Outcome of breech delivery at term.
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Breech Presentation↗
Biomedical subjects
R Cleary
Publications and source records attributed to R Cleary.
Effects of colour substitutions upon motion detection in spatially random patterns.
To investigate the effects of colour upon motion detection, the short-range motion displacement limit (Dmax) was determined using two-frame kinematograms in which the two classes of square comprising the pattern differed both in luminance and in colour. In the second motion frame, the squares retained either the same luminance and colour as in the first frame, or they changed their colour while retaining their luminance. The experiment was repeated at three different viewing distances to investigate the effects of element angular size. Two of the four observers had normal trichromatic colour vision; the other two were dichromats (protanopes). For the trichromatic observers, the change of colour between frames made motion displacements harder to detect when the squares were large, but not when they were small. The result accords with an input of colour into motion detection at low but not at high spatial frequencies. For the dichromats, the colour change had little effect at any of the viewing distances, thus ruling out the possibility that the deleterious effects of colour substitution upon motion detection in trichromats was due to chromatic aberration or other artefacts.
Color Perception↗
Ambiguous motion in a two-frame sequence.
We measured the ability of ten different observers to identify the direction of motion displacement of a 1.77 c/deg grating in a two-frame sequence. One subject showed virtually errorless performance, in agreement with Derrington and Cox [(1992) Vision Research, 32, 2191-2193], but most subjects found the task difficult and made many errors. One subject saw motion in the reverse of the actual direction of displacement. We conclude that the two-frame sequence contains motion signals in both directions, and that selective attention to the forward direction is necessary for errorless performance.
Attention↗
Effects of contrast substitutions upon motion detection in spatially random patterns.
We report the effects of contrast changes between frames in a random-square kinematogram. When the contrasts of both frames are too low to permit directional discrimination, increasing the contrast of either the first or the second frame alone makes directional discrimination possible. However, at suprathreshold contrasts for motion detection, increasing the contrast of either the first or the second frame alone makes discrimination more difficult. We conclude that motion detection is a special case of contrast discrimination, in agreement with the Reichardt model of motion detection.
Contrast Sensitivity↗
Is there a half-cycle displacement limit for directional motion detection?
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Humans↗
Direction discrimination for band-pass filtered random dot kinematograms.
When an array of random dots is displaced, the ability to report the direction of apparent motion is subject to an upper spatial limit (dmax). As the size of the displacement is increased, direction discrimination errors show a monotonic increase that becomes asymptotic at a chance level. We have measured direction discrimination using spatially band-pass filtered random dots. These stimuli do not yield a monotonic increase in errors. Rather, for displacements greater than around 1 cycle of the stimulus centre frequency (Fc), performance oscillates about chance, with displacements of 1 1/4 cycles of Fc yielding systematic errors in perceived direction. We analyse this pattern of performance in terms of the stimulus autocorrelation function and conclude that dmax can be taken as lying on the initial rising portion of the displacement versus error function. Using this definition we find, in line with the results of Chang and Julesz (1985), that dmax scales inversely with Fc. Contrary to the results of Chang and Julesz, we find that this scaling holds beyond 4 c/deg.
Discrimination, Psychological↗
Masking of low frequency information in short-range apparent motion.
When an array of random dots is displaced, the ability to report the direction of apparent motion is subject to an upper spatial limit (dmax). Using spatially low-pass filtered random dot kinematograms we show that dmax is dependent on the upper cut-off frequency of the stimulus (Fh). The extent of this dependence is critically dependent on the size of the stimulus. Our results suggest a process whereby low spatial frequency motion information is masked by the presence of high spatial frequencies in the same region of the field, analogous to phenomena occurring in the perception of static form (e.g. the Abraham Lincoln effect). The effects of stimulus size on dmax, found for broad-band stimuli by ourselves and others, result from a loss of high frequency sensitivity at increased retinal eccentricities; this loss reduces the masking effect of high frequencies, as stimulus size increases.
Discrimination, Psychological↗
Contrast dependence of short-range apparent motion.
The apparent motion of band-pass filtered random dot kinematograms was assessed by measurements of two alternative direction discrimination performance. In the case of two-dimensional (isotropically filtered) stimuli, the results were largely independent of contrast: at Michelson contrasts of 5 and 50%, near-perfect direction discrimination was obtainable for a limited range of displacements. However for one-dimensional (grating) stimuli, apparent motion seems to be highly dependent on contrast. At 5% contrast, performance was comparable to that obtained with the two-dimensional stimuli. At 50% contrast the motion percept broke down to a large extent, with consistently poor direction discrimination being obtained. The breakdown of apparent motion is interpreted in terms of a decreased signal-to-noise ratio in the pooled response of motion detectors that are tending to contrast saturation. Here, "noise" refers to the sampling components present in any apparent motion sequence. Evidence relating to the sampling frequency of the motion sequence is presented to support the hypothesis. It is argued that the discrepancy between the results obtained with one- and two-dimensional stimuli support the idea that motion is initially encoded by orientationally tuned mechanisms.
Contrast Sensitivity↗