Depth discrimination of a crowded line is better when it is more luminant than the lines crowding it

Observers usually cannot discriminate the relative depth of a crowded feature with respect to crowding features about 2 arc min distant if all the features have the same luminance. However, stereo thresholds significantly less than 20 arc sec are obtained when the crowded feature is about twice as luminant as the features crowding it. The thresholds depend only upon the ratio of the luminance of the target feature to the luminance of the crowding features and are independent of the absolute luminance of the features. With further increase in the relative luminance of the target feature, the performance eventually deteriorates and this deterioration is not due to difficulty in seeing the features which were individually clearly visible for all the luminances tested. The closest spacing of local crowded features that still allows good stereo discrimination is about the same as the spatial resolution attainable for many luminance-based non-stereo tasks.

Stereopsis due to luminance differences in the two eyes

A local region in an image is seen as slanted when the two eyes are shown different luminance values in that region. The steepness of the slant depends upon the size of the region and the difference in the luminance values in the two eyes. Three examples where this phenomenon influences depth perception are given: (1) stereopsis without corresponding binocular luminance edges is shown to be a limiting case of the phenomenon; (2) edges less than 1 min arc apart can be seen in relative depth with respect to each other; and (3) regions that appear transparent or translucent can be seen in depth despite having all the luminance edges at zero disparity in simple stereo images.

[Do physicians know if their patients are satisfied? A study in general practice carried out by medical students]

A doctor's ability to assess to what degree his patients are satisfied may indicate how well the patient and the doctor communicate. The main intention of this study was to evaluate the doctor's ability to register a patient's level of satisfaction after a consultation. 19 doctors from six medical centres in Eastern Norway participated. In 50% of the 216 consultations the doctors were able to state exactly how satisfied the patients were. In most of the cases with a mismatch between the doctor's and his patient's ratings the patients were more satisfied than the doctors realized. In some of these cases, however, the doctors failed to observe that the patients were dissatisfied. Male doctors were able to state the patients' level of satisfaction more accurately than female doctors were. Doctors with more experience were also more accurate in their assessments. The method of research used in this area can be problematic. A more reliable and valid questionnaire should be developed.

Some temporal aspects of stereoacuity

Stereoacuity thresholds improve considerably with practice when measured using three vertical lines 15' apart horizontally and presented briefly. For experienced observers, these thresholds are relatively independent of exposure time for stimulus durations smaller than 100 msec. The thresholds are 2-3 times larger when the outer flanking lines are shown continuously than when they and the central target line are turned on and off simultaneously. When the target and flanking lines are shown sequentially, stereoacuity thresholds can be predicted from the number of times the configuration is presented. Changes in thresholds can be measured for intervals as small as 5 msec between successive presentation of the relative disparity configuration. The underlying mechanism is modeled well by a first order auto-regression process.

Initial performance, learning and observer variability for hyperacuity tasks

Psychophysically-experienced and -inexperienced human observers were tested on 34 different non-stereo and 49 stereo hyperacuity stimuli. Performance reached hyperacuity levels within the first five trials for the non-stereo stimuli. For stereoacuity tasks the results were very different. Even extremely experienced observers with very low thresholds for certain stereo tasks required considerable practice to achieve their best performance for slightly different stereo tasks. Performance on both types of tasks showed considerable observer variability. These results suggest that adults do not synthesize new visual modules for hyperacuity tasks early in the visual pathway on a task-driven basis. We also interpret these results to suggest that there can be many equally general models of hyperacuity performance that show only the qualitative general trends of a "standard" observer. Incorporating individual variability might provide sufficient constraints on such models to provide clues about physiological mechanisms.

Perceived motion of a colored spot in a noisy achromatic background

It is shown that human observers can use color both for detecting and for discriminating motion. The contributions of chromaticity and luminance to the detection and discrimination of motion are investigated with a high-contrast, nonisoluminant stimulus. The motion stimulus is a rectangular 'particle' defined by its luminance and chromaticity, which moves against a background containing luminance noise. Although the luminance noise is found to make achromatic particles undetectable over a large range of luminances, the addition of color to a particle can render it detectable and also enable accurate speed discriminations to be made. The contributions of both luminance and chromaticity were measured. The effect of changing the hue angle of the particle as it moves was also examined, and it was found that the detectability of motion is low in that circumstance.

Comparison of human performance with algorithms for estimating fractal dimension of fractional Brownian statistics

Five standard methods for estimating fractal dimension were compared by means of one-dimensional fractional Brownian series generated by four different algorithms yielding series with different statistics. The same one-dimensional series were also displayed as jagged lines and as one-dimensional luminance patterns for judgments by human observers. Only the algorithm implementing the maximum-likelihood method, which required that the generation statistic for the fractal series be known, gave better performance than human observers in estimating fractal dimension. However, when the method of generation is not known, one of the four other standard methods for estimating fractal dimension must be used, and these performed significantly worse than human observers.

Temporal aspects of depth contrast

Depth contrast is a contrasting change in the depth of a feature that results from changes in the disparities of other objects in the field of view, even though the disparity of the original feature remains unchanged. Depth contrast effects decrease during continuous viewing of the stimuli and may disappear altogether after several minutes unless the disparities of the inducing features change with time. This fading occurs whenever the inducing features have constant disparity, whether they are stationary or oscillating laterally. Depth contrast effects occur whenever the inducing features are visible within half a second before or after presentation of the test features. When test features are enclosed by a rectangle which is just inside of a circumscribing outer trapezoid, the inner rectangle "shields" the test features from the depth-inducing effects of the outer trapezoid. Surprisingly, this shielding effect persists if the inner rectangle and outer trapezoid have the same slant direction, but fades with time if the slants are opposite in direction.

Depth discrimination of a line is improved by adding other nearby lines

Depth discrimination thresholds are shown to be lowered by up to a factor of 10 when a few reference lines are added to a stimulus containing a single isolated test line. Four reference lines are better than two, which are better than one, and the improvement in performance is greater when the test line lies between the two reference lines in depth. Stereoacuity for the relative depth of a target line, relative to other nearby reference lines, is shown to be insensitive to changes of disparity of the whole pattern of up to about +/- 5 arc min and is only weakly sensitive to larger displacements of up to +/- 10 arc min.

Shape analysis and stereopsis for human depth perception

The perceived relative depth of two isolated short parallel lines in the center of a scene is known to depend on the disparities and positions of other items in the scene, as well as on their own disparities. We demonstrate here that the shapes of these other items also contribute significantly to the perceived depth, and that these non-disparity influences on depth judgements may already be evident when only three dots are presented as stimuli. When two short vertical test lines are surrounded by a trapezoidal "picture frame", the perceived relative depth of the test lines is affected by the shape of the trapezoid as well as by the disparities assigned to its vertical parallel sides. The influence of the trapezoidal frame can be interpreted as an effect of perspective. The induced relative depth of the test lines is measured by recording the amount of "compensating disparity" that must be given to one of the lines in order for observers to judge the two test lines to be equidistant from the observer's viewing position. Surprisingly, for fixed disparities of the vertical edges of the surrounding picture frame, the induced depth of the test lines increases as the difference in the lengths of the vertical sides increases, regardless of whether the perspective interpretation of the difference in the lengths is consistent with or in conflict with the disparity-defined slant. Shape-related apparent depth changes are especially sensitive to the shape of the trapezoid if it is nearly rectangular, and are comparable in magnitude to those resulting from changes in disparity of the surrounding frame. When a pair of short vertical parallel test lines is presented alone, without a surrounding frame or any other items in the scene, excellent relative depth discrimination is displayed by most subjects. However, if the lines are replaced by squares, trapezoids, triangles, single horizontal lines, or other figures of about the same size as the original test lines, the slant discrimination threshold for these plane figures for naïve observers become poorer by a factor of 20-100. By the use of a feedback signal, observers can be trained to use only disparity cues and ignore shape effects. Some observers have difficulty ignoring the shapes of some figures, the "difficult" figures being different for each observer. After training, the relative depth thresholds for most figures approach those of the original unconnected parallel test lines.

Influence of remote objects on local depth perception

The perceived relative depth of two test dots displayed within the fovea is shown to be influenced by other features in the surrounding area. These features can be as far apart as 51 deg and can have relative disparities as large as 20 deg, much larger than the disparities of the test dots. Since this effect is seen for stimuli presented for 100 msec or less, changes in direction of gaze cannot play a role. The effect varies inversely as the spatial separation between the test dots and the remote features, and is insensitive to the relative disparities of these remote features when they are greater than 2 deg. Observers sometimes differ significantly from each other in their responses to various configurations of the outlying features. This appears to rule out response mechanisms which depend only on the stimulus; some characteristics of the observer must be involved in determining the response. For these briefly presented stimuli, observers are unable to report accurately the relative depth of the central foveated test items if they are also required to report the depths of distant peripheral features.