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Biomedical subjects

R P Power

Publications and source records attributed to R P Power.

At least 19 recordsLinked to original sources
PubMed2005

Long-term outcome of cadaveric pediatric en bloc transplantation--a 15-year experience.

Pediatric en bloc transplantation of infant organs into adult recipients is a recognized technique to expand the number of kidneys available for transplantation. We reviewed our experience with this technique over a 15-year period to determine the long-term outcomes. Twelve patients underwent pediatric en bloc transplantation from donors aged <4 years. All transplants functioned immediately with no graft thrombosis. Two patients died 12 and 10 years posttransplant with functioning grafts. The remaining 10 recipients experienced excellent graft function with a mean follow-up time of 73.8 months (range, 10 to 169 months) with no evidence of hyperfiltration injury. We conclude that pediatric en bloc transplantations achieve excellent long-term allograft function in selected recipients.

Adult↗
PubMed1994

Motion aftereffects of wagon wheels: motion aftereffects follow apparent rather than real movement.

Power and Moulden have proposed a model which accounts for the movement of gratings in apertures including the barber pole illusion. It predicts the direction of motion aftereffects which follow from perceived veridical motion and the direction of these aftereffects which follow from the illusory movement experienced during the barber pole illusion. At a perceptual level, the model predicts motion aftereffects will follow direction of apparent movement rather than veridical direction. Four experiments tested this prediction. In Exp. 1 a spiral was viewed under flickering light so it appeared to be moving in the direction opposite to true motion, and the aftereffect was opposite to the apparent direction. In Exp. 2 the spiral was viewed through a narrow aperture so that it was effectively a grating appearing to move in the opposite direction to veridical motion. Again, the motion aftereffect was opposite to the apparent rather than true direction of rotation. In Exp. 3 a sectored disc was used, and similar results were obtained. In Exp. 4 the sectored disc was videotaped so that it appeared to be rotating in the direction opposite to true motion. The after motion to this "wagon wheel" effect was opposite to its apparent direction of rotation on the screen. In all experiments the predictions were confirmed, thereby confirming the general principle that motion aftereffects follow apparent rather than real direction of movement.

Adolescent↗
PubMed1993

Tests of the dipole model of perceived movement in apertures.

Power and Moulden (1992) have proposed a dipole model to account for the apparent movement of gratings in apertures. This includes movement orthogonal to the orientation of the grating, and the barber pole illusion: the illusion that a grating drifting diagonally across a narrow aperture appears to be moving along it. The essence of the model is that movement is signalled by a large number of dipoles, of many orientations and lengths. These dipoles respond if, and only if, one end is stimulated, and then the other. Three experiments intended to test predictions from the model are reported here. In each case a horizontal grating drifted across an aperture and subjects fixated outside the aperture. In experiment 1 subjects fixated just above or below the aperture, and reported the motion aftereffect (MAE) shown by a set of test spots. As predicted by the model, the spots further from the fixation point showed a strong MAE. Experiment 2 combined both viewing conditions of experiment 1, so that test spots above and below the fixation point were viewed simultaneously. The predictions were confirmed, since test spots further from the fixation point exhibited a stronger MAE than test spots closer to the fixation point. In experiment 3 the fixation point in all conditions was below the aperture, and, as predicted, the MAE of a spot near the bottom of the aperture was diagonally upward, although stimulation was horizontal. Again, as predicted, the MAE of a spot in the middle of the aperture appeared to move horizontally. Finally, it was predicted that a test spot at the top of the aperture would appear to move diagonally downwards, but subjects were unable to report unequivocally the direction of motion, since the MAE was occurring too far from the fovea for clear vision. Overall, then, the predictions from the model were confirmed, although there are associated phenomena the model cannot as yet account for.

Adolescent↗
PubMed1992

Spatial gating effects on judged motion of gratings in apertures.

Wallach has described in qualitative terms the movement of lines behind apertures. We related the data he obtained to the aperture problem, constructed a model of movement perception, and carried out tests of the model. Experiment 1 was a parametric study, and showed the conditions under which a reliable illusion (the barber pole illusion) of diagonal movement of lines along an aperture could be obtained, and when fluctuating judgements or veridical percepts were obtained. On the basis of this study a dipole model was constructed. The model was further developed and tested. In experiment 2 the effects of total area of stimulation were examined: diagonal gratings were viewed behind multiple apertures. In experiment 3 the effects of local signs were examined: diagonal gratings were viewed in an aperture which had edges cut in small steps and stairs, with the risers parallel to the grating, and the treads parallel to the direction of motion of the grating. Experiment 4 was designed to test a prediction about the motion aftereffect of dots near and far from the point of fixation, and the results confirmed the model. It was concluded that the model accounts for the barber pole illusion and, generally, for the movement of gratings in apertures.

Adult↗
PubMed1985

Apparent reversals in trapezia: confirmation of the 'tan apex ratio' heuristic.

A new model for predicting the frequencies of apparent reversals in rotating trapezia, proposed by the authors in 1983, is validated and extended in two studies. Experiment 1 showed that the model retained its predictive efficiency over an increased range of shapes and viewing distances, and with trapezia made of sheet metal and not, as previously, of wire. Experiment 2 was designed to investigate an unexpected effect, in which the specificity of prediction was found to decrease as the corners of the stimulus objects were slightly rounded.

Humans↗
PubMed1983

Apparent reversal frequencies in squares and trapezia: the effect of invariant and variant information.

Three studies of apparent reversals (ARs) in rotating rectangular and trapezoid arrays are reported, together with a mathematical analysis. Study 1 explores the effect of different degrees of outline suppression on the frequency and distribution of ARs in squares and trapezia. Study 2 examines how modifications in both types of array reduce AR frequency, demonstrates a manipulation whereby ARs in rotating squares are eliminated, and suggests that ARs in squares and trapezia are of different origins. In the mathematical analysis, an invariant characteristic of the transformations of rotating rectangles is reported, and from this a numerical index is developed that expresses the extent to which a rotating trapezium may appear to pause just before each point of oscillation. In study 3 it is shown that this measure is a better predictor of AR frequencies than the heuristic proposed by Braunstein.

Adult↗
PubMed1983

Apparent movement induced by afterimages.

A small light source viewed in the dark through an afterimage produced by an electronic flash may appear to move along short random paths. Experiment 1 shows that this is not an instance of the autokinetic phenomenon, and experiment 2 shows that it is not related to the 'fluttering hearts' phenomenon. On the basis of experiments 1 and 3 it is concluded that small-scale ocular drifts, microsaccades, and possibly tremors cause the whole afterimage to move. The afterimage serves as a frame of reference and induces movement of the light source.

Adolescent↗
PubMed1982

Echo perception of shape and texture by sighted subjects.

Experienced blind subjects have previously demonstrated good echo perception of size and distance and some echo-discrimination of shapes and textures. In three experiments untrained sighted subjects also proved able to echo-detect and recognize three simple shapes and to recognize fabric and wooden but not carpet and Plexiglas discs at significantly above chance levels. Some improvement occurred over the first few trials but little thereafter, suggesting that this sort of echo perception requires very little training. A blind subject exhibited over-all accuracy comparable to those of sighted subjects. There were, however, interesting differences between the blind subject and the sighted subjects in echo perception of specific stimuli and in approach to the task.

Adolescent↗
PubMed1981

The dominance of touch by vision: occurs with familiar objects.

In a study of visual dominance based on Rock and Victor's experiments, observers examined well-known objects while receiving conflicting tactual and visual information. In experiment 1 observers simultaneously felt a circular 20 cent coin and looked at it through a lens which transformed the visual image to an oval with axes in the ratio 1 : 1.8. In experiment 2 another sample of observers felt well known cubic objects-dice-and looked at them through lens systems made the visual images: (i) rectangular with sides in the ratio 1 : 1.3; (2) diamond-shaped with 72 degrees smaller angles; (iii) rectangular with side in the ratio 1 : 1.8; and (iv) a parallelogram with side lengths in the ratio 1 : 1.8 and with 60 degrees smaller angles. Although there was some conflict between the observers' cognitions and their percepts, effectively all reported that the target objects were like or felt like their visual images, demonstrating that the dominance of vision over touch is far stronger than has been recognised.

Adolescent↗
PubMed1980

The dominance of touch by vision: sometimes incomplete.

It is generally agreed that, when the two senses are in conflict, vision dominates touch. In the series of experiments reported here the discrepancy between the visual and the haptic images was progressively increased. In each experiment half of the observes were adults and half children, and half of each group wore a glove which concealed the distortion produced by the lens. In experiment 1 the visual image of a haptic square was transformed by a week or strong cylindrical lens so that the sides were in the ratio 1:1.5 or 1:1.8. Gloved observers selected a comparison image close to the visual image. Bare-handed observers gave compromise judgements with the strong lens, but judgements similar to those of the gloved observers with the weak lens. In experiment 2 the square had a visual image which was either a diamond or a parallelogram, the parallelogram having angles of 70 degrees and 110 degrees. Observers described and drew their percepts, which were generally in accord with the visual image. There were no significant differences due to age, a finding contrary to that of Page and Locke, nor were there differences in experiment 2 due to the gloved-hand-bare-hand dimension. It is concluded (i) that repeated observations do not destroy the effect, (ii) tht considerable discrepancies between the visual and haptic images do not destroy the effect, but (iii) that feedback from a bare hand can reduce the effect when it is of considerable magnitude.

Adult↗
PubMed1978

Hypotheses in perception: their development about unambiguous stimuli in the environment.

A series of seven photographs which depict a real scene as it is approached by an observer were shown, in sequence, to ninety-six university students. The photographs were either projected colour transparencies or black-and-white prints. Two sixes of photographs were used in each set and the series was presented with or without the first photograph. In earlier studies in which blurred photographs were used as stimuli, observers were unsure about their hypotheses. That was not the case in this study. Although individual observers differed widely in their initial hypotheses, the initial photographs were not perceived as ambiguous. It required on average from 2.9 photographs (large, black-and-white, full series) to 4.5 photographs (small, colour, full series) for correct identification. In general, colour is a hindrance to correct identification. For the full series approximately three hypotheses are put forward before the scene is correctly identified.

Color Perception↗
PubMed1977

Characteristics of items in the Eysenck Personality Inventory which affect responses when students simulate.

A large sample of students completed Form A of the Eysenck Personality Inventory, and four subgroups were later asked to simulate extraversion, introversion, neuroticism or stability. It was found that subjects could simulate these four personalities successfully. The changes in individual item responses were correlated with the items' factor loadings, validity, response bias, and detectability. The different scales and types of item were considered separately. In some cases the changes in item responses when simulating introversion and extraversion were related to the extraversion validities and factor loadings of the items. More often, however, the behaviour of items under simulation was correlated with aspects of the items that made them more like an item from another scale and thus lessened their susceptibility to a particular type of simulation.

Deception↗
PubMed1976

Dominance of touch by vision: generalization of the hypothesis to a tactually experienced population.

It is now held that in conditions where there is conflicting sensory information vision dominates touch. The present study was designed to place boundary conditions on the generality of the hypothesis by comparing the performance of tactually experienced subjects - potters - with university students on a form-perception task in which a square was optically distorted so as to appear rectangular. Subjects examined the square haptically and visually, and matched it by touch or vision. Although potters were more accurate than students when not permitted vision, tactual judgements of both were dominated by vision when they were presented with conflicting information. In a supplementary study longer, more active exploration and emphasis on the necessity of matching by touch had no differential effect. In a third study it was found that a range effect produced by the comparison stimuli could explain the lesser accuracy of the students when they were making haptic judgements. The generality of the hypothesis that vision dominates touch is strengthened by our failure to refute it.

Adolescent↗