Biomedical subjects

E T Davis

Publications and source records attributed to E T Davis.

15 recordsLinked to original sources

PubMed2002

Antinuclear autoantibodies (ANA) in Gulf War-related illness and chronic fatigue syndrome (CFS) patients.

It is established that veterans of the 1991 Gulf War have an increased frequency of experiencing multiple symptoms. The underlying mechanism of these ailments is unclear, although they do not correspond to any clearly defined syndrome. The most common symptoms overlap with those of chronic fatigue syndrome (CFS). CFS was recently associated with a novel subtype of antinuclear autoantibody (ANA) that reacts with nuclear envelope (NE) antigens. NE autoantibodies are not known to be linked with any distinct clinical condition, but have been observed in patients with unusual mixed chronic autoimmune disorders and connective tissue diseases. In this study we examined whether NE ANAs are a feature of patients with CFS and symptomatic Gulf War veterans (sGWV). We studied the prevalence of ANA in 130 sGWV, 90 well Gulf War veterans (wGWV), 128 symptomatic Bosnia and Era veterans (sBEV), 100 CFS patients, and 111 healthy control subjects matching for age and sex. We found no significant difference in the prevalence of ANAs between any of the groups. None of the patients/or veterans we studied had ANA of the NE type. Our results show that multisymptom illness due to CFS or related to Gulf War service is not associated with antinuclear autoimmunity.

Adult↗

PubMed1995

A labeled lines explanation of the perceived spatial frequency of moderate-, near-threshold- and zero-contrast spatial patterns.

We tested the predictions of a multiple-channels model about the appearance of spatial patterns. Specifically we tested how encoding the perceived spatial frequency of a near-threshold pattern compared with encoding of a zero-contrast or moderate-contrast pattern. For example, the model predicts that the mean perceived spatial frequency of a near-threshold pattern is a weighted average of the response to the stimulus and the noise. Six subjects used the method of adjustment procedure to match a peripherally viewed test stimulus (or a blank) with a foveally viewed grating. For near-threshold patterns we found a smooth perceived spatial-frequency function, with a smaller range of perceived spatial frequencies than obtained for 0.16 contrast patterns. These results are consistent with the predictions of the model: noise can affect the appearance of near-threshold and zero-contrast patterns.

Contrast Sensitivity↗

PubMed1995

Flash visual evoked potential (VEP) in amblyopia and optic nerve disease.

Amblyopia and optic atrophy are two very different causes of unilateral long-standing visual impairment. Yet, in some patients the differential diagnosis is not always manifest and standard clinical tests may fail to provide accurate information. We tested the efficacy of a nonstandard clinical test [flash visual evoked potentials (VEP's)] and quantitative multivariate statistical techniques as aids in the assessment of this differential. Thirty-three patients were separated into four groups (normal, anisometropic amblyopia, strabismic amblyopia, and unilateral optic atrophy). Non-patterned flash VEP's were obtained using several different temporal frequency rates. Patients with optic atrophy had significantly reduced VEP's in the affected eye at all temporal frequencies. Strabismic amblyopes, but not anisometropic amblyopes, often showed supranormal responses in the affected eye at the higher temporal frequencies. Finally, by using discriminant analysis (DA) we were able to classify correctly almost 70% of the patients, well above chance level of 25%. This DA provided very good sensitivity and specificity. We have shown that the use of flash VEP's and of multivariate statistical techniques may provide an effective way to diagnose amblyopia differentially from optic atrophy.

Amblyopia↗

PubMed1992

Reduction of foveal desensitization with blurred backgrounds.

Photopic spatial desensitization and sensitization (Westheimer) functions were measured with sharp edged and blurred edged backgrounds. Data show that for 6-min backgrounds, desensitization is reduced for test flashes presented against blurred backgrounds compared to test flashes presented against backgrounds with sharp edges. We suggest that the transients from edges of retinal images, caused by high frequency fixational eye movements, contribute to the mechanisms of spatial desensitization and sensitization; the effect of transients is reduced by blurring the backgrounds.

Adaptation, Ocular↗

PubMed1991

Effects of stimulus context on magnitude estimations and category ratings of perceived loudness--the spacing of intensity intervals.

Effects of stimulus context on magnitude estimations and on category ratings were examined for a range of stimulus intensities of a 1-kHz tone. The stimuli were distributed in equal-interval steps of energy so they formed a perceptual cluster of high-intensity tones with a perceptual outlier at the lowest intensity. According to the Invariance Principle, the shape of the response function should not be affected by the distribution of stimulus intensities. However, neither magnitude estimations nor category ratings yielded the linear functions predicted from the Invariance Principle when plotted on log-log axes. Instead, both procedures yielded concave-upward response functions for the group data as well as for the individual data sets of the six subjects. Moreover, unlike previous reports of a nonlinear relationship, we found a linear relationship between magnitude estimations and category ratings. Rather than implying an equivalence of the underlying sensory scales, however, our results may imply subjects used a similar attention strategy for both procedures. We consider some theoretical suggestions, including an attention-band concept, for modification of a multistage stimulus-response (S-R) transformation model.

Adult↗

PubMed1990

Modeling shifts in perceived spatial frequency between the fovea and the periphery.

A quantitative multiple-channels model can account for changes in perceived spatial frequency between foveally and peripherally viewed patterns; these frequency shifts are as large as 0.47 octave (38.5%). Factors that change as a function of eccentricity and that can alter the appearance of visual patterns include (1) scaling of receptive-field sizes, (2) overall sensitivity, and (3) steepness and nonlinearity of contrast-transfer functions (CTF's). Computations based on receptive-field size scaling, combined with fixed perceptual labels, predict higher perceived spatial frequency for peripheral patterns. Computations based on reduced sensitivity, however, predict the opposite result and thus partially offset the effect of receptive-field size scaling. The predicted perceived-frequency shifts based on changes in CTF's are quite small and can be ignored. Both individual differences and visual-field-location differences in magnitudes of perceived-frequency shifts are predicted by the model. An alternative perceptual explanation based on size-distance invariance and size constancy is also evaluated.

Biomechanical Phenomena↗

PubMed1990

The effects of temporal modulation and spatial location on the perceived spatial frequency of visual patterns.

The perceived spatial frequency of a visual pattern can increase when a pattern drifts or is presented at a peripheral visual field location, as compared with a foveally viewed, stationary pattern. We confirmed previously reported effects of motion on foveally viewed patterns and of location on stationary patterns and extended this analysis to the effect of motion on peripherally viewed patterns and the effect of location on drifting patterns. Most central to our investigation was the combined effect of temporal modulation and spatial location on perceived spatial frequency. The group data, as well as the individual sets of data for most observers, are consistent with the mathematical concept of separability for the effects of temporal modulation and spatial location on perceived spatial frequency. Two qualitative psychophysical models suggest explanations for the effects. Both models assume that the receptive-field sizes of a set of underlying psychophysical mechanisms monotonically change as a function of temporal modulation or visual field location, whereas the perceptual labels attached to a set of channels remain invariant. These models predict that drifting or peripheral viewing of a pattern will cause a shift in the perceived spatial frequency of the pattern to a higher apparent spatial frequency.

Adult↗

PubMed1987

Normative data and control studies of flash VEP's for comparison to a clinical population.

The effects of aging, interocular comparisons, pupil dilation, iris pigmentation, and simulated cataracts on bright flash visual evoked potentials (VEP's) of normal observers were studied to assess the flash VEP as a potential predictor of postsurgical visual function in cataract patients. Seventy-six observers (from 20 to 80 years old) were tested for age differences in flash VEP's. Implicit time measures for transient responses increased significantly as a function of age, but neither transient nor steady-state response amplitudes were altered significantly. Moreover, no significant changes in VEP's occurred as a function of interocular comparisons, pupil dilation, or differences in iris pigmentation of normal observers. Simulation of cataracts with 14 observers did not affect implicit time measures, but did change response amplitudes significantly. Therefore, implicit time measures of cataract patients should be compared either to those of normal age-matched controls or, in the case of a unilateral problem, to the normal fellow eye. Delayed implicit time measures, after the appropriate comparison, suggest optic nerve/pathway dysfunction. However, comparisons of amplitude measures require compensation for opacity density.

Adult↗

PubMed1987

Comparison of perceived spatial frequency between the fovea and the periphery.

A grating viewed in the periphery usually appears to be of higher frequency than the same grating viewed at the fovea, for frequencies below the Nyquist frequency of the periphery. Systematic shifts in perceived frequency between fovea and periphery were found under several experimental conditions: spatially localized or extended sine-wave patterns, test stimuli presented in the superior or the inferior visual field, and stimuli of high or low mean luminance at several different contrasts. A multiple-spatial-frequency channels model can qualitatively account for our results, if it is assumed that each channel has a receptive-field size that increases as a function of eccentricity but has a perceived frequency label that does not change as a function of eccentricity.

Fovea Centralis↗

PubMed1986

Shifts in perceived spatial frequency of low-contrast stimuli: data and theory.

We studied shifts in perceived spatial frequency of foveally viewed stimuli as a function of contrast, using lower contrasts and/or a lower range of spatial frequencies than used previously by others. Reliable shifts in perceived spatial frequency were found, but the direction of the shift at low frequencies was not the same as at high frequencies for some observers. Models based on either light scatter or an early compressive nonlinearity (transducer function) probably cannot account for these results. A multiple spatial-frequency-channels model in which a nonlinear contrast-transfer function (CTF) follows the output of each channel, however, is consistent with most of the results for suprathreshold contrasts. We considered several versions of this latter model, differing in their assumptions about the peak frequencies and bandwidths of the underlying channels, the shape of the CTF, and the combination rule by which the outputs of the channels are labeled and combined according to a weighted average.

Humans↗

PubMed1983