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Biomedical subjects

W W Dawson

Publications and source records attributed to W W Dawson.

17 recordsLinked to original sources
PubMed1992

Optic nerve aplasia.

Optic nerve aplasia is a rare congenital defect invariably associated with other ocular or systemic disorders. We examined a 3-year-old girl with monocular microphthalmos who had optic nerve aplasia on histopathologic examination of the eye after enucleation. Magnetic resonance imaging verified the presence of unilateral optic nerve aplasia, and demonstrated hemichiasmal hypoplasia on the affected side and bilateral optic tracts. The visually evoked cortical response revealed increased signals over the occipital cortex ipsilateral to the aplastic nerve, suggesting misdirection of axons from the temporal retina of the normal eye. The visual pathway in unilateral optic nerve aplasia may assume a primitive form of neuronal organization characterized by an increase in contralateral retinogeniculostriate projection.

Child, Preschool↗
PubMed1992

A primate model for age related macular drusen.

A closed colony of semi-free-ranging rhesus monkeys maintained in isolation since 1938 by the Caribbean Primate Research Center (CPRC) is being studied as a model for age related macular drusen. Of examined colony animals 57.7% of the monkeys and 47.3% of their eyes have drusen. The prevalence and severity of drusen are linearly related to increasing age and are significantly higher in specific maternal lineages (matrilines). An electrophysiological estimate indicates loss of function associated with drusen. Prevalence of drusen in CPRC females is almost twice that of males, while the prevalence among CPRC animals in general appears to be several times that of monkeys from continental US facilities. Evidence suggests that the frequency of endstage lesions is also similar to that in human populations. The CPRC matriline monkeys appear to provide the best model yet reported for human age related macular drusen.

Aging↗
PubMed1991

Pigment epithelial function in canine retina.

The lateral distribution and temporal changes in the eye standing potential of 15 dogs with normal eyes (as determined by use of an ophthalmoscope and electroretinography) were measured by use of noninvasive methods. The standing potential was converted to an alternating potential by controlled eye movement. The light peak occurred 6 minutes after a stimulus intensity increase of 4 log units. The ratios of the highest measured voltage after the light step divided by the voltage measured immediately before the light step ranged from 1.27 to 2.07 (mean 1.74 +/- SEM, 0.064). The responses typically decayed slowly after the light peak. The potential after the light peak did not return to prelight step values during the observation period. The field potential of the standing potential decreased nonlinearly in temporal direction from the outer canthus.

Animals↗
PubMed1979

Static and kinetic properties of the dolphin pupil.

In dim illumination, adult human pupil area becomes asymptotic at about 51 mm2 whereas the dolphin pupil achieves an area of about 70 mm2 under equivalent conditions. At moderate illumination levels (greater than 100 lx), the dolphin pupil becomes a horizontally oriented ellipse and develops a pronounced central constriction with additional light. The relative relationship between pupil area and illumination copmares closely with published human data. Rates of pupil response to step increments in illumination are very similar for the two species. Although the pupil has been proposed as a mechanism for correction of the dolphins' high aerial myopia, correlation of the laboratory findings on static and kinetic pupil properties with visually controlled aerial behaviors disclosed no special interdependence.

Adaptation, Ocular↗
PubMed1979

Improved electrode for electroretinography.

Corneal electrodes useful for clinical electroretinography require topical anesthesia, interfere with vision, can abrade, and are not well accepted by most children and many adults. A low mass conductive thread, corneal (DTL) electrode is described and comparatively tested against the Burian-Allen electrode. The DTL electrode was found to have few of the limitations of the hard contact lens electrode. Furthermore, the DTL electrode signal quality was comparable to that of the Burian-Allen electrode and provided less between-patient variability.

Adult↗
PubMed1978

The transscleral VER: prediction of postoperative acuity.

In 33 patients with corneal or lens opacities, the VER was recorded in response to a series of retinal stimuli delivered by the transscleral route (TVER). Based on the preoperative TVER characteristics, predictions of the postoperative acuities were made on a 1 to 5 scale. The predicted acuities were compared with those measured 6 months after surgery and showed that the TVER is a good predictor of postoperative acuity (r = 0.8). The criteria used for acuity estimates are presented.

Cataract Extraction↗
PubMed1978

The transscleral VER: normal responses.

Normative data taken on a sample of 10 eyes indicate that transscleral visual evoked response (TVER) stimulation of the cortical evoked response with a range of lights and at lower adaptation levels produces replicable, low-variance responses which may be of sufficient quality for the clinical evaluation of retinal and visual pathway condition. Response amplitudes were related to the radiant peak-power of white stimuli but were greater for red stimuli. Attenuation of the subjective brightness of the stimulus delivered through the inferior lid and sclera was about 0.4 log units compared to corneal delivery. Typical signal:noise ratios were 5:1.

Adolescent↗
PubMed1978

Cortical responses evoked by laser speckle.

Human cortical evoked responses elicited by patterns are very size and focus dependent, but those produced by moving laser speckle patterns (LASCERs) remain similar in amplitude and subjective sharpness over a wide (+/- 20 D) variation in added lens power. Moreover, cortical signal amplitudes evoked by speckle displays with high average spatial frequencies are significantly larger than those reported for check stimuli equivalent in spatial frequency.

Evoked Potentials↗
PubMed1977

An in vitro and in vivo analysis of anodized tantalum capacitive electrodes: corrosion response, physiology, and histology.

Oxidation-reduction reactions which can destroy high current-density metal-stimulating electrodes are avoided when using capacitive electrodes. The results of in vitro and in vivo testing of anodized, high surface area, sintered tantalum electrodes are presented. The corrosion response of the electrodes is excellent; there is no evidence of dissolution of the electrode. A deposit forms on the surface of the electrodes, but has little effect on the voltage response to constant current stimulation. The physiological and histopathological results indicate the capacitive tantalum electrode to be the safest yet tested.

Animals↗
PubMed1977

The visual cortex during chronic stimulation.

The lateral and suprasylvian gyris of cats have been stimulated anodically by surface electrodes for a total of 40 h by biphasic square waves. Peak currents were +/-6.0 mA, O to peak. Control electrodes were never stimulated. Electrode materials were low-temperature isotropic carbon and metals (Rh, Pt, Au). Reductions in tissue impedance quickly followed the onset of stimulation. The impedance change was related to stimulus current and to the incidence of motor seizures but not clearly to the electrode material. Utilizing concentric electrodes, the impedance change was not localized to the stimulating electrode - tissue interface. No protection from stimulation-associated impedance modifications was provided by the systemic administration of a material of high osmolarity (Mannitol) but the usual impedance decrease was not seen after systemic administration of a glucocorticoid.

Animals↗
PubMed1977

Cortical histopathology following stimulation with metallic and carbon electrodes.

Stimulation of the cat striate cortex with polished 1 mm2 gold, platinum, rhodium or carbon electrodes for 40h, at 50 Hz, and 0.5 msec duration, results in tissue damage at current densities of 0.6 A/cm2. At 0.1 A/cm2 rhodium and carbon were toxic to brain. In vitro gold and platinum corroded after 1,500-2,300 h of testing in simulated cerebrospinal fluid. The materials utilized are not suitable for long-term chronic electrodes for neural prosthetics.

Animals↗
PubMed1977

Retina viability in the Necturus.

Electroretinograms (ERG's) were recorded from the eyes of paralyzed normal and decapitated mudpuppies (Necturus maculosus). Fully dark-adapted responses to a range of stimulus intensities were compiled hourly under both conditions. Statistical analyses indicate no significant change in responsiveness in paralyzed normal animals during periods up to 6 hours. After decapitation, there was a significant decline of sensitivity to light during the first 3 hours. An oxygen-sensitive component similar to the c-wave was observed with DC recording but only in normal eyes. Anoxic mudpuppy eyes show an ERG decline with an average time constant of 1.5 hours.

Animals↗
PubMed1977

The electrical stimulation of the retina by indwelling electrodes.

Chronically implanted intraocular electrodes were used to stimulate visual cortical responses to current passage through the retina. Electrical current threshold for the light-adapted cat retina lies in the region of 30 to 100 X 10(-6) amps. for the conditions used. Dark adaptation caused a large threshold increase. Retinal excitability also decreased when electrical stimuli were delivered more often than one per 5 seconds. Effective charge-density threshold was moderately greater than reported for frogs and humans.

Animals↗
PubMed1977

Marijuana and vision--after ten years' use in Costa Rica.

Several tests of visual function were applied to an abstaining user (10 years or more) group and a nonuser group carefully preselected to be free of clinical signs of eye disease. The groups were matched on several criteria. The results show that all findings from both groups are within established limits of normalcy. Small differences and trends were found between the groups. These would have been undetected without large samples. Relative to the nonuser group, tests showed these user trends: increased basal lacrimation, increased intraocular pressure, increased photosensitivity, decreased dark adaptation, decreased color-match limits, and decreased Snellen acuity. These differences were associated with statistical probabilities, p=0.07 to p=0.001. There were no significant differences or clear trends between the user and nonuser groups in incidence of pathological fundus signs, conjunctival hyperemia, pterygia, or color-match midpoints.

Adult↗