Biomedical subjects

F I Porter

Publications and source records attributed to F I Porter.

8 recordsLinked to original sources

PubMed1991

Validation of physiologic predictors of successful telescopic spectacle use in low vision.

A group of 32 patients with low vision who were considered clinically appropriate candidates for visual rehabilitation with telescopic spectacles were prospectively studied before the first attempted use of these visual aids. Laboratory measurements were made of: (1) rotational head stability in pitch and yaw during quiet standing; (2) sensitivity of visual acuity with telescopic spectacles to imposed yaw head motion; and (3) ocular stabilization reflexes during passive, whole-body rotation in the horizontal plane. Predicted likelihood of successful use of telescopic spectacles was prospectively computed for each patient using the measurement of head stability in the pitch axis and the sensitivity of visual acuity with telescopic spectacles to head motion using a previously described statistical method. Patients were then given telescopic spectacles, and functional success was evaluated in the field at least 6 weeks later by independent masked observers. Although corrected visual acuities did not differ in the 24 patients in whom rehabilitation was successful or in the 8 patients in whom it was not, successful patients had statistically significantly less (P less than 0.05) angular head instability in pitch and yaw, as well as less impairment of visual acuity with telescopic spectacles during head motion. This finding was confirmed in a more clinically homogeneous subgroup of 16 patients who had low vision due to maculopathy. Gains of the 0.1 Hz horizontal vestibulo-ocular reflex (VOR) and visual-vestibulo-ocular reflex (VVOR) with 4X telescopic spectacles did not differ between patients in whom rehabilitation was successful and those in whom it was not.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1989

Eye movements in cocaine abusers.

Using electro-oculography, we quantitatively investigated eye movements in nine heavy cocaine abusers and three groups of controls. Plasma levels of 3-methoxy-4-hydroxyphenylglycol (MHPG), a major metabolite of brain norepinephrine, in cocaine abusers were reduced to an average of 53% of normal. Cocaine abusers had normal smooth pursuit, optokinetic nystagmus, vestibulo-ocular reflex, visual suppression of the vestibulo-ocular reflex, and caloric nystagmus. Data were suggestive of a significant reduction in the degree of immediate enhancement of visual-vestibulo-ocular reflex gain by magnified vision in the cocaine abusers. However, adaptive plasticity of the vestibulo-ocular reflex was normal in the cocaine abusers. These results suggest that, despite animal data linking vestibulo-ocular reflex plasticity to central norepinephrine, this neurotransmitter may not be important to plasticity in human beings.

Adult↗

PubMed1989

Predictors of functional success in telescopic spectacle use by low vision patients.

Telescopic spectacles can theoretically improve function of low vision patients by enlarging retinal images. However, unintended head movement may produce sufficient instability of enlarged retinal images to negate the visual benefit. We investigated this phenomenon as a cause of failure in 38 low vision patients who had previously attempted use of telescopic spectacles. Patients underwent evaluation of the vestibulo-ocular reflex, visual-vestibulo-ocular reflex, head stability in the pitch and yaw axes, and sensitivity of magnified visual acuity to head motion. Although it was impossible to distinguish successful from unsuccessful telescopic spectacle users by means of clinical or historical data, multiple logistic regression analysis was used to derive a useful predictive function based on measurements of sensitivity of magnified vision to head motion, and head instability in the pitch axis. Predictive performance was found to be superior to conventional clinical judgement. These findings support the hypothesis that retinal image stability is important to functional vision, and suggest that head-stabilizing strategies may improve function with telescopic spectacles in certain low vision patients.

Adult↗

PubMed1989

Adaptation to telescopic spectacles: vestibulo-ocular reflex plasticity.

The vestibulo-ocular reflex (VOR) is a mechanism for the production of rapid compensatory eye movements during head movements. To investigate the adaptation of this reflex to spectacle magnifiers, the effect on the VOR of a brief period of wearing telescopic spectacles during head rotation was studied in normal subjects. VOR gain, as measured in darkness, was defined to be the ratio of compensatory slow phase eye velocity to head velocity. Initial VOR gain as measured for vertical axis sinusoidal head rotation at 0.1 Hz, amplitude 60 degrees/sec, was about 0.7. After 15 min adaptation by sinusoidal rotation during the viewing of a remote video display through X2, X4, or X6 binocular telescopic spectacles, 47-70% of subjects exhibited significant VOR gain increases of 7-46%. These increases were measured with occlusion of the unmagnified visual field peripheral to the telescopes during adaptation. There was considerable interindividual variability in adaptation to telescopic spectacles. Telescopic spectacle power had little or no effect on the amount of VOR change after adaptation, although all telescope powers produced a greater VOR gain change than did adaptation without telescopes. Testing of VOR gain at multiple frequencies indicated that adaptation to telescopic spectacles by rotation at a single sinusoidal frequency induces VOR gain changes across a broad spectrum of frequencies of head rotation. When the unmagnified peripheral visual field was unobstructed during adaptation, VOR gain increases were significantly less than when the unmagnified peripheral visual field was occluded, and were similar to those observed during adaptation without the wearing of telescopic spectacles at all. VOR gain adaptation was associated with amerlioration of symptoms of oscillopsia and motion discomfort initially experienced by about 20% of subjects wearing telescopic spectacles.

Adaptation, Physiological↗

PubMed1988

Dynamic visual acuity with telescopic spectacles: improvement with adaptation.

Telescopic spectacles are used as aids for the visually impaired in order to increase effective visual acuity. Because ocular stabilization reflexes are not fully compensatory when telescopic spectacles are worn, head motion would be expected to produce retinal image motion which could decrease visual acuity. Using 1.0 Hz sinusoids of vertical axis head rotation, we investigated the effect of head velocity and telescopic spectacle magnification on binocular dynamic visual acuity (DVA), the acuity during head motion, in 34 normally sighted subjects. The visual field peripheral to the telescopes was masked. Up to a head velocity amplitude of 30 degrees/sec, DVA was insensitive to head velocity for X2 telescopic spectacles. For X4 and, to a greater degree, X6 telescopic spectacles, DVA decreased progressively as head velocity increased. DVA measurements were repeated after a 15 min adaptation period, during which a distant video monitor was viewed using telescopic spectacles. For X4 telescopic spectacles, DVA increased significantly after adaptation. With an unobstructed peripheral visual field, initial DVA with X4 telescopic spectacles was equal to adapted DVA with peripheral vision occluded, but adaptation produced no further improvement in DVA with the peripheral field unobstructed. These data indicate that the visual acuity obtained with telescopic spectacles is substantially reduced under conditions where head motion occurs, potentially reducing the functional value of these devices in low vision rehabilitation. The adverse effect of head motion on DVA may be reduced by adaptation.

Adaptation, Physiological↗

PubMed1987

Vestibulo-ocular reflex during magnified vision: adaptation to reduce visual-vestibular conflict.

This report describes the short-term effect of 2.2X telescopic spectacles on the vestibulo-ocular reflex (VOR) in seven volunteers. VOR gain was initially measured in darkness and light during passive sinusoidal rotations. Subjects were then rotated in light for 15 min while wearing telescopic spectacles. Dynamic visual acuity (DVA), vision during head rotation, was measured with telescopic spectacles. Initial VOR gain in darkness was 0.74 +/- 0.10 (mean +/- S.D.); VOR gain with unmagnified vision was 1.07 +/- 0.04. Initial VOR gain with magnified vision was 1.37 +/- 0.53. DVA was poorer than static acuity in three of four subjects. After adaptation, VOR gain in darkness increased to 0.83 +/- 0.12, with six of seven subjects exhibiting a gain increase of 7-23% (p less than 0.05 for 5). Adapted VOR gain with magnified vision was 1.54 +/- 0.25. Adapted performance was more consistent and oscillopsia was reduced. Adapted DVA improved 30-100% in four subjects. These changes indicate VOR adaptation to telescopic spectacles decreases visual-vestibular conflict.

Adaptation, Physiological↗

PubMedDate not supplied

Effect of telescopic spectacles on head stability in normal and low vision.

Telescopic spectacles, highly magnifying visual aids mounted in spectacle frames, markedly alter the visual consequences of head movements. To evaluate the effect of this altered visual feedback on head stability, angular head velocity of normally sighted and low vision subjects was measured in the roll, pitch, and yaw axes. Measurements were made under two postural conditions: (1) quiet standing; and (2) walking in place, as well as three visual conditions: (1) eyes closed; (2) unmagnified vision; and (3) vision with 4 x binocular telescopic spectacles. For normal subjects during quiet standing, both unmagnified vision and vision with telescopic spectacles tended to reduce spontaneous head velocity in all axes as compared to the eyes-closed condition. However, in low vision subjects neither unmagnified vision nor vision with telescopic spectacles produced significant changes in values of head velocity relative to those measured with eyes closed. Spontaneous head velocities for standing low vision subjects tended to be higher than in normal subjects, although not all differences were statistically significant. During walking in place, Fourier analysis demonstrated prominent frequency components related to harmonics of the walking frequency under all viewing conditions. In normal subjects, vision with telescopic spectacles, to a greater degree than unmagnified vision, reduced head velocity during walking in the roll and yaw, but not the pitch, axes. For low vision subjects, significant reductions in head velocity during walking were observed only during vision with telescopic spectacles. These findings indicate that vision reduces angular instability of the head during standing and walking. Magnification produced by telescopic spectacles further improves head stability under some conditions, although the effect of vision is least evident in the pitch axis. The stabilizing effect of vision is reduced in low vision subjects.

Adult↗

PubMedDate not supplied

Visual-vestibular interaction with telescopic spectacles.

Vestibularly and visually driven eye movements interact to compensate for head movements to maintain the necessary retinal image stability for clear vision. The wearing of highly magnifying telescopic spectacles requires that such compensatory visual-vestibular interaction operate in a quantitative regime much more demanding than that normally encountered. We employed electro-oculography to investigate the effect of wearing of 2x, 4x, and 6x binocular telescopic spectacles on visual-vestibular interactions during sinusoidal head rotation in 43 normal subjects. All telescopic spectacle powers produced a large, immediate increase in the gain (eye velocity/head velocity) of compensatory eye movements, called the visual-vestibulo-ocular reflex (VVOR). However, the amount of VVOR gain augmentation became limited as spectacle magnification and the amplitude of head velocity increased. Optokinetic responses during wearing of telescopic spectacles exhibited a similar nonlinearity with respect to stimulus amplitude and spectacle magnification. Computer simulation was used to demonstrate that the nonlinear response of the VVOR with telescopic spectacles is a result of nonlinearities in visually guided tracking movements. Immediate augmentation of VVOR gain by telescopic spectacles declined significantly with increasing age in the subject pool studied. Presentation of unmagnified visual field peripheral to the telescopic spectacles reduced the immediate VVOR gain-enhancing effect of central magnified vision. These results imply that the VVOR may not be adequate to maintain retinal image stability during head movements when strongly magnifying telescopic spectacles are worn.

Adaptation, Ocular↗