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At least 127 records · Page 7Linked to original sources
PubMed1989

Differential contributions of circular and elongated spatial filters to the Café Wall illusion.

The Café Wall illusion is induced in part by the spatial filtering properties of circular-symmetric receptive fields at the retina, and in part by cortical processes, presumably that have elongated receptive fields. Two experiments are reported that address these two contributions. The first shows a reversal effect that is consistent with brightness-induction and related theories. The second shows extinction of the effect in a manner predicted by recent observations of gating nonlinearity in cortical cells.

Corpus Striatum↗
PubMed1989

Projective invariance and visual shape constancy.

As one moves about a table, the projection of its shape on the retina varies enormously, yet the table's shape appears constant. The various retinal images of a single object are nearly congruent in projective geometry. To explain apparent constancy, standard theories of vision assume that the visual system has access to this projective congruence. We present four experiments that undermine this assumption (i.e., the projective thesis). The basic result is that observers' estimates of shape in a simple production task represent gross departures from correct projection, even when observers are given aids to fixation. We manipulate both observer sample and experimental procedure in an attempt to find a source of these persistent errors. Our present hypothesis is that observers lack the sensitivity or implicit knowledge of projective geometry that has been attributed to them.

Attention↗
PubMed1989

Compressive axial strain distributions in cancellous bone specimens.

The compressive axial strain distribution in cylindrical trabecular bone specimens was studied using digitized images of the specimen surface. Specimens were tested with strain rate 0.00015 s-1. Images were taken at 0, 1, 2, 3, 4, 6, 8 and 10% strain. Using an optical illusion of movement by rapidly changing succeeding images, failures were classified as transverse (33%) or oblique collapses (67%). The location of failure was not determined by the specimen density gradient. Local axial strain in the distal, intermediate and proximal third was measured throughout the compression in the transversely failing specimens, whereas local strain in the obliquely failing specimens was measured in the pre-failure phase only. Axial strain inhomogeneity was observed in the pre-failure as well as in the post-failure phase. In the pre-failure phase the intermediate third was strained significantly less than the thirds near the ends. In the post-failure phase specimen strain occurred solely in the collapsed part. Ultimate strain of the transversely failing specimens was 2.5% and ultimate strain of the failing third was 3.7%. At failure less than 1% strain was observed in the intermediate third and at 10% specimen strain 1.5% local strain was found in the intermediate third. The results indicate unreliability of conventional stiffness and strain measurements in trabecular bone specimens probably due to lack of trabecular constraint at the end surfaces. Conventional measurements tend to underestimate stiffness and, by giving an average value of strain in spite of considerable strain inhomogeneity, to underestimate failure strain.

Bone Density↗
PubMed1983

A Ponzo-like illusion left and right of fixation: a failed prediction.

Ponzo-like figures, with two parallel horizontal test lines superimposed on a pattern of converging background lines suggesting either a floor or a ceiling were presented in a tachistoscope unpredictably left or right of fixation, and the subjects had to compare the test lines as to length. The classical relative overestimation of the line closest to the apex of the background lines was obtained, but it was not stronger in the left visual field, as seemed to be predicted by at least two current generalizations concerning right hemisphere capacities: (a) that the right hemisphere is better at extracting spatial relations; (b) that its processing mode is more "holistic" and less "analytic" than that of the left hemisphere.

Distance Perception↗