Biomedical subjects

Raymond van Ee

Publications and source records attributed to Raymond van Ee.

23 records · Page 2Linked to original sources

PubMed2002

The role of the cyclopean eye in vision: sometimes inappropriate, always irrelevant.

During binocular fixation, the eyes usually point in different directions, and yet, each object is judged to lie in a single direction. It is commonly believed that a particular location in the head serves as the origin for such directional judgments. This location is known as the cyclopean eye. We argue here that observers can judge visually perceived directions from angular information alone, and do not require positional information supplied by a cyclopean eye. We show that experimental findings reported as evidence for the cyclopean concept can also be explained solely by angular information without the need for a cyclopean eye. Recent findings concerning binocular shape perception and the cyclopean illusion demonstrate that binocular perception is incompatible with vision from a single vantage point. The concept of the cyclopean eye is sometimes inappropriate and always irrelevant as far as vision is concerned.

Depth Perception↗

PubMed2002

Multi-coloured stereograms unveil two binocular colour mechanisms in human vision.

Two different colours, one presented to one eye and the other presented to the other eye, often create the impression of a third colour. This percept is known as binocular colour mixture. Here we use coloured stereograms to study binocular colour appearance. Vivid pastel colours are induced in monocular, achromatic patches, if these are placed in stereograms whose left and right images differ in colour. The build-up of the colours is slow and takes tens of seconds or even minutes in certain individuals. The induced colours remain visible during monocular viewing of the patch and decay gradually. The same colours are induced irrespective of whether the patches are placed in fusible or rivalrous stereograms. We show that these colour effects cannot be induced by monocular colour mechanisms, either alone or in combination with binocular colour mixing. We suggest that the colours are induced by a binocular feedback mechanism, which reduces colour differences between the colour appearances of two monocular images. Induced colours are not observed if the achromatic patches are binocular. However, induced colours are apparent if one switches to monocular viewing after prolonged binocular viewing of the binocular patches. This aftereffect suggests that binocular colour induction acts on the monocular representations of binocular images. We suggest that during binocular viewing the fast process of binocular colour mixing masks the changes in colour appearance produced by the much slower process of binocular colour induction.

Adult↗

PubMed2002

A planar and a volumetric test for stereoanomaly.

Stereoanomaly is the failure to see differences in depth when the viewer is presented with stimuli having different magnitudes of stereoscopic disparity. In the absence of eye movements, everyone suffers from stereoanomaly for extremely large disparities. Typically, such disparities are seen at the same depth as monocular stimuli. However, about 30%, of the population exhibit some form of stereoanomaly even for very small disparities, provided eye movements are avoided. In some cases, the sign of the disparity will be confused, and the perceived depth will be incorrectly seen as 'behind' rather than 'in front of' the fixation point, for example. Because anomalies provide useful information about perceptual mechanisms, tests that measure and quantify the extent of a blindness are important investigative tools for research. Here we offer two easy-to-administer tests for stereoanomaly. The first test is based on depth judgments of two bars relative to a fixation point. The second test involves judgments of volumetric stimuli, seen stereoscopically. In each case, subjects indicate depth by setting a rectangle (with fixed base) to match the perceived depth. Although both tests are correlated, some differences in stereo processing are seen, depending upon whether or not the stimuli are presented near the point of fixation.

Depth Perception↗

PubMed2002

Vertical disparity can alter perceived direction.

It has been well established that vertical disparity is involved in perception of the three-dimensional layout of a visual scene. The goal of this paper was to examine whether vertical disparities can alter perceived direction. We dissociated the common relationship between vertical disparity and the stimulus direction by applying a vertical magnification to the image presented to one eye. We used a staircase paradigm to measure whether perceived straight-ahead depended on the amount of vertical magnification in the stimulus. Subjects judged whether a test dot was flashed to either the left or the right side of straight-ahead. We found that perceived straight-ahead did indeed depend on the amount of vertical magnification but only after subjects adapted (for 5 min) to vertical scale (and only in five out of nine subjects). We argue that vertical disparity is a factor in the calibration of the relationship between eye-position signals and perceived direction.

Adaptation, Physiological↗

PubMed2002

Bi-stability in perceived slant when binocular disparity and monocular perspective specify different slants.

We examined how much depth we perceive when viewing a depiction of a slanted plane in which binocular disparity and monocular perspective provide different slant information. We exposed observers to a grid stimulus in which the monocular--and binocular-specified grid orientations were varied independently across stimulus presentations. The grids were slanted about the vertical axis and observers estimated the slant relative to the frontal plane. We were particularly interested in the metrical aspects of perceived slant for a broad spectrum of possible combinations of disparity--and perspective-specified slants. We found that observers perceived only one grid orientation when the two specified orientations were similar. More interestingly, when the monocular--and binocular-specified orientations were rather different, observers experienced perceptual bi-stability (they were able to select either a perspective--or a disparity-dominated percept).

Depth Perception↗