Biomedical subjects

D Regan

Publications and source records attributed to D Regan.

At least 55 records · Page 3Linked to original sources

PubMed1997

Alternatives to ceramic brackets: the tensile bond strengths of two aesthetic brackets compared ex vivo with stainless steel foil-mesh bracket bases.

The mean tensile/peel bond strengths were evaluated for three types of aesthetic brackets (a ceramic-reinforced bracket and two generations of a ceramic/polycarbonate combination bracket). These were found to be significantly lower than the mean tensile/peel bond strength of a convention foil-mesh stainless steel bracket base. Failure of the ceramic-reinforced polycarbonate brackets occurred predominantly by fracture of the tie wings during testing. With the ceramic/polycarbonate combination brackets, the majority of the specimens failed due to separation of the ceramic and polycarbonate parts of the bracket.

Analysis of Variance↗

PubMed1997

Babe Ruth: with vision like that, how could he hit the ball?

Unfortunately, it is unlikely that a definitive answer will be known. We believe that it is most likely that Ruth was not amblyopic. This seems possible only if Dr. Kara had missed some amblyogenic factor such as strabismus or a significant refractive error. Our favored solution is that Ruth's unilateral vision loss was a complication of his cancer, and that Dr. Kara's examination occurred before the optic nerve damage became detectable. Of course, this is in disagreement with the ophthalmologist who examined his eyes.

Amblyopia↗

PubMed1997

Development of motion-defined figure-ground segregation in preschool and older children, using a letter-identification task.

BACKGROUND: Three-month-old infants can discriminate motion-defined (MD) form, but we do not know the age at which this ability reaches adult levels. Previous psychophysical evidence suggests that different neural mechanisms are involved in the processing of luminance-defined (LD) and MD spatial form in adults. This difference may be reflected in the development of LD versus MD form identification in children. METHODS: We measured speed threshold for identifying MD letters, letter-chart (i.e, Snellen) acuity for high-contrast LD letters and single-letter acuity for high- and low-contrast LD letters. Forty-seven children between 3 and 12 years of age and 20 adult subjects were tested. RESULTS: Development to the adult level was observed as follows: low-contrast single-letter acuity before 3 years; high-contrast single-letter acuity by 5 to 6 years; the ability to identify MD letters by 7 to 8 years; letter-chart acuity by 9 to 10 years. CONCLUSIONS: MD form identification continues to mature in preschool children. LD form identification also matures in this age group but with a different time course. MD letters are not equivalent to low-contrast letters developmentally. Our findings provide further support for the hypothesis that the spatial aspects of MD and LD form are processed separately to some extent.

Adult↗

PubMed1997

Judging the time to collision with a simulated textured object: effect of mismatching rate of expansion of object size and of texture element size.

We measured the accuracy with which subjects estimated the time to collision with a simulated textured object approaching at constant speed along the line of sight. The independent variable was the ratio R, where R = (rate of dilation of the texture elements that covered the simulated object)/ (rate of dilation of object size). When matching was perfect (i.e., R = 1.0), the mean of 12 settings was close to the nominal value of 2,000 msec for the 2 subjects. In addition, the standard error of 12 settings was only 25 and 52 msec in 2,000 msec for the 2 subjects. Discrimination threshold for time to collision was not significantly affected by R over the range investigated between R = 0 and R = 2.0. However, the accuracy of estimating time to collision was significantly affected by R. Estimated time to collision was a monotonic function of R. For example, when the mismatch was only 10% (i.e., R = 0.9) subjects judged time to collision would occur 178 msec later than the true time to collision of 2,000 msec.

Adult↗

PubMed1996

Evidence for a neural mechanism that encodes angles.

We measured the discrimination threshold (delta theta)Th for angle theta, where theta was either the angle of a Vee composed of two straight lines contained within the frontoparallel or the angle intersection of two straight lines contained within the frontoparallel plane. The two-line pattern was rotated bodily through a random angle between trials with the aim of eliminating the absolute orientation of one or the other line as a reliable cue to the task. We report evidence that this aim was achieved. Our main conclusion is that the ability to discriminate a change in angle theta cannot entirely be explained in terms of the ability to discriminate changes in the orientations of the individual lines that comprise the Vee. We propose that the human visual pathway contains a neural mechanism that encodes the difference in the orientations of two simultaneously-presented straight lines. Discrimination threshold for angle (delta theta)Th is roughly twice orientation discrimination threshold for an isolated line. When subjects cannot use the orientation of one or another line as a cue to the task, the plot of (delta theta)Th vs theta is approximately flat between the delta = 20 and 160 deg.

Cues↗

PubMed1996

Magnetic and electrical brain responses to chromatic contrast in human.

Differences between magnetic responses to red-green chromatic gratings and yellow-black luminance gratings were: (1) response waveforms differed considerably; (2) at some recording sites the chromatic grating response was considerably greater than the sum of responses to the red and green components of the chromatic grating; (3) the latencies of the successive peaks in the response to the onset of chromatic contrast were greater than the latencies of the corresponding peaks in the response to luminance contrast onset; (4) chromatic grating responses were lowpass with respect to spatial frequency while luminance grating responses were bandpass; (5) chromatic grating responses attenuated more steeply with increasing frequency above 2 c/deg than did luminance grating responses. Items (3)-(5) above are consistent with well-known psychophysical findings that contrast sensitivity is lowpass for chromatic gratings and chromatic responses are more sluggish than luminance responses. In subsidiary experiments we found that magnetic responses to red-green and blue-yellow equiluminant gratings had similar waveforms in all six subjects tested, but the topographical distributions were different in three subjects. The results of comparing magnetic and electrical responses to the onset and offset of contrast can be understood in terms of the considerable intersubject variability in the relation between neuroanatomy and cortical function that has been demonstrated by other techniques.

Adult↗

PubMed1996

Cyclopean motion perception produced by oscillations of size, disparity and location.

UNLABELLED: For cyclopean and monocularly-visible targets we measured psychophysical thresholds for perceptions produced by the following three stimuli: oscillations of disparity (DO), oscillations of size (SO) and oscillatory motion within the frontoparallel plane (FPO). RESULTS: thresholds for motion in depth perception produced by DO were similar for cyclopean and non-cyclopean targets over the entire 0.25-8 Hz frequency range investigated. Thresholds for perceiving motion in depth produced by SO were considerably (up to 2.5 times) higher for cyclopean targets than for monocularly-visible targets, as were thresholds for perceiving size oscillations. For both cyclopean and monocularly-visible target the perception of motion in depth could be canceled by pitting DO vs SO. Thresholds for perceiving FPO were similar to DO thresholds for the monocularly-visible target, but for the cyclopean targets FPO thresholds rose more steeply than DO thresholds for oscillation frequencies above 1 Hz. CONCLUSIONS: (1) for our subjects, the effective binocular stimulus for motion in depth perception is a rate of change of disparity; an interocular velocity difference is significant only to the extent that it produces a rate of change of disparity. (2) The sensations of motion in depth produced by DO and SO are qualitatively identical. (3) Neural signals produced by DO and SO converge onto a single neural mechanism that signals motion in depth.

Adaptation, Ocular↗

PubMed1996

Cyclopean discrimination thresholds for the direction and speed of motion in depth.

We measured just-noticeable differences in the direction and speed of motion in depth of cyclopean and monocularly visible targets. Our stimulus set comprised different combinations of (d phi/dt)/(d delta/ dt), d delta/dt, d phi/dt and delta delta, where d phi/dt was the angular frontal plane speed of the binocularly-fused target, d delta/dt was its rate of change of disparity and delta delta was its disparity displacement. Our three subjects based their direction discriminations entirely on the task-relevant variable (d phi/dt)/(d delta/dt), and based their speed discriminations entirely on the task-relevant variable d delta/dt. They ignored all task-irrelevant variables in both tasks. Performance on both tasks was the same for motion within the horizontal and vertical meridians. Direction discrimination threshold rose significantly as the reference direction grew more oblique with respect to a line passing midway between the eyes and perpendicular to the frontal plane. Performance on the direction discrimination task was significantly better for the noncyclopean than for the cyclopean target, but the difference was not great. For the cyclopean target, the lowest value of the direction discrimination threshold was 0.70 deg (mean of three observers and two meridians). The Weber fraction for discriminating speed was not significantly different for the cyclopean and monocularly visible targets, and did not depend on the direction of motion in depth. The lowest values (mean of three observers and two meridians) were 0.12 (cyclopean) and 0.10 (noncyclopean). Findings did not scale for viewing distance. We propose that the human visual pathway contains: (a) a cyclopean mechanism sensitive to variations in the ratio (d phi/dt)/(d delta/dt) that is comparatively insensitive to both d phi/dt and d delta/dt; and (b) a speed-sensitive cyclopean mechanism that responds to variations in the value of d delta/dt, but is comparatively insensitive to d phi/dt. We also propose that a single speed-sensitive mechanism determines speed discrimination thresholds for both cyclopean and monocularly visible targets.

Adult↗

PubMed1996

Two-dimensional aspect ratio discrimination for shape defined by orientation texture.

A pattern of 12,860 short (0.15 x 0.05 deg) texture lines contained an orientation texture-defined (OTD) rectangle of aspect ratio a/b (a was the rectangle's height and b was its width). All the lines within the rectangle had the same orientation and all the lines outside the rectangle had the same orientation. These two orientations were theta deg symmetrically clockwise and anticlockwise of the vertical. The rectangle's visibility was created entirely by the orientation difference 2 theta. Aspect ratio discrimination threshold for the texture-defined rectangle was a U-shaped function of theta that was approximately symmetrical about theta = 45 deg. The lowest values of aspect ratio discrimination threshold were 2.8% (SE = 0.1%), 2.7% (SE = 0.1%) and 5.1% (SE = 0.3%) for three observers. A luminance-defined (LD) rectangle with the same spatial sampling as the OTD rectangle was created by removing all texture lines outside the rectangle. Aspect ratio discrimination threshold for the LD rectangle was 1.1% (SE = 0.1%), 1.7% (SE = 0.1%) and 2.2% (SE = 0.1%)) for the same three observers. Although these discrimination thresholds were not greatly lower than discrimination thresholds for the OTD rectangle, they were significantly lower at the P < 0.001 level. Discrimination thresholds for the OTD rectangle are comparable with the lowest aspect ratio discrimination thresholds for motion-defined (MD) rectangles (2 and 3% for two observers), and for disparity-defined (DD) rectangles (3.1, 3.4, 4.0 and 7.4% for four observers), even though the MD and DD rectangles were much smaller than the 185 deg2 OTD rectangle used in the present study.

Adult↗

PubMed1996

Accuracy of reproducing angles: is a right angle special?

The ability to set the angle of a 'V' to a designated value in the following three conditions was compared: (1) verbal designation of V angle; (2) initial 30 s visual demonstration of the designated V angle; (3) verbal designation of V angle plus feedback after every setting. The designated angles were 90 degrees and 45 degrees plus three arbitrary angles (65 degrees, 125 degrees, and 145 degrees). Each run comprised thirty consecutive settings. To ensure that our observers based their settings entirely on V angle it was arranged that line orientation did not provide a reliable cue to V angle. In condition (1), accuracy of setting V angle was significantly worse when the designated angle was other than 90 degrees or 45 degrees. This was not the case in condition (2), indicating that observers maintained a memory of the initial demonstration throughout a run of thirty settings. Setting error was not significant in condition (3) for any of the five angles. However, even in condition (3), setting-to-setting variability was significantly lower for the 90 degrees angle than for the other angles.

Adult↗

PubMed1996

Iodine-131-anti-B1 radioimmunotherapy for B-cell lymphoma.

PURPOSE: The CD20 B-lymphocyte surface antigen expressed by B-cell lymphomas is an attractive target for radioimmunotherapy, treatment using radiolabeled antibodies. We conducted a phase I dose-escalation trial to assess the toxicity, tumor targeting, and efficacy of nonmyeloablative doses of an anti-CD20 monoclonal antibody (anti-B1) labeled with iodine-131 (131I) in 34 patients with B-cell lymphoma who had failed chemotherapy. PATIENTS AND METHODS: Patients were first given tracelabeled doses of 131I-labeled anti-B1 (15 to 20 mg, 5 mCi) to assess radiolabeled antibody biodistribution, and then a radioimmunotherapeutic dose (15 to 20 mg) labeled with a quantity of 131I that would deliver a specified centigray dose of whole-body radiation predicted by the tracer dose. Whole-body radiation doses were escalated from 25 to 85 cGy in sequential groups of patients in 10-cGy increments. To evaluate if radiolabeled antibody biodistribution could be optimized, initial patients were given one or two additional tracer doses on successive weeks, each dose preceded by an infusion of 135 mg of unlabeled anti-B1 one week and 685 mg the next. The unlabeled antibody dose resulting in the most optimal tracer biodistribution was also given before the radioimmunotherapeutic dose. Later patients were given a single tracer dose and radioimmunotherapeutic dose preceded by infusion of 685 mg of unlabeled anti-B1. RESULTS: Treatment was well tolerated. Hematologic toxicity was dose-limiting, and 75 cGy was established as the maximally tolerated whole-body radiation dose. Twenty-eight patients received radioimmunotherapeutic doses of 34 to 161 mCi, resulting in complete remission in 14 patients and a partial response in eight. All 13 patients with low-grade lymphoma responded, and 10 achieved a complete remission. Six of eight patients with transformed lymphoma responded. Thirteen of 19 patients whose disease was resistant to their last course of chemotherapy and all patients with chemotherapy-sensitive disease responded. The median duration of complete remission exceeds 16.5 months. Six patients remain in complete remission 16 to 31 months after treatment. CONCLUSION: Nonmyeloablative radioimmunotherapy with 131I-anti-B1 is associated with a high rate of durable remissions in patients with B-cell lymphoma refractory to chemotherapy.

Adult↗

PubMed1996

Collision avoidance: a helicopter simulator study.

METHOD: Six pilots flew a helicopter flight simulator with a helmet-mounted display. The pilot's task was to judge the time to collision with a second helicopter that disappeared at either 2, 2.5, or 3 s before collision. The field of view (FOV) was 66 degrees (vertical) x 127 degrees or 19 degrees x 25 degrees or 3 degrees x 3 degrees. RESULTS: With a stationary target and ownship forward speed of 60 kt, pilots estimated that collision with a stationary target would occur roughly 200 ms before the actual time to collision in the large and intermediate FOV conditions. In the case of ownship stationary and the target approaching at 60 kt, estimates of collision time showed considerably less lead in the large and intermediate FOV conditions. When the smallest FOV was used, estimates were the same whether closing speed was produced entirely by ownship motion or entirely by target motion. CONCLUSION: We suggest that the too-early errors in estimating time to collision were caused by the impression of self-motion produced by stimulating the peripheral retina with a radially-expanding flow pattern.

Accident Prevention↗

PubMed1995

An audio-visual convergence area in the human brain.

By recording the magnetic field of the human brain while simultaneously presenting light to the eye and sound to the ear we have identified a brain region where auditory and visual signals converge. The location of this region is close to primary auditory cortex and far from primary visual cortex.

Auditory Pathways↗

PubMed1995

Orientation discrimination in cyclopean vision.

Using a dynamic random-noise display we measured orientation discrimination threshold for two kinds of cyclopean bar and for a cyclopean edge. Ocular vergence was monitored by means of nonious lines. Orientation discrimination threshold for a cyclopean bar or cyclopean edge fell to a minimum at some disparity between about 2 and 50 min arc. For the three subjects tested with bars, the minimum value of discrimination threshold lay between 0.6 and 1.5 deg, and for the two subjects tested with the edge between 0.7 and 2.0 deg. The lowest discrimination thresholds for the cyclopean bars were similar for crossed and uncrossed disparities for all three subjects tested. Matched depth increased smoothly with increasing disparity through a range over which orientation discrimination threshold fell and then levelled out. We conclude that the processing of the depth of a cyclopean form is dissociated from the processing of the orientation of that same cyclopean form. We suggest that orientation discrimination of cyclopean form is determined by the relative activity of binocular, disparity-sensitive, orientation-tuned neurons.

Adult↗

PubMed1995

Visual processing of looming and time to contact throughout the visual field.

We measured discrimination threshold for time to contact with a simulated approaching object at 20 locations between 0 and 32 deg eccentricity in the left, right, upper, and lower visual fields. We also measured discrimination threshold for rate of expansion at the same 20 locations. At 0 deg eccentricity, discrimination of trial-to-trial variations in time to contact was virtually unaffected by simultaneous trial-to-trial variations of both rate of expansion and starting size, discrimination of trial-to-trial variations in rate of expansion was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and starting size, and discrimination of trial-to-trial variations in starting size was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and rate of expansion. We conclude that, in foveal vision, time to contact, rate of expansion and size can be processed simultaneously, independently and in parallel. Our main finding was that this independence progressively decreased as eccentricity increased. For example, in peripheral, but not in foveal vision, variations in rate of expansion produced illusory variations in time to contact. A secondary finding was that the effect of eccentricity on discrimination threshold for the task-relevant variable (whether time to contact or rate of expansion) was considerably less than the effect of eccentricity on visual acuity and on several other aspects of visual performance. We suggest that visual processing of time to contact is developed by exposure to optic flow patterns created by self-locomotion.

Adult↗

PubMed1995

Visual processing of the motion of an object in three dimensions for a stationary or a moving observer.

A rate of change of relatively disparity is a sufficient binocular stimulus for the perception of motion in depth. For motion within the meridian that contains the eyes, disparity change associated with approaching motion is processed through four channels, each tuned to a different direction of motion in depth. Directional discrimination is most acute but detection sensitivity falls to a minimum for a trajectory passing approximately midway between the eyes. This can be explained if discrimination depends on the relatively of the four channels. Two binocular retinal-image correlates of the direction of the motion of an object in depth are the ratio between the velocities of the retinal images of the object in the left and right eyes [(d phi/dt)R/(d phi/dt)L], and the ratio between the translational velocity of the binocularly fused images and the rates of change of disparity [(d phi/dt)/(d gamma/dt)]. Directional discrimination is possible by using the second cue alone. An isotropic rate of expansion of the retinal image is a sufficient monocular stimulus for the perception of motion in depth. There is no evidence that expansion is processed through channels tuned to the direction of motion in depth. Two monocular correlates of the direction of the motion of an object in depth are the ratio between the translational velocity and the rate of expansion of the retinal image of the object [(d phi/dt)/(d theta/dt)], and the ratio between the velocities of opposite edges of the retinal image [(d alpha 1/dt)/(d alpha 2/dt)]. Subjects are able monocularly to discriminate the direction of motion in depth with high acuity (better than 0.1 deg threshold) in the vertical, horizontal, or oblique meridians, even when the direction and the speed of translational motion are removed as cues. Visual discrimination of time to contact with an approaching object can be disconfounded from discrimination of its rate of expansion and vice versa with a threshold separation ratio of as much as 100:1.

Cues↗

PubMed1995

Parallel independent encoding of orientation, spatial frequency, and contrast.

Subjects were presented with a set of 216 test gratings in random order. Each had a different combination of orientation, spatial frequency, and contrast. For each test grating, subjects were instructed to judge whether or not orientation was clockwise of the mean of the stimulus set, whether or not spatial frequency was higher than the mean of the stimulus set, and whether or not contrast was higher than the mean of the stimulus set. Each of the three sets of button presses was analyzed with respect to each of the three parameters, giving nine psychometric functions from one response set. It is concluded that, for gratings of high visibility, changes of orientation, spatial frequency, and contrast are encoded independently and in parallel, at least for small changes in these three visual parameters. In another experiment only one of the three parameters was varied at a time. Neither orientation-discrimination threshold, nor spatial-frequency-discrimination threshold, nor contrast-discrimination threshold was appreciably, if at all, lower than when all three parameters were varied simultaneously. It is concluded that interactions between the processing of small changes in orientation, spatial frequency, and contrast are negligible when all three are processed simultaneously. It is proposed that trial-to-trial variations of orientation, spatial frequency, and contrast are unconfounded by opponent processing within a population of neurons, each of which confounds the three variables.

Adult↗

PubMed1995

Orientation discrimination for bars defined by orientation texture.

A texture pattern consisting of short (0.2 deg) lines contained a 5.0 deg x 1.4 deg texture-defined bar. The bar was rendered visible by the difference in orientation (2 theta) between the lines inside the bar and outside the bar. Orientation-discrimination threshold for the texture-defined bar was a U-shaped function of 2 theta over the range 2 theta = 0 degree to 2 theta = 180 degrees. The lowest threshold was at 2 theta = 90 degrees, and was 0.57 degrees for both subjects tested. This threshold was little different from the lowest values of threshold for motion-defined bars, disparity-defined bars, and colour-defined gratings reported elsewhere. A luminance-defined bar was created by switching off all texture lines outside the texture-defined bar. Orientation-discrimination threshold fell to a limiting value as the luminance contrast of this bar was progressively increased. The lowest value of orientation discrimination for the luminance-defined bar (0.42 degrees and 0.35 degrees for the two subjects) was not greatly less than the lowest value for the texture-defined bar.

Adult↗