Biomedical subjects

R van Ee

Publications and source records attributed to R van Ee.

7 recordsLinked to original sources

PubMed1997

Capture of visual direction: an unexpected phenomenon in binocular vision.

Binocular perception of visual direction is based on laws which were formulated more than 100 years ago. These laws govern the directions in which human beings perceive objects visible to both eyes (binocular objects) and objects visible to only one eye (monocular objects). We report here that the laws do not hold for monocular objects adjacent to binocular objects. The perceived directions of these monocular objects are captured by those of nearby binocular objects. Capture of binocular visual direction is an unexpected phenomenon because it refutes the generally accepted notion that a particular retinal location gives rise to a particular subjective visual direction. The practical consequence is that the subjective techniques for measuring eye position which are widely used in fundamental research and clinical practice are unreliable if they are used in densely structured stimuli. We suggest that capture results from a mechanism of lateral interaction between adjacent visual directions. This mechanism ensures that, despite eye movements, objects have the same spatial order in monocular and binocular vision. This conservation of spatial order also explains why retinal blind spots are not manifest in binocular vision.

Convergence, Ocular↗

PubMed1997

Neurological impairment during long-term intrathecal infusion of bupivacaine in cancer patients: a sign of spinal cord compression.

Adequate pain relief in patients with far advanced cancer sometimes requires intrathecal (IT) administration of a combination of opioids and local anesthetics. Tumor progression as well as the IT administration of local anesthetics can lead to neurologic dysfunction during treatment. Five patients showed symptoms of compression of the cauda equina or spinal cord shortly after the start of combined IT administration of morphine and bupivacaine in a dosage usually not associated with neurologic symptoms. Unexpectedly, neurologic evaluation suggested compression of the cauda equina and spinal cord, which was confirmed radiographically. Manifestation of new neurologic symptoms during low dose bupivacaine infusion intrathecally might therefore be an early indicator of space-occupying processes within the spinal canal in cancer patients.

Aged↗

PubMed1996

Binocular alignment in different depth planes.

A generally accepted notion in binocular vision is that we see the world as if viewed by a single eye, the cyclopean eye. A consequence of seeing the world from a single point in space is that the outlines of occluding and occluded surfaces have the same shape. We designed stereograms in which subjects aligned binocularly visible lines to each other. The lines were lying in different depth planes. In the vicinity of occluded areas, binocular alignment was achieved by alignment of the lines in the eye that viewed the monocularly visible details. Stereograms in which shapes of surfaces lying in different depth planes were compared to each other show that occluding and occluded surfaces do not have the same shape: a square surface occludes rectangular surfaces in other depth planes of which the horizontal widths are smaller than the vertical widths. This difference is perceived shape is not possible if the centre of binocular direction has a fixed position in the head.

Depth Perception↗

PubMed1996

Anisotropy in Werner's binocular depth-contrast effect.

We investigated Werner's binocular depth-contrast effect. Subjects viewed stereograms consisting of a test pattern and an inducing pattern. The half-images of the inducing pattern were either horizontally scaled or sheared relative to each other. Subjects judged the (induced) perceived slant of the test pattern. We were interested in what influence the spatial configuration of the test pattern and the inducing pattern had on the depth-contrast effect. We conclude that the depth-contrast effect is a global effect. In other words, it is not restricted to the location of the inducing pattern. The effect decreases with distance, however, in an anisotropic way. The depth-contrast effect was present most prominently when the test pattern was positioned in the direction along the slant (rotation) axis of the inducing pattern. We suggest that Werner's depth-contrast effect can be explained by the (previously reported) findings that: (1) stereopsis is relatively insensitive to whole-field horizontal scale and shear; and (2) stereopsis is very sensitive to horizontal scale and shear of two stimuli relative to each other.

Adult↗

PubMed1996

Stability of binocular depth perception with moving head and eyes.

We systematically analyse the binocular disparity field under various eye, head and stimulus positions and orientations. From the literature we know that certain classes of disparity which involve the entire disparity field (such as those caused by horizontal lateral shift, differential rotation, horizontal scale and horizontal shear between the entire half-images of a stereogram) lead to relatively poor depth perception in the case of limited observation periods. These classes of disparity are found to be similar to the classes of disparities which are brought about by eye and head movements. Our analysis supports the suggestion that binocular depth perception is based primarily (for the first few hundred milliseconds) on classes of disparity that do not change as a result of ego-movement.

Depth Perception↗

PubMed1995

Binocular perception of slant about oblique axes relative to a visual frame of reference.

From the literature it is known that the processing of disparity for slant is different in the presence and in the absence of a visual frame of reference. The experimental finding that vertical disparity is not processed for slant perception in the presence of a visual reference is elaborated. This theoretical analysis results in a reduction of the three basic first-order transformations between the retinal half images (divergence, rotation, and deformation) to only two basic orthogonal transformations. The first of these, horizontal scale, results in slant perception about the vertical axis, whereas the second, horizontal shear, results in slant perception about the horizontal axis. These transformations are based primarily on horizontal disparity. It is shown experimentally that in the presence of a frame of reference the amount of vertical transformation that is added to the two basic transformations (horizontal scale and shear) of a random-dot stimulus is indeed irrelevant for slant perception. It is suggested that, in the presence of a visual reference, slant perception about oblique axes is based solely on linear combinations of the horizontal-scale and horizontal-shear transformations. Subjects are able to reproduce slants about oblique axes experimentally merely by combining horizontal scale and shear.

Adult↗

PubMed1992

Numerical analysis of the magnetic field for arbitrary magnetic susceptibility distributions in 2D.

We describe a numerical technique for calculating the 2D magnetic field in arbitrary magnetic susceptibility distribution. The technique we used is the explicit finite difference method with an addition of the Du Fort-Frankle algorithm. The proposed algorithm is unconditionally stable and has excellent convergence properties. For simple geometries, numerical results were compared against analytical solutions and appeared to be in excellent agreement.

Algorithms↗