require dissociation in rats.

Calcium absorption is thought to occur only if calcium is in a soluble or dissociated form, although experimental evidence is lacking. The intestinal absorption of calcium oxalate, a small, neutral and virtually insoluble calcium salt, was elucidated in the whole body of awake rats. Suspensions of 45Ca ascorbate, 14C-oxalic acid and doubly labeled 45Ca-[14C]-oxalate were given by gavage to separate groups of rats. Following dosing, blood samples were drawn for up to 240 min through a previously inserted intravenous catheter. Serum was assayed for radioactive tracers, and data were then plotted as fraction of dose over time. Calcium absorption was 15% [with a loading of 0.3 mmol (15 mg) calcium], oxalic acid absorption was 22% and Ca-oxalate absorption was <2%. Appearance of 45Ca from calcium ascorbate and 14C from oxalic acid differed, whereas 45Ca and 14C from doubly labeled Ca-oxalate had identical serum appearance profiles. Therefore, we conclude that calcium oxalate was absorbed intact. Addition of excess, unlabeled calcium to the doubly-labeled calcium oxalate did not alter the relationship of the serum level of the two tracers, confirming absorption of calcium oxalate as the intact salt. Thus, calcium bound as a small, neutral, calcium salt such as calcium oxalate does not have to be dissociated prior to absorption. Possibly other small compounds would be similarly absorbed. These results alter our current understanding of calcium bioavailability from foods and therapeutic agents.