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D Regan

Publications and source records attributed to D Regan.

At least 73 records · Page 4Linked to original sources
PubMed1995

Test-retest variability and correlations between tests of texture processing, motion processing, visual acuity, and contrast sensitivity.

PURPOSE: To compare the test-retest variability (reliability) and the relations among clinical tests of texture and motion processing, visual acuity for high- and low-contrast letters, and the Pelli-Robson contrast sensitivity test. METHODS: In 20 normally sighted subjects, monocular visual acuity for letters of 96% and 11% contrast, Pelli-Robson contrast sensitivity, and motion-defined and texture-defined letter recognition thresholds were measured on each of two different days. RESULTS: Test-retest correlation coefficients were 0.75, 0.91, 0.61, 0.90, and 0.84 and bivariate test-retest regression slopes were 1.0, 1.1, 0.8, 1.0, and 1.2 for high- and low-contrast acuity, contrast sensitivity, and motion and texture processing, respectively. The inter-test correlations with both test and retest significant were as follows: visual acuity for high-contrast letters vs. visual acuity for low-contrast letters; and recognition threshold for texture-defined letters vs. acuity for letters of both high and low contrast. CONCLUSION: Test-retest variability for the tests of motion and texture processing were at least as low as for established clinical tests of high and low contrast acuity and contrast sensitivity. We conclude that these new tests offer a reliable means of obtaining clinical information which complements that provided by conventional tests with luminance-defined letters.

Adolescent↗
PubMed1995

Magnetic and electrical responses of the human brain to texture-defined form and to textons.

1. We searched for a neurophysical correlate of preattentive texture discrimination by recording magnetic and electric evoked responses from the human brain during the first few hundred milliseconds following the presentation of texture-defined (TD) checkerboard form. The only two textons that changed when the TD checkerboard appeared or disappeared were the local orientation and line termination textons. (Textons are conspicuous local features within a texture pattern). 2. Our evidence that the magnetic response to TD form cannot be explained in terms of responses to the two associated textons is as follows: 1) by dissociating the two responses we showed that the magnetic response to TD form is almost entirely independent of the magnetic response to the local orientation texton; 2) a further distinction between the two responses is that their distributions over the head are different; and 3) the magnetic response to TD form differs from the magnetic response to the line termination texton in both distribution over the head and waveform. We conclude that this evidence identifies the existence of a brain response correlate of preattentive texture discrimination. 3. We also recorded brain responses to luminance-defined (LD) checkerboard form. Our grounds for concluding that magnetic brain responses to the onset of checkerboard form are generated by different and independent neural systems for TD and LD form are as follows: 1) magnetic responses to the onset of TD form and LD form had different distributions over the skull, had different waveforms, and depended differently on check size; and 2) the waveform of the response to superimposed TD and LD checks closely approximated the linear sum of responses to TD checks and LD checks alone. 4. One possible explanation for the observed differences between the magnetic and electric evoked responses is that responses to both onset and offset of TD form predominantly involve neurons aligned parallel to the skull, whereas that is not the case for responses to LD form.

Adolescent↗
PubMed1995

Multiple sclerosis can cause visual processing deficits specific to texture-defined form.

We performed the following tests in 25 patients with multiple sclerosis (MS) and 25 age-matched control subjects: recognition of texture-defined (TD) letters; recognition of motion-defined (MD) letters; and recognition of luminance-defined (LD) letters of 96% and 11% contrasts. Six patients with normal visual acuity were abnormal on recognizing TD letters, of whom one gave normal results on all other tests. Eleven patients were abnormal on MD letter recognition, of whom four gave normal results on all other tests. Visual acuity for letters of 11% contrast were abnormally low in seven patients, of whom two gave normal results on all other tests. We conclude that the neural mechanisms underlying recognition of TD, MD, and low-contrast LD letters in subjects with normal visual acuity are sufficiently different that they can be differentially damaged by MS. Therefore, TD, MD, and LD letter tests provide complementary information. We suggest that the detection of TD letters can be disrupted by demyelination of long-range horizontal connections between orientation-tuned neurons in the striate cortex.

Adult↗
PubMed1995

Spatial vision in adults and infants: a tribute to Russell Harter.

To be recognized, the retinal image of an object must first be segregated from the retinal image of its surroundings. Luminance contrast is only one way by which an object can be rendered visible. Motion contrast alone, texture contrast alone, colour contrast alone and disparity contrast alone can also render an object visible. This paper notes that psychophysical studies of patients with brain lesions show that motion-defined form and luminance-defined form are processed separately and describes how magnetic (MEG) or electrical (VEP) recording can be used to compare brain responses to luminance-defined form, motion-defined form, colour-defined form, texture-defined form and disparity-defined form. At an early stage in visual processing, the retinal images of luminance-defined objects pass through neural filters, each of which is selectively sensitive to a limited range of orientations, spatial frequencies and temporal frequencies. The properties of individual filters can be isolated and quantified by means of the two-sinewave VEP approach. The two-sinewave method also allows binocular function to be tested objectively and its normality assessed quantitatively even when one or both eyes have low acuity. Evoked potential studies of visual development in human infants and studies of labile phenomena in adults are both facilitated by the speed of our VEP sweep method.

Adult↗
PubMed1994

Shape discrimination for rectangles defined by disparity alone, by disparity plus luminance and by disparity plus motion.

We measured the just-noticeable difference in aspect ratio a/b for dotted rectangles defined entirely by disparity (DD rectangles), dotted rectangles defined by both disparity and luminance contrast (DD+LD rectangles) and rectangles defined by both disparity and relative motion (DD+MD rectangles) over a wide range of uncrossed (0-126 min arc) and crossed (0-126 min arc) disparities. The rectangle's height and width were, respectively, a and b. Vergence was monitored by nonious lines, fusion was monitored by a fusion line, and subjects were instructed to look at the rectangle. We conclude that for DD rectangles, aspect ratio discrimination for crossed and uncrossed disparities cannot be compared accurately unless (a) ocular vergence is monitored, (b) a wide range of crossed and uncrossed disparities are investigated and (c) trials are repeated until threshold has stabilized for the entire range of both crossed and uncrossed disparities. Intersubject variations in fixation disparity is the reason for (a). The reason for (b) is that the effect of disparity on aspect ratio discrimination threshold is different for crossed and uncrossed disparities. The reason for (c) is that, for some subjects, the lowest discrimination threshold continues to fall with practice for uncrossed disparities beyond the point at which the lowest threshold has effectively stabilized for crossed disparities. We report that aspect ratio discrimination threshold for a DD rectangle first decreased and then levelled out as its disparity was progressively increased from zero, while the perceived depth of the rectangle increased smoothly and approximately linearly. We found that the lowest value of aspect ratio discrimination threshold was the same for both crossed and uncrossed disparities. This lowest value occurred at or just before the disparity at which fusion was lost for the dots surrounding the rectangle. The lowest value of discrimination threshold was lower for DD+LD and for DD+MD rectangles than for DD rectangles. The lowest value of discrimination threshold for DD rectangles was 4.0, 3.4, 7.4 and 3.1% for our four subjects. If we assume that a and b are encoded directly, then a 3.1% discrimination threshold implies that the precision of encoding a and b is better than 1 min arc--considerably better than the 9 min arc mean dot separation, and considerably better than the 3-5 c/deg estimate for cyclopean grating acuity.

Adult↗
PubMed1994

Recognition and detection of texture-defined letters.

We quantified texture segregation by measuring psychophysically the percent correct detection scores for a set of 10 texture-defined (TD) letters using two-alternative forced-choice, and at the same time quantified spatial discrimination of TD form by measuring psychophysically the percent correct letter recognition scores for the 10 letters using 10-alternative forced-choice. Ten levels of task difficulty were created by adding noise dots to the texture patterns. Two kinds of letters were used. Static textures had the same letter and the same texture pattern throughout any given 1-sec presentation. Dynamic textures had the same letter, but a different texture pattern for every one of the 70 frames during any given 1-sec presentation. For both static and dynamic textures, letter recognition scores fell to chance level from a lower number of noise dots than did letter detection scores. Both recognition and detection scores were generally better for dynamic than for static texture patterns. We suggest that, for dynamic textures, subjects were able to enhance the signal-to-noise ratios of the noisy letters by signal averaging.

Discrimination, Psychological↗
PubMed1994

Monocular discrimination of the direction of motion in depth.

The direction of motion in depth of a monocularly-viewed rigid sphere can be quantified in terms of the distance by which the sphere's centre will miss the centre of the pupil of the observing eye. If we express this distance as ns (where s is the sphere's radius and n is a scaling factor), then n approximates the ratio (d phi/dt)/(d theta/dt) between the translational velocity (d phi/dt) and the rate of expansion (d theta/dt) of the object's retinal image. To use this monocular information alone as a basis for motor action, prior knowledge of s would be necessary. (However, the value of s is available from binocular information, so that the distance by which the sphere's centre would miss the eye is, in principle, available from retinal image information alone and, in particular, without knowing the object's size or distance from the eye). We measured the just-discriminable difference in the direction of motion in depth for a monocularly-viewed simulated object. Thresholds were measured for trajectories contained within the horizontal, vertical and two oblique meridia. The translational speed of the retinal image was removed as a reliable cue to the direction of motion in depth by randomly varying the simulated object's speed on a trial to trial basis. The direction of translational motion was also removed as a reliable cue. Discrimination threshold for the stimulated direction of motion ranged from 0.03 to 0.12 deg for our seven subjects, and did not vary appreciably with the direction of motion relative to the line of sight over the range investigated, nor did it depend on whether trajectory was contained within the horizontal, vertical or oblique meridia. We conclude that subjects are able monocularly to discriminate differences in the direction of motion in depth, even when both the direction and speed of retinal image translation are removed as reliable cues.

Adult↗
PubMed1993

Binocular correlates of the direction of motion in depth.

Two binocular cues to the direction of an object's motion in depth are the ratio (phi R/phi L) between the velocities of the object's retinal images in the right and left eyes and the ratio (phi/gamma) between the velocity of the binocularly-fused image of the object and the rate of change of disparity. We report that the apparent direction of motion in depth of a monocularly-camouflaged target can be varied by altering the ratio phi/gamma. Because no monocular motion signal is available in this case, we conclude that the ratio phi/gamma is a sufficient cue to the direction of motion in depth. This is not to deny that the phi R/phi L cue might be used in the everyday visual situation where monocular velocities phi R and phi L are available to the brain.

Cues↗
PubMed1993

Dissociation of discrimination thresholds for time to contact and for rate of angular expansion.

It is well known that, if a rigid sphere is moving at constant speed towards the eye along the line of sight then, for small values of theta, T = theta/theta, where T is the time to contact, theta is the instantaneous angular size and theta is the rate of increase of angular size. We describe a rationale and an experimental procedure for demonstrating empirically when subjects base discrimination of time to contact on trial-to-trial variations of (theta/theta) rather than on variations of theta or on variations of delta theta (the change of angular size during a presentation). Discrimination threshold for the ratio (theta/theta) was 0.070-0.13, and was independent of mean time to contact over a range of at least T = 1.0-4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to the ratio (theta/theta) rather independently of the values of theta and theta. Using a different procedure we demonstrated empirically that subjects based discriminations of rate of expansion on trial-to-trial variations of theta rather than variations of time to contact or on variations of delta theta. Discrimination threshold for rate of expansion was 0.85-0.14, and was independent of mean time to contact over a range of at least T = 1.0-4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to rate of expansion rather independently of time to contact or absolute change in size. When rate of expansion and time to contact were both available as cues, discrimination threshold was on average lower than when only one of the two cues was present. We conclude that there is some summation of the two cues. Our data can be explained by a small modification to a model previously put forward to account for data on threshold elevations and aftereffects caused by adapting to changing-size stimulation. This model incorporates a filter that is strongly activated by isotropic, homogeneous two-dimensional expansion of the retinal image and whose output is inversely proportional to time to contact (i.e. the more urgent the demand for evasive action, the stronger the output of the filter).

Discrimination, Psychological↗
PubMed1993

The tensile bond strength of new and rebonded stainless steel orthodontic brackets.

The study investigated the effect on the tensile/peel bond strength of the variables associated with the bracket base, the enamel surface, and the type of adhesive when both new and used brackets were rebonded to a previously bonded enamel surface. The tensile/peel bond strength was firstly evaluated for three different types of stainless steel orthodontic bracket/base combinations. The cast integral base gave a significantly lower bond strength than the foil-mesh and photo-etched bases. Following debonding, a group of new brackets were bonded to the teeth using a chemically-activated or a light-cured adhesive. The old adhesive had been removed from the enamel by either a hand scaler or a tungsten-carbide bur. The rebonded new brackets demonstrated a small, but statistically significant fall in bond strength. No differences were found between the enamel preparations or the adhesives. A further group of previously debonded brackets were rebonded to the same teeth. The bracket bases were prepared by either smoothing with a green stone or heating in a bunsen flame followed by sandblasting and electropolishing. Highly significant falls in bond strength were obtained with all the bases. No significant differences were found between the two methods of bracket preparation.

Analysis of Variance↗
PubMed1993

Crowding and contrast in amblyopia.

The crowding effect, defined as the ratio of visual acuities for letters presented in Snellen (i.e., line) format and isolated-letter format, was measured in the amblyopic eyes of 15 children and 15 adults with unilateral amblyopia. Normal limits were based on data from 20 children and 20 adults with no history of amblyopia. The crowding effect was compared for letters of high (96%) contrast and for letters of low (11%) contrast. We report that amblyopia can differentially affect line and isolated-letter acuity for both high- and low-contrast letters. For the patients that show an abnormal crowding effect, this effect can be: (1) significantly stronger, (2) significantly weaker, or (3) not significantly different for high- than for low-contrast letters. These findings are consistent with the hypothesis that the crowding effect is contrast-dependent in some amblyopic eyes of both child and adult unilateral amblyopes.

Adult↗
PubMed1993

Measurement of glare susceptibility using low-contrast letter charts.

The Committee on Ophthalmic Procedures Assessment of the American Academy of Ophthalmology suggested either contrast sensitivity or low-contrast visual acuity (VA) measured before and after adding a glare source as a test for assessing overall visual disability from immature cataracts. We have developed a test that follows the Committee's three principles of design, and we report that the effect of glare on VA is considerably greater for recognizing low-contrast letters than for recognizing high-contrast letters. The effect of glare on VA increases progressively as letter contrast is reduced from 96% through 50%, 25%, 11%, and 4%. The 25% chart (and possibly the 11% chart) gives the most suitable sensitivity for eyes with immature cataracts. Neither age-related brunescence and aging itself nor even the presence of a distinct opacity necessarily produces high sensitivity to glare. The test is sensitive enough to detect an ageing effect even before the age of 45 to 55 years and to reveal large intersubject differences among normally sighted 19- to 25-year-old subjects.

Adult↗
PubMed1993

Measurement of glare sensitivity in cataract patients using low-contrast letter charts.

The Committee on Ophthalmic Procedures Assessment of the American Academy of Ophthalmology suggested using low-contrast visual acuity measured before and after adding a glare source as a test for assessing overall visual disability from immature cataracts. We have developed a test that follows the Committee's three principles of design, and we report that the effect of glare on visual acuity is considerably greater for recognizing low-contrast letters than for recognizing high-contrast letters. The effect of glare on visual acuity increases progressively as letter contrast is reduced in the stages 96%, 50%, 25%, 11% and 4%. The 25% chart (and possibly the 11% chart) gives the most suitable sensitivity for eyes with immature cataracts. Age-related brunescence and aging itself do not necessarily produce high sensitivity to glare. Sensitivity to glare was markedly different in eyes with different kinds of cataract.

Aged↗
PubMed1993

Dissociation of achromatic and chromatic processing of spatial form and temporal modulation by the titration method.

Using a titration method to attempt to silence the achromatic contrast subsystem, we tested the assumption that an equiluminant chromatic pattern does not stimulate the physiological achromatic contrast subsystem. We also examined the question whether contrast threshold is entirely or even partially determined by the chromatic contrast subsystem at equiluminance. We report a method for demonstrating when contrast threshold is entirely determined by the chromatic contrast subsystem.

Adult↗
PubMed1993

Nonlinear terms produced by passing amplitude-modulated sinusoids through a hair cell transducer function.

We derive mathematically the output of Corey and Hudspeth's hair cell transducer function for the following cases: (1) the input is a single carrier whose output is modulated by the sum of two sinusoids; (2) the input is the sum of two carriers, each of which is amplitude-modulated by a single sinusoid. The theoretical results are similar to the results of an experiment in which field potentials were recorded from the human scalp while one ear was stimulated with auditory waveform (1) and with auditory waveform (2) of the cases above.

Acoustic Stimulation↗
PubMed1992

Motion-defined letter detection and recognition in patients with multiple sclerosis.

Two important distinctions in visual perception are (1) between the detection and recognition of shape (e.g., letters), and (2) between the recognition of shapes defined by a difference in brightness and the recognition of shapes defined by a difference in motion. We report that 6 of 10 patients with multiple sclerosis showed impaired recognition for motion-defined (MD) letters, although the detection of MD letters was normal as were both detection and recognition of luminance-defined letters. We have shown that this was not a function of acuity loss or the loss of ability to detect motion or a general failure of recognition per se, but was confined to a loss of ability to recognize MD letters. The neurological implications of these findings are discussed, and it is suggested that the MD letter test be used by others interested in the central pathology of visual disorders.

Adult↗