Search PubMed⌕ Search

Biomedical subjects

D Regan

Publications and source records attributed to D Regan.

At least 109 records · Page 6Linked to original sources
PubMed1989

Bond strengths of two integral bracket-base combinations: an in vitro comparison with foil-mesh.

The tensile/peel and shear/peel bond strengths of a cast integral bracket-base were compared with a machined integral base and a foil-mesh base. The bases were tested with both a 'no-mix' and a two-paste adhesive. The cast base performed significantly better than the other two. All the bases produced a higher bond strength when subjected to a shear mode of loading and greater bond strengths were obtained with both loading configurations when using the two-paste adhesive. These differences in bond strength between the bases cannot be explained in terms of the differences in the surface areas of the bases, so that other factors will need to be considered.

Dental Bonding↗
PubMed1988

Low-contrast visual acuity test for pediatric use.

Single-letter flash cards were made at contrast values of 96%, 7% and 4% to compare high-, intermediate- and low-contrast visual performance in amblyopic children young enough to benefit from occlusion therapy. The single-letter format was intended to differentiate the effect of contrast on reading performance from any effects of nearby contours. Two groups of patients were tested: 37 children aged 3 to 8 years who had completed occlusion therapy and 15 children aged 4 to 8 years who were still receiving occlusion therapy. Their results were compared with the results of 45 control subjects aged 3 to 8 years. Three patterns of visual loss were identified in the patients: predominantly for high-contrast acuity, fairly uniform at high-, intermediate- and low-contrast levels, and, in two patients, loss at low- and intermediate-contrast levels with relative sparing at the high-contrast level. The incidence of high-contrast acuity loss was not significantly different between the two groups, but the proportion of children with no acuity loss at any contrast level was 54% in the group that had completed treatment, compared with 17% of those who were still receiving treatment.

Amblyopia↗
PubMed1988

Objective evidence for phase-independent spatial frequency analysis in the human visual pathway.

Electrophysiological responses in human index an interaction between responses to two gratings that is relatively independent of the distribution of light in the retinal image. Two 5 cycle/deg sinewave gratings were superimposed, one counterphase-modulated at F1 Hz and the other at F2 Hz. Nonlinear interaction terms of frequency (nF1 +/- mF2) were recorded that could not be produced by superimposing the F1 Hz grating on blank-field 7 Hz flicker. A local luminance origin could be excluded for the (2F1 + 2F2) term and for the suppression of 2F1 and 4F1, but not for the (F1 + F2), (F1 + 3F2) and (5F2 - F1) terms. The relative spatial phase of the two gratings was varied, thus altering the light distribution in the retinal image without changing its spatial power spectrum. The (2F1 + 2F2) Hz contrast-specific grating-grating interaction term was almost unaffected by these substantial changes in retinal image light distribution providing that the spatial frequency power spectrum of the retinal image was held constant. The (2F1 + 2F2) term and the suppression of 2F1 were both tuned to spatial frequency.

Evoked Potentials, Visual↗
PubMed1987

Nonlinearity in human visual responses to two-dimensional patterns, and a limitation of Fourier methods.

Subjects viewed a pattern consisting of two superimposed gratings: a vertical grating that was counterphase-modulated at Fl Hz and a variable-orientation grafting modulated at F2 Hz. A nonlinear orientation-tuned cross-modulation term of frequency (2F1 + 2K2)Hz in the evoked potential was large when the gratings were parallel and had a half-height full bandwidth of about 12 deg. But a strong (2F1 + 2F2) term was also produced by orthogonal gratings. The application of Fourier methods to analyzing human visual processing of patterns modulated in two dimensions assumes that there is no nonlinear interaction between visual responses to orthogonal gratings. The existence of a strong cross-modulation term in the orthogonal-grating response violates this crucial requirement. Our findings could not result from the stimulation of independent, linear, orientation-selective mechanisms.

Evoked Potentials, Visual↗
PubMed1987

Opponent model for line interval discrimination: interval and vernier performance compared.

Vernier and spatial interval thresholds were determined as a function of contrast, using thin bars with a Gaussian luminance profile. The previously reported contrast dependence of vernier acuity was confirmed, but contrast had a much smaller effect upon interval acuity. A subsidiary experiment showed that the relative contrast of the two bars in an interval discrimination could be randomly perturbed over trials without substantially reducing acuity. A model of interval discrimination is developed, including an opponent stage between hypothetical spatial coincidence detectors.

Discrimination, Psychological↗
PubMed1987

Orientation-selective visual loss in patients with Parkinson's disease.

Visual contrast sensitivity was measured using 2 cycle/deg sinewave gratings of different orientations in 10 patients with Parkinson's disease and in 15 age-matched controls. Loss of visual contrast sensitivity was found in 6 patients, all of whom had normal visual acuity. Visual loss depended on grating orientation; in all cases the maximum sensitivity loss was for the horizontal. Sensitivity loss was most marked at a temporal frequency of 4 to 8 Hz. Visual fields gave no hint of the orientation selectivity. We conclude that orientation selectivity implicates visual cortical cells in Parkinson's disease. We tentatively suggest that a preferential loss of contrast sensitivity to horizontal gratings might be due to a functional abnormality in the striate cortex that relatively spares the extrastriate cortex. The dependence of visual loss on temporal frequency combined with the sparing of visual acuity might possibly be understood if Parkinson's disease preferentially affects the visual pathway leading from the retina to cortex via the magnocellular layer of the lateral geniculate nucleus. There is an intriguing similarity between the pattern of visual loss in Parkinson's disease and in multiple sclerosis.

Aged↗
PubMed1987

Origin of notches in CSF: optical or neural?

Grating contrast sensitivity was measured across a range of 1 to 32 cycles per degree (c/deg) in normal observers with a computer-automated method of ascending limits. Monocular contrast sensitivity functions (CSF) were obtained for vertical, oblique and horizontal orientations, with or without full refractive correction. Small amounts of astigmatic error resulted in loss of sensitivity at selective spatial frequencies. Coincident with these CSF "notches" was the presence of monocular diplopia induced, in this study, by the condition of astigmatic error. Experimental manipulation of the selective spatial frequency losses was possible by the introduction of slight cylindrical defocus and by changes in grating orientation. Determination of the angular displacement and orientation of the monocular double images allowed prediction of the spatial frequencies which would show reduced sensitivity due to partial cancellation of contrast. The close fit between the predicted and measured sensitivity loss supports the suggestion that refractive error can affect narrowly-tuned notches. These results indicate that before the presence of a notch in the CSF can be attributed to neural abnormality, an optical cause must be eliminated.

Adult↗
PubMed1986

Visual processing of four kinds of relative motion.

Evidence is presented supporting the idea that the human visual system has several specific sensitivities to different kinds of relative motion. These specific sensitivities include: sensitivity to a velocity difference between two different points A and B on one eye's retinal image, the two velocities being directed along the line AB; sensitivity to the velocity difference at A and B between velocity components perpendicular to the line AB (i.e. shearing motion); sensitivity to rotary motion; sensitivity to the ratio between the velocities of the left and right retinal images of an object that is moving in depth. These specific sensitivities can be attributed to relatively hardwired neural filters that are "tuned" to different retinal image correlates of the three-dimensional structure and motion of solid objects in the environment. Such filters may be of use in distinguishing rigid nonrotating objects from nonrigid or rotating objects. They may also be of use in recovering information from the two-dimensional retinal image, including information about object boundaries, the three-dimensional structure of the environment, self-motion and object motion in depth. An alternative way of regarding certain of these specific sensitivities is that they might provide rough physiological equivalents of the values of div V and curl V at every point in the instantaneous velocity field of the retinal image and thus crudely analyze the retinal image flow pattern in terms of mathematical quantities that have the useful property of being relatively invariant against bodily translations of the whole retinal image caused by eye rotation.

Adaptation, Ocular↗
PubMed1986

Periodicity in orientation discrimination and the unconfounding of visual information.

Orientation discrimination is a periodic function of mean orientation. Discrimination sensitivity (i.e. threshold-1) was measured at 7.5 deg increments around the clock for an 8 c/deg grating subtending 1.0 deg dia located 1.25 deg from the foveal centre. Discrimination sensitivity was best for horizontal and vertical orientations, but did not fall monotonically to minima at 45 deg and 135 deg. Instead it fell precipitously to minima at angles of only 20 deg to the horizontal and vertical, and there was a weak submaximum near 45 deg. This finding is consistent with the proposal that orientation discrimination is determined by the relative activity of broadly-tuned, orientation-sensitive neural elements, and that only a small number of elements are effective in any small retinal region. This idea can also explain why subjects do not confound a change of orientation with a simultaneous change of contrast or spatial frequency.

Discrimination, Psychological↗
PubMed1986

Human ocular vergence movements induced by changing size and disparity.

Human subjects viewed an electronically generated bright square. Horizontal movements of the two eyes were recorded with the scleral coil method. The dynamic properties of vergence movements induced by movement of the bright square were investigated for the following three kinds of stimulus motion: (a) both the size and the binocular disparity of the square changed together, in such a way as to exactly mimic the retinal image changes produced by a real object's motion in depth; (b) the changing-size component in (a) was present with no disparity component; (c) the changing-disparity component in (a) was present with no size component. The gain and phase of the ocular vergence responses to these three stimuli were computed. Ocular vergence movements were induced by changing size in all five subjects. Responses during binocular viewing were higher and less variable than responses during monocular viewing. Size oscillations induced ocular vergence oscillations with a phase lead of up to 65 deg relative to target size for frequencies of stimulation below 1.0 Hz. Vergence oscillation amplitudes were of the order of 10 min of arc and maximal for frequencies of 0.4-0.7 Hz. Ocular vergence movements were not induced by changes in target size in one dimension nor by flickering a stationary square. Ocular vergence movements induced by size changes were entirely transient with no sustained component: vergence responses to disparity were sustained. When the stimulus combined size change with disparity change in the ratio characteristic of a real moving object, vergence tracking was more accurate and less noisy than when the eyes were stimulated with the disparity component alone. The ocular vergence response induced by the combination of size change with disparity change was accurately predicted by linearly adding the vergence response produced by the size change alone to the vergence response produced by the disparity change alone: combined stimulation produced no evidence of non-linear interaction between responses to size change and to disparity change. The properties of vergence responses induced by changing size and by changing disparity showed several close correlations with the corresponding data on psychophysical sensitivity for motion-in-depth sensation. We suggest that responses to changing size contribute to the accuracy with which ocular vergence tracks real objects moving in depth.

Adult↗
PubMed1986

Form from motion parallax and form from luminance contrast: vernier discrimination.

UNLABELLED: Some objects are perfectly camouflaged when stationary, but are clearly visible when moving; the boundaries of such an object are defined entirely by motion parallax. Little is known about the eye's ability to make spatial discriminations between motion-defined objects. In this study, subjects viewed a pseudo-random pattern of dots within which a camouflaged bar was made visible by relative motion of dots. Vernier acuity for the motion-defined bar was 27-45 sec arc for three subjects, much less than the interdot separation of 360 sec arc, much less than the 2 deg receptive field size for motion, and comparable with the foveal intercone separation of 30 sec arc. It is proposed that an opponent-orientation process and an opponent-position process can both contribute to vernier judgements for motion-defined objects. Real-world motion contrast commonly confounds the following cues for figure-ground segregation: (1) different texture velocities on either side of the figure's boundary; (2) in any given time interval, texture in figure and ground moves different distances; and (3) texture continually appears and disappears along the figure's boundary. When cues (2) and (3) were eliminated, thus ensuring figure-ground segregation was achieved entirely by motion-sensitive neural elements, vernier acuity was 44 +/- 5 sec arc compared with 36 +/- 8 sec arc for a dotted bar defined by luminance contrast. CONCLUSION: Vernier acuity for a dotted bar whose boundary was defined entirely by motion-sensitive neural elements was similar to vernier acuity for a dotted bar whose boundary was defined by luminance contrast.

Contrast Sensitivity↗
PubMed1986

Visual field defects for vergence eye movements and for stereomotion perception.

An objective visual field can be mapped in terms of stimulus-induced eye movement. The authors used the scleral coil technique to record vergence and conjugate eye movements while stimulating different visual field locations with a 3 X 3 deg target whose image vergence was oscillated. For each of three subjects tested there was a visual field location where vergence eye movements were much weaker than in a control location of equal retinal eccentricity. On the other hand, conjugate eye movements driven from these two locations by lateral motion were similar. Field defects for ocular vergence coincided with regions in which oscillating retinal disparity failed to produce a sensation of motion in depth, although visual responses to static disparity were normal, and psychophysical thresholds for lateral motion showed no defect with either binocular or monocular viewing. It was concluded, therefore, that the perceptual stereomotion scotomata were not due to a monocular loss, but to a defective binocular interaction between motion signals from the left and right eyes, and that this defective interaction was specific for opposed rather than parallel motion in the two eyes. Furthermore, the visual loss was specific for motion rather than for position. The correlation between the field defects for ocular vergence and stereomotion perception leads the authors to suggest that the same defect in binocular interaction is responsible for both the eye movement and sensory abnormalities. Two candidate hypotheses are proposed: one is framed in terms of a single population, and the other in terms of two populations of cortical neurons.

Blindness↗
PubMed1986

Necessary conditions for the perception of motion in depth.

This study investigated the relation between the perception of motion in depth and ocular vergence movements for a single foveally viewed dot, and for a 30 deg X 30 deg pattern of many dots. When the target's disparity was changed, it appeared to move in depth relative to stationary reference marks, but removing the reference marks completely abolished the sensation of motion in depth for the multi-dot target and left only a weak sensation of motion in depth for the single dot target. However, it is not the case that motion-in-depth sensation per se depends on the presence of reference marks; motion in depth generated by changing-size stimulation was unaffected by removing reference marks. Possible explanations for the loss of motion-in-depth sensation include ocular vergence exactly tracked stimulus motion; vergence changes and disparity changes, though unequal, produced equal and opposite motion-in-depth signals; vergence changes, though producing no motion-in-depth signals, suppressed the signals produced by disparity changes; motion-in-depth sensation requires relative motion. Explanation is rejected because vergence tracking errors were large. Explanation is rejected because vergence changes do not in themselves induce a sensation of motion in depth. Explanation is rejected because motion-in-depth threshold is not affected by vergence changes. Conclusions are as follows. For a single-dot target, visual sensitivity to motion in depth is much higher for changes in relative retinal disparity than for changes in absolute retinal disparity, while for a multi-dot target any residual sensitivity is abolished by an interaction between neighboring coherently moving dots. The authors suggest that the relative velocity elements proposed to explain sensitivity to changing size feed the stereomotion mechanism also.

Depth Perception↗