Effects Of Partial Occlusion Research Articles

Effects of partial occlusion on perceived slant difference

When two slanted surfaces are placed in proximity, the perceived slant difference between them is exaggerated. This effect has been called slant contrast. When a partial occluder is presented in front of the gap between them, the perceived slant difference between the surfaces is reduced. We refer to this reduction in perceived slant difference as stereo-slant assimilation. We investigated two properties of the occluder that might affect perceived stereo-slant difference. Three vertically aligned random-dot patterns were presented either with a partial occluder over the gaps between them or without it. Observers judged the perceived slant difference between the center pattern and two surround random-dot patterns that had the same slant. The perceived slant difference was reduced when the partial occluder was present. We varied stereo-depth and slant of the occluder and found that the decreased perceived slant difference was not due to either of these. Note that the surfaces were all simulated and presented on a computer screen and the results may not apply to real surfaces. The effect of the occluder on perceived slant differences could have resulted from either a reduction of slant contrast or an increase of slant assimilation.

Time-to-contact judgments of an approaching object that is partially concealed by an occluder.

Prior studies of time-to-contact (TTC) focused on judgments of unoccluded approaching objects. P. R. DeLucia, M. K. Kaiser, J. M. Bush, L. E. Meyer, and B. T. Sweet (2003) showed that partial occlusion decreases an object's optical size and expansion rate and that the value of tau derived from the reduced optical size (relative rate of accretion; RRA) does not necessarily correspond to TTC. In the present study, a computer-generated object approached the observer while unoccluded or partially concealed by a stationary or moving occluder. In some scenes, the occluder's motion nullified the object's optical expansion. Results indicated that stationary and moving occluders affected TTC judgments. RRA predicted directional changes in TTC judgments but predicted larger changes in such judgments than were observed. Tau did not predict effects of occlusion. When developing models of perceived collision, it is important to consider effects of partial occlusion on optical TTC information and on TTC judgments.

Effects of Partial Occlusion on Perceived Surface Segregation

Tests are reported of the possibility that local information from contour junctions and from corners of intersecting surfaces is used for perceived surface segregation. Stimuli were two intersecting squares with small disks occluding different parts of the squares. The perceived segregation of the squares from one another decreased as the amount of occlusion of parts of the squares increased. This segregation was less when disks occluded parts of contours than when disks occluded parts entirely inside the squares. Occlusion of parts of contours reduced segregation independently of whether contour junctions or corners were visible or invisible, both where the intersecting surfaces were transparent squares and when they were outlined squares. The present findings show that local information from contour junctions or from corners is not used for surface segregation, and confirm that this segregation is determined by global processes of grouping of areas and of extrapolation of contours.

Cardiovascular responses to graded reductions in leg perfusion in exercising humans

Our objective was to determine whether the chemoreflex from human muscle is elicited by small graded reductions in muscle blood flow (MBF) during mild exercise or whether this reflex has an obvious threshold associated with large changes in femoral venous lactate and H+ levels (i.e., as in dogs with high muscle oxidative capacity). Seven subjects exercised supine at 40, 87, and 142 W; lower body positive pressure (LBPP) was applied in 3-min steps at 25, 35, 45, and 50-60 mmHg with the lower body and the cycle ergometer in a sealed box. Estimated MBF (Fick) fell by 5.3 +/- 4.3 to 19.9 +/- 3.8% at four levels of LBPP over three work rates. Mean arterial pressure (MAP), heart rate (HR), and plasma norepinephrine (NE) concentration rose with increasing LBPP. MAP was significantly correlated with femoral venous pH, lactate, O2 tension, and O2 content during moderate and heavy exercise, without an apparent threshold. Percentage decreases in muscle vascular conductance exceeded the decreases in MBF twofold, indicating significant opposition to reduction in MBF by the chemoreflex. Approximately 50% of the correction of MBF back toward control (i.e., at 0 LBPP) could be explained by increased cardiac output, calculated from the rise in HR; the remaining correction could be attributed to both sympathetic vasoconstriction (indicated by high NE levels) and to mechanical effects of partial occlusion. Results suggest that in humans stepwise reductions in MBF gradually elicit muscle chemoreflexes with no apparent threshold at these levels of exercise.