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At least 937 records · Page 52Linked to original sources
PubMed1978

Fresnel prisms and their effects on visual acuity and binocularity.

1. The visual acuity with the Fresnel membrane prism is significantly less than that with the conventional prism of the same power for all prism powers from 12 delta through 30 delata at distance and from 15 delta through 30 delta at near. 2. The difference in the visual acuity between base up and base down, and between base in and base out, is not significantly different for either the Fresnel membrane prism or for the conventional prism. 3. For both Fresnel membrane prism and the conventional prism, the visual acuity when looking straight ahead. 4. Using Fresnel membrane prisms of the same power from different lots, the visual acuity varied significantly. The 30 delta prism caused the widest range in visual acuity. 5. When normal subjects are fitted with the higher powers of the Fresnel membrane prism, fusion and stereopsis are disrupted to such an extent that the use of this device to restore or to improve binocular vision in cases with large-angle deviations is seriously questioned. 6. Moreover, the disruption of fusion and stereopsis is abrupt and severe and does not parallel the decrease in visual acuity. The severely reduced ability to maintain fusion may be related to the optical aberrations, which, in turn, may be due to the molding process and the polyvinyl chloride molding material. 7. Through the flexibility of the membrane prism is a definite advantage, because of its proclivity to reduce visual acuity and increase aberrations its prescription for adults often must be limited to only one eye. 8. For the same reasons in the young child with binocular vision problems, the membrane prism presently available should be prescribed over both eyes only in powers less than 20 delta. When the membrane prism is to be used as a partial occluder (over one eye only), any power can be used. 9. The new Fresnel "hard" prism reduces visual acuity minimally and rarely disrupts binocularity, thus increasing the potential for prismotherapy to establish binocularity. This prism is currently available only for use as a trial set. Since the cosmetic appearance of the Fresnel "hard" prism is similar to that of the Fresnel membrane prism and it is easier to maintain, it would be the prism of choice (over all other types) for bilateral prescriptions in the young patient with emmetropia. The manufacturer is urged to make these prisms available to fit a special round adjustable frame, such as that developed in Europe for use with the wafer prism.

Adolescent↗
PubMed1994

Three-dimensional versus two-dimensional video system for the trained endoscopic surgeon and the beginner.

Because of the lack of depth perception in current two-dimensional endoscopic video systems, the performance of fine motor movements is impaired. Endoscopic surgeons are generally able to cope with this limitation, and the performance of complex procedures becomes easy with experience. To assess the value of three-dimensional vision, two groups of surgeons, with and without experience in endoscopic surgery, underwent dexterity tests at a simulator. Data from use of two- and three-dimensional video systems were analysed and are discussed in this paper. The three-dimensional system improved the ability to perform endoscopic procedures, particularly when the highest degree of hand-eye coordination was needed. A common sensation of ocular fatigue, related to the prolonged use of this equipment, may limit its usefulness in advanced endoscopic procedures.

Clinical Competence↗
PubMed1994

[Laparoscopic cholecystectomy].

The author presents a new surgical procedure-laparoscopic cholecystectomy. Comparing the time interval from introduction to widespread adaption of other new operations this century, such as vagotomy and open cardiac procedures, it took remarkably little time for the surgical community to accept laparoscopic cholecystectomy. To perform endoscopic surgery, surgeons had to improve eye-hand coordination, adapt to a new tactile sensation and lack of depth perception. The laparoscopic cholecystectomy is "here to stay". The author reports preintervention program as well as the procedure and its technique with necessary instruments. Laparoscopic cholecystectomy is a recently developed alternative to open cholecystectomy that is rapidly gaining popularity because patients have experienced reduced stay in hospital, decreased wound pain and early return to normal activity.

Cholecystectomy, Laparoscopic↗
PubMed1994

Three-dimensional computed tomography imaging in the evaluation of craniofacial abnormalities.

A retrospective study was undertaken to assess the value of three-dimensional computed tomography (3-D CT) in evaluating craniofacial abnormalities. Over a 2-year period, 145 children evaluated with routine two-dimensional (2-D) CT had 3-D CT reformatted images assessing a variety of anomalies including craniosynostosis, midface clefts, trauma, and craniofacial syndromes. There were 62 females and 83 males ranging in age from 1 month to 17 years (median: 5 months). Both soft-tissue and bone algorithms were used. In all 145 cases, the 3-D CT images confirmed or provided additional information of depth perception, contours, volumes, and extent of an abnormality. This was especially important and useful in the settings of complex anomalies and preoperative planning. We concluded that 3-D CT reconstructed images in conjunction with routine 2-D CT should be an integral part of the examination in evaluating craniofacial abnormalities.

Adolescent↗
PubMed1996

Misoprostol and ranitidine in the prevention of NSAID-induced ulcers: a prospective, double-blind, multicenter study.

OBJECTIVE: To compare ranitidine to misoprostol with respect to the prevention of gastric and duodenal ulcers in patients on chronic NSAID therapy. METHODS: A multi-center, 8-wk, randomized, double-blind study. Eligible patients were on chronic NSAID therapy and were experiencing NSAID-related upper gastrointestinal (UGI) pain without UGI endoscopic evidence of gastric or duodenal ulcers. Patients enrolled in the study were randomized to either misoprostol 200 micrograms q.i.d. or ranitidine 150 mg b.i.d.. Follow-up UGI endoscopy was performed after 4 and 8 wk of treatment. Therapeutic failure was considered the development of a gastric or duodenal ulcer > or = 0.3 cm in diameter with perceptible depth. RESULTS: Gastric ulcers were found in only 1/180 (0.56%) patient on misoprostol and in 11/194 (5.67%) patients on ranitidine, a difference that was statistically significant (p < 0.01). Duodenal ulcer rates were similar for the ranitidine (2/185 or 1.08%) and misoprostol (2/181 or 1.10%) groups. CONCLUSION: Misoprostol is significantly more effective than ranitidine in the prevention of NSAID-induced gastric ulcers. Ranitidine was as effective as misoprostol for the prevention of NSAID-induced duodenal ulcers. Misoprostol should be used for prophylaxis against both gastric and duodenal ulceration in patients on chronic NSAID therapy.

Administration, Oral↗
PubMed1996

Receptive field asymmetries and sensitivity to random dot stereograms.

The differences between the two monocular receptive fields of cortical cells were measured and compared to their disparity tuning in the awake behaving monkey. Several receptive field properties (direction selectivity, orientation preference, eye preference and response modulation) were determined for each eye using sweeping bright bars. The disparity sensitivity of these cells was also assessed by plotting their response profile, determined for each cell under strictly binocular cyclopean stimulation (dynamic random dot stereograms, RDS). We have found that large differences between the two monocular receptive fields were infrequent and, apparently, not related with the disparity sensitivity profile. We conclude that the monocular asymmetries tested in this study and the sensitivity to positional binocular disparities present in RDS, might be linked to different mechanisms involved in depth perception in the visual system.

Animals↗
PubMed1996

[How good is spatial orientation with rigid endoscopic optics? Study of micromanipulation in the nasal cavity].

Rigid endoscopes are used as an alternative to the operating microscope for visual control during paranasal sinus surgery. Since the view through the endoscope is monocular, perception of depth (stereopsis) through the operating microscope must be expected to be superior due to the binocular view afforded. To investigate whether monocular visualization through an endoscope is a disadvantage in paranasal sinus surgery, we compared stereoacuity in a model of the nasal cavity, using a head lamp, an operating microscope and a 0 degree Hopkin's endoscope. Twenty volunteers had to touch defined points in a spatial model of the nasal cavity. Under binocular vision with a head lamp, performance was significantly better than with the microscope or the endoscope. Manipulations were performed with the same speed under microscopic and endoscopic control, although under microscopic guidance more faults in point sequence were made than with the endoscope. Various monocular phenomena seem to allow sufficient spatial orientation through the endoscope, so that monocular visualization does not appear to be a disadvantage for quick and safe manipulations during functional endoscopic sinus surgery.

Depth Perception↗
PubMed1977

Iridocapsular lenses versus iris-clip lenses: comparison of the results and complications of 100 of each.

The results and complications of implantation of 100 iridocapsular and 100 iris-clip lenses are evaluated. The follow-up period for the first group varied from one to 25 months (mean 12 months) and from one to 43 months (mean 27 months) for the second group. I performed all of the operations and most of the follow-up examinations. Of the patients with iridocapsular lenses, 86 percent and of those with iris-clip lenses, 87 percent obtained a visual acuity of 0.5 (6/12) or better. Binocular vision, positive depth perception, and a small degree of aniseikonia (3% or less) are evident optical benefits. The complicatioons most frequently necessitating a second operation were retained lens material (17%) and repositioning the pseudophakos (6%) in the iridocapsular group. The four-loop lenses caused fewer problems; in one case, posterior luxation necessitated its removal.

Adolescent↗
PubMed1976

[The bilateral paralysis of the abducens nerve. A contribution to industrial medical judgement (author's transl)].

After a brief statistical survey the anotomic basis is explained in a broad outline. In the description of a case with bilateral paralysis of the abducens nerve, the suffering of the patient in his working life is examined and the phenomenon of inacurrate depth projection, of "pointing past", is discussed. As a result the suitability for employment is more highly reduced for those suffering from bilateral paralysis of the abducens nerve, as compared to persons with one eye. During the report a 16 mm Color film was shown, showing the patient before and after the O'Connor operation. The possibility of keeping a progress or control report, of patients suffering from inacurrate depth perception, was demonstrated.

Abducens Nerve↗
PubMed1997

Artificial vision workbench.

Machine vision is an important component of medical systems engineering. Inexpensive miniature solid state cameras are now available. This paper describes how these devices can be used as artificial retinas, to take snapshots and moving pictures in monochrome or color. Used in pairs, they produce a stereoscopic field of vision and enable depth perception. Macular and peripheral vision can be simulated electronically. This paper also presents the author's design of an artificial orbit for this synthetic eye. The orbit supports the eye, protects it, and provides attachment points for the ocular motion control system. Convergence and image fusion can be produced, and saccades simulated, along with the other ocular motions. The use of lenses, filters, irises and focusing mechanisms are also discussed. Typical camera-computer interfaces are described, including the use of "frame grabbers" and analog-to-digital image conversion. Software programs for eye positioning, image manipulation, feature extraction and object recognition are discussed, including the application of artificial neural networks.

Artificial Organs↗
PubMed1998

Proton-beam irradiation of subfoveal choroidal neovascular membranes in presumed ocular histoplasmosis syndrome: a case report.

BACKGROUND: Presumed ocular histoplasmosis syndrome (POHS) refers to a choroidopathy that is characterized by the presence of multiple peripheral atrophic chorioretinal scars, peri-papillary atrophy, and choroidal neovascular membranes (CNVM), usually in or adjacent to the fovea. In the United States, POHS is an important cause of loss of central visual acuity in patients between the ages of 20 and 50 years. A number of treatment options for subfoveal and juxtafoveal CNVMs in POHS have been under investigation, including laser photocoagulation, surgical excision of the CNVM, and radiation therapy. CASE REPORT: A 28-year-old women was referred to our office reporting decreased depth perception and finger-counting vision in the right eye for the duration of 1 month. A diagnosis of POHS with subfoveal CNVM was made and the patient was referred for an experimental protocol of proton-beam irradiation. Four months after her initial visit, the patient returned, reporting blurry vision with a blind spot in her left eye. A subfoveal CNVM in the left eye was subsequently treated with irradiation as well. Seven months after the initial treatment, visual acuities were 20/20 in each treated eye. CONCLUSION: Although is currently an experimental procedure, proton-beam irradiation appears to be a promising treatment for subfoveal CNVM in patients with POHS.

Adult↗
PubMed1998

The hazard of spatial disorientation during helicopter flight using night vision devices.

INTRODUCTION: Night Vision Devices (NVDs) provide an enormous advantage to the operational effectiveness of military helicopter flying by permitting flight throughout the night. However, compared with daytime flight, many of the depth perception and orientational cues are severely degraded. These degraded cues predispose aviators to spatial disorientation (SD), which is a serious drawback of these devices. METHODS: As part of an overall analysis of Army helicopter accidents to assess the impact of SD on military flying, we scrutinized the class A-C mishap reports involving night-aided flight from 1987 to 1995. The accidents were classified according to the role of SD by three independent assessors, with the SD group further analyzed to determine associated factors and possible countermeasures. RESULTS: Almost 43% of all SD-related accidents in this series occurred during flight using NVDs, whereas only 13% of non-SD accidents involved NVDs. An examination of the SD accident rates per 100,000 flying hours revealed a significant difference between the rate for day flying and the rate for flight using NVDs (mean rate for daytime flight = 1.66, mean rate for NVD flight = 9.00, p < 0.001). The most important factors associated with these accidents were related to equipment limitations, distraction from the task, and training or procedural inadequacies. CONCLUSIONS: SD remains an important source of attrition of Army aircraft. The more than fivefold increase in risk associated with NVD flight is of serious concern. The associated factors and suggested countermeasures should be urgently addressed.

Accidents, Aviation↗
PubMedDate not supplied

Image rotation and reversal--major obstacles in learning intracorporeal suturing and knot-tying.

BACKGROUND AND OBJECTIVES: A major stumbling block to teaching and learning the finer skills of laparoscopy is related to the "optical illusions" the video camera plays on the surgeon's eyes. Until now, the belief was that lack of coordination was the result of depth perception deficiencies resulting from the two dimensional plane of the video monitor. In reality, this is a minor problem that is easily surmounted with practice. A closer analysis of how organ orientation at the operative site compares to the video camera's fields of focus reveals the real problem: the major optical difference between laparotomy and laparoscopy involves rotation of the images received by the brain. CONCLUSIONS: There are four major operating positions in laparoscopy: camera position, right camera position, left camera position and opposite camera position. The object in front of the camera has two components; the first, a reality image, which results from light reflected off the object as it exists in time and space. The second, a visual image, which represents the actual light entering our eyes. At right camera position the visual image is a 90 degrees counter-clockwise rotation of the reality image. At the left camera position the visual image is a 90 degrees clockwise rotation of the reality image. At opposite camera position, a 180 degrees rotation and complete reversal of the reality image occurs. It is only at camera position that the visual image is equal to the reality image, and we approach a scenario similar to that found in laparotomy. Every other position will be unlike what we were accustomed to in open surgery.

Clinical Competence↗
PubMed1999

Organization of disparity-selective neurons in macaque area MT.

Neurons selective for binocular disparity are found in a number of visual cortical areas in primates, but there is little evidence that any of these areas are specialized for disparity processing. We have examined the organization of disparity-selective neurons in the middle temporal visual area (MT), an area shown previously to contain an abundance of disparity-sensitive neurons. We recorded extracellularly from MT neurons at regularly spaced intervals along electrode penetrations that passed through MT either normal to the cortical surface or at a shallow oblique angle. Comparison of multiunit and single-unit recordings shows that neurons are clustered in MT according to their disparity selectivity. Across the surface of MT, disparity-selective neurons are found in discrete patches that are separated by regions of MT that exhibit poor disparity tuning. Within disparity-selective patches of MT, we typically observe a smooth progression of preferred disparities (e.g. , near to far) as our electrode travels parallel to the cortical surface. In electrode penetrations normal to the cortical surface, on the other hand, MT neurons generally have similar disparity tuning, with little variation from one recording site to the next. Thus disparity-tuned neurons are organized into cortical columns by preferred disparity, and preferred disparity is mapped systematically within larger, disparity-tuned patches of MT. Combined with other recent findings, the data suggest that MT plays an important role in stereoscopic depth perception in addition to its well known role in motion perception.

Animals↗
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↗
PubMed2006

Dichoptic motion perception limited to depth of fixation?

When counterphase spatio-temporal flicker is presented to the left and right eye continuous directional motion can be perceived. Here, we investigate whether this type of dichoptic motion can be observed at different depth planes. Four observers indicated direction of motion for dichoptic motion stimuli, presented in a context containing crossed and uncrossed disparity information in different conditions. Our results show that despite the presence of disparity cues in the stimulus, discrimination of motion direction remained maximal at interocular phase offsets that correspond to binocular motion perception at zero disparity. This constraint brings into question perception of dichoptic motion as the result of an early binocular motion system. We compared our results with predictions of a computational stereo-motion model [Qian, N. (1994). Computing stereo disparity and motion with known binocular cell properties. Neural Computations, 6, 390-404; Qian, N., & Andersen, R. A. (1997). A physiological model for motion-stereo integration and a unified explanation of Pulfrich-like phenomena. Vision Research, 37, 1683-1698]. In contrast to our empirical results, simulations of cell activation in this hybrid energy model predict maximal activation at non-zero disparities. It is concluded that perception of dichoptic motion is a by-product of early interocular combination at low contrasts rather than the result of a dedicated stereo-motion system.

Computer Simulation↗