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Biomedical subjects

W L Larson

Publications and source records attributed to W L Larson.

At least 19 recordsLinked to original sources
PubMed1997

Collet-Sicard syndrome mimicking neuralgic amyotrophy.

A man with shoulder pain, wasting, and weakness had ipsilateral cranial nerve abnormalities. Electrodiagnostic studies supported a diagnosis of neuralgic amyotrophy, but we later demonstrated a spinal accessory mononeuropathy with ipsilateral hypoglossal weakness (Collet-Sicard syndrome). Magnetic resonance imaging demonstrated an inaccessible occipital condyle mass, and disseminated adenocarcinoma was subsequently diagnosed. Although cranial mononeuropathies can occur in neuralgic amyotrophy, this case illustrates the importance of identifying a focal lesion, and highlights the localizing value of electrodiagnosis.

Aged↗
PubMed1996

Relationships between motor-unit number estimates and isometric strength in distal muscles in ALS/MND.

Measurement of progression in amyotrophic lateral sclerosis/motor neuron disease (ALS/MND) is useful for charting the natural history and assessing efficacy in drug trials. Common measures are maximal voluntary isometric contraction (MVIC) which mainly focuses on-proximal muscles, and electrophysiologic measures of the compound muscle action potential (CMAP) and motor-unit number estimation (MUNE) which focus on distal muscles. We have undertaken a study to compare the relationships between MVIC, CMAP and MUNE recorded in intrinsic hand muscles of 10 subjects with ALS/MND. Grip and pinch (between the first and fifth digits) MVIC were recorded. The CMAP and MUNE (determined by the multipoint stimulation technique) were recorded from thenar and hypothenar muscle groups. To facilitate comparisons between strength and electrophysiologic measures, the MUNE values and the CMAP values from the thenar and hypothenar muscle groups were each summed. Test-retest correlations were high for all measures (r = 0.75-0.99). Pinch and grip MVIC were highly correlated (r = 0.83). However, MVIC measures showed weaker correlations with summed MUNE values (pinch r = 0.56, grip r = 0.64) and summed CMAP values (pinch r = 0.58 and grip r = 0.65). The weak correlations between MVIC and electrophysiologic measures are due to two factors. First, grip and pinch MVIC are correlated through co-activation of agonist muscle groups, for we recorded strong concomitant muscle activity from forearm flexor muscles during pinch. In contrast, CMAP and MUNE reflect measurement from isolated muscle groups in the hand. Second, MVIC and the CMAP are affected by collateral reinnervation and only indirectly assess motor-unit loss, while MUNE is uninfluenced by reinnervation and directly addresses the degree of motor-unit loss. These factors determine the information available from endpoint measures. We offer guidelines for choosing useful measures based upon the goals of a given study.

Action Potentials↗
PubMed1995

Foot drop as a harbinger of aortic occlusion.

A man presented with foot drop and 2 months later an infrarenal occlusion of his abdominal aorta was discovered. Initial evaluation found no structural etiology for his nerve injury. His neurologic deficits progressed until gangrenous changes developed in his feet leading to the discovery of aortic occlusion. This case demonstrates that peripheral nerve injury in the legs may signal underlying large vessel occlusive disease. We discuss the localization and potential mechanisms of ischemic nerve injury.

Adult↗
PubMed1995

Disabled stereopsis may be the norm among well-educated people.

Most well-educated people, where the author lives, have lost the ability to perceive the third dimension in space during everyday life. This has passed unnoticed because poor depth perception is not a serious handicap in an urban environment and because clinical depth perception tests have not been designed to detect this deficit. The author's hypothesis is that disabled stereopsis is a global phenomenon. A simple way of testing for disabled stereopsis is presented, in the hope that investigators in other parts of the world will test a sample of their local population and send the results to the author.

Depth Perception↗
PubMed1994

An investigation of prism adaptation latency.

PURPOSE: The latency of adaptation of the human vergence system to a change in convergence or divergence forced upon it by a prism was investigated in this pilot study. METHODS: Adaptation was stimulated by a 5-s period of binocular vision through a prism of 0 delta, -8 delta, or 8 delta. Immediately thereafter and for a further period of 45 s, lateral heterophoria was monitored subjectively by an automated version of Duane's screen and parallax test. Adaptation was calculated from the phoria 40 s after the end of binocular vision. Tests were performed at 0.4 m. In the first test session, there was screening during the binocular period to test for flaws in the screening method. In the second session, there was no screening during the binocular period and phoria measurement was started without changing the power of the prism. In the third session, the prism was restored to 0 delta after the period of binocular vision. Immediately after the last 5-s test, tests were repeated with 1 s of binocular vision. The subject had excellent visual acuity, stereoacuity, and stereolatency. RESULTS: The test functioned correctly and showed good repeatability. The greatest adaptation to 8 delta was 59%. This was obtained with only 1 s of binocular vision. There was adaptation to -8 delta with 5 s of binocular vision but it was obscured by adaptation to 0 delta or 8 delta which persisted from previous tests. DISCUSSION: The reason why the subject's latency was not found more precisely is explained. Additional evidence is presented in support of the finding that prism adaptation can take place within 1 s (perhaps less) of binocular vision, and the persistence and dominance of adaptation to base-out prism at near is pointed out. Guidelines are proposed for the experimental measurement of prism adaptation latency.

Adaptation, Ocular↗
PubMed1992

Clinical measurement of depth perception by means of motion parallax: a case report.

An automated 2-rod stereoacuity test was modified to permit lateral head movement during testing so that depth perception by means of motion parallax could be assessed. The patient, a 48-year-old truck driver with one blind eye, found the test easy to perform. In 24 presentations of a 40 sec arc disparity he was 96% correct. With the same number of presentations at 20 sec arc he was 83% correct. In the latter case, the probability of success by chance alone was p = 0.0004.

Depth Perception↗
PubMed1992

Prevalence of disabled stereopsis in a class of optometry students.

Stereopsis is said to be disabled when the third dimension (3-D) cannot be perceived during everyday life but can be when viewing conditions are appropriately arranged. To discover a test suitable for detecting disabled stereopsis, I measured stereoacuity, stereolatency, and the subjective difference in depth perception between monocular and binocular vision of a group of 41 optometry students. Only 2 students showed no sign of this disability. A predictive value for each test was calculated from the results of 14 students whose disability was confirmed by another means. Stereoacuity had the worst predictive value (43%) and stereolatency the best (100%). The prevalence of disabled stereopsis in this group was no less than 34% and might have been as great as 95%.

Depth Perception↗
PubMed1992

Stereolatency: a stereopsis test for everyday depth perception.

An instrument is described which tests for the use of stereopsis by measuring how long it takes for depth perception to begin (its latency) after a transition from monocular to binocular vision. Starting at 1 s, the duration of binocular vision is reduced progressively toward a limit of 16 ms (or increased to a limit of 4 s) until the latency of stereopsis is found. The preliminary period of monocular vision acts as a probe for suppression of either eye and allows time for accommodation and vergence to stabilize before binocular vision begins. Typical results are presented.

Depth Perception↗
PubMed1991

Stereopsis after unilateral anterior temporal lobectomy. Dissociation between local and global measures.

In man, an impairment in stereopsis may occur following damage outside the occipital lobes. If local and global stereopsis can be dissociated, this may imply that different mechanisms mediate these functions. We tested 44 patients with unilateral anterior temporal lobe excisions and 23 normal control subjects on two tasks. In the local stereopsis task, subjects indicated which of two pins varying in disparity between 4 and 512 s of arc was closer. Results showed no threshold impairment in any group. In the global stereopsis task, random-dot stereograms varying in binocular correlation were presented in random order, and subjects indicated if the squares perceived in depth were in front of or behind the screen. At binocular correlations between 50% and 70%, left and right temporal lobectomy resulted in a deficit, which was more marked following right-sided excisions. These results are concordant with earlier work (Ptito and Zatorre, 1988), and with other evidence of a right hemisphere superiority in treating binocular depth information. Moreover, the fact that global stereopsis is impaired in the presence of intact local stereopsis suggests that closely related but not identical mechanisms are involved, and fits the notion that there is a hierarchical organization of the visual pathways originating in the striate cortex leading into temporal cortex. Global stereopsis would thus be mediated in temporal lobe areas that receive information about local disparities from more posterior regions.

Adult↗
PubMed1991

Effect of induced blur on visual acuity and stereoacuity.

The acuities of three subjects with good visual acuity and stereoacuity were reduced by the addition of diffusing filters placed before one or both eyes; these subjects then underwent automated stereoacuity and visual acuity tests. Results were idiosyncratic and differed from subject to subject to such an extent that no general rule sufficed to describe the effect of different degrees of blur on visual acuity and stereoacuity. It had been thought that blur degraded stereoacuity more than visual acuity but most of the results from one subject indicated the opposite effect. The effects of monocular vs. binocular blur are discussed.

Form Perception↗
PubMed1991

A definitive case of paradoxical stereopsis.

Author JF had perfect stereoacuity but was unaware that he did not make much use of stereopsis under everyday conditions of seeing, i.e., his stereopsis was paradoxical. We describe how this condition was diagnosed and everyday depth perception restored. The objective of this report is to demonstrate convincingly that paradoxical stereopsis exists as a clinical entity. A combination of signs and symptoms is presented as a syndrome for the diagnosis of paradoxical stereopsis.

Depth Perception↗
PubMed1990

An investigation of the difference in stereoacuity between crossed and uncrossed disparities using Frisby and TNO tests.

Is stereoacuity better, on average, with a crossed test disparity? To answer this question, 15 subjects with good stereoacuity performed Frisby and TNO stereotests with test plates in each of 2 possible orientations, crossed and uncrossed. Stereoacuities and response times were measured. The answer was that a subject's stereoacuity with crossed and uncrossed disparity could be different, e.g., 30 and 480 sec arc, respectively, but that, on average, a crossed disparity was no easier to detect than an uncrossed one. There were more and greater differences with the TNO than with the Frisby test and Frisby stereoacuity was, on the average, 4 times better. It is recommended that tests which can have two orientations be tested in both and the best result taken as the stereoacuity.

Depth Perception↗
PubMed1990

Prism adaptation without binocular vision.

The assumption that vergence adaptation to a prism could only take place during single binocular vision was investigated by having subjects wear a prism while fusion was dissociated. Adaptation was evaluated by its effect on lateral heterophoria. Dissociation and measurement of heterophoria were by means of an automated version of Duane's screen and parallax test. A preliminary investigation with 37 subjects fixating at 6 m while wearing 4 delta base-out and with 33 other subjects fixating at 50 cm while wearing 3 delta base-in showed that at least 25% adapted to the prism. Results of an investigation with 2 subjects fixating at 6 m (4 delta base-out and base-in) and with 3 subjects fixating at 50 cm (3 delta base-in) are presented as evidence that prism adaptation can take place. It was also found that the power of the added prism could just as well be provided by the phoria test's variable prism. In other words, there is a possibility that adaptation will occur when heterophoria is measured with a variable prism.

Adaptation, Ocular↗
PubMed1990

A simple method for improving stereopsis.

Presented here is a method for the improvement of stereopsis. It is based on covering and uncovering one eye in a particular way. A complete program for the improvement of stereopsis is described. This includes an uncovering technique, the use of an Orthofuser, and the observation of specular reflections. Examples are given which demonstrate the effectiveness of the method. Persons who are capable of stereopsis but who use it selectively or not at all in everyday life (paradoxical stereopsis) are particularly easy to cure.

Adult↗
PubMed1988

Effect of TNO red-green glasses on local stereoacuity.

Local stereoacuity was reduced when red-green anaglyph glasses were worn. Glasses were from the TNO stereotest. Without these glasses, all 15 subjects had good or almost good local stereoacuity. With the glasses, all stereoacuities were reduced but one subject still had a good stereoacuity of 8 sec arc. Reductions with the TNO anaglyph glasses ranged from 2 to 34 sec arc. The TNO test was also performed to see if there was a difference between local and global stereoacuity while wearing the glasses. Results were similar for 6 subjects, all of whom had the best TNO stereoacuity of 15 sec arc. Global (TNO) stereoacuity was worse than local stereoacuity for the other subjects with the most extreme difference being 464 sec arc. If aniseikonia was induced by the red-green glasses, this could not have contributed to a reduction in either local or global stereoacuity.

Color↗