Depth perception in disparity-defined objects: finding the balance between averaging and segregation.

Deciding what constitutes an object, and what background, is an essential task for the visual system. This presents a conundrum: averaging over the visual scene is required to obtain a precise signal for object segregation, but segregation is required to define the region over which averaging should take place. Depth, obtained via binocular disparity (the differences between two eyes' views), could help with segregation by enabling identification of object and background via differences in depth. Here, we explore depth perception in disparity-defined objects. We show that a simple object segregation rule, followed by averaging over that segregated area, can account for depth estimation errors. To do this, we compared objects with smoothly varying depth edges to those with sharp depth edges, and found that perceived peak depth was reduced for the former. A computational model used a rule based on object shape to segregate and average over a central portion of the object, and was able to emulate the reduction in perceived depth. We also demonstrated that the segregated area is not predefined but is dependent on the object shape. We discuss how this segregation strategy could be employed by animals seeking to deter binocular predators.This article is part of the themed issue 'Vision in our three-dimensional world'.

Selenium deficiency alters thyroid hormone metabolism in guinea pigs.

In guinea pigs, activity of glutathione peroxidase in most organs is markedly lower than in organs of other rodents despite comparable dietary intakes and tissue levels of selenium. To determine if metabolism of selenium with respect to other selenoproteins also differs in guinea pigs, we measured the effects of selenium intake on thyroid hormone metabolism. Weanling male Hartley Albino guinea pigs were fed a selenium-deficient Torula yeast-based diet, or the same diet supplemented with 0.5 mg selenium/kg diet as sodium selenate for 72 d. Growth was impaired in guinea pigs fed the unsupplemented diet. Activity of glutathione peroxidase was higher in tissues and plasma of supplemented guinea pigs than in selenium-deficient animals. However, it was still far lower than reported values for other rodent species. In selenium deficiency, activity of type 1 5'-iodothyronine deiodinase was 60% less in liver and 45% less in kidney. Concentration of thyroxine was 68% lower in kidney of selenium-deficient animals, and levels of 3,3',5-triiodothyronine in kidney and plasma were 44 and 31% lower, respectively. Thus, with the exception of thyroxine concentrations, thyroid hormone metabolism responds to selenium deficiency in guinea pigs as it does in rats, although the magnitude of that response is not as great.

Bar calcification of the mitral anulus. A risk factor in mitral valve operations.

Between Jan. 1, 1979, and Jan. 1, 1986, 72 septuagenarians had open heart operations for disease of the mitral valve. Thirty-two (44%) had additional operative procedures. Overall seven patients (9.7%) died within 30 days of operation. Eleven patients had bar calcification of the posterior mitral annulus as defined by three criteria and 61 did not. No differences between these two groups were present except for hospital mortality. Three of the 11 patients (27.3%) died at or soon after operation of complications resulting from the calcified annular bar. Only four of 61 patients (6.6%) without bar calcification died early. The difference in early mortality between the two groups is significant (p less than 0.05) and identifies the presence of bar calcification of the posterior mitral annulus as an independent risk factor of mitral valve operations in elderly patients.