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N Denny

Publications and source records attributed to N Denny.

12 recordsLinked to original sources
PubMed2002

Evidence for an interaction between p-glycoprotein and cadmium toxicity in cadmium-resistant and -susceptible strains of Drosophila melanogaster.

The role of an ATP-dependent membrane protein, p-glycoprotein (pgp), in cadmium toxicity and resistance in Drosophila melanogaster was investigated. Two strains were compared, a wild-type and a cadmium-resistant strain. Verapamil (1, 10, and 100 mciroM) was added to the larval diet of Drosophila in both the presence and the absence of 80 ppm cadmium chloride. Adult emergence of wild-type but not of cadmium-resistant flies was reduced in the presence of cadmium. Verapamil in the larval diet without cadmium had no effect on adult emergence in either strain. Verapamil in the diet spiked with cadmium significantly reduced adult emergence in both strains. This result is indicative of an interaction between the efflux of cadmium and verapamil and demonstrates that pgps have a role in regulating the toxicity of cadmium in Drosophil. These results do not rule out pgps as putative cadmium resistance genes.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1997

Suppressive rod-cone interactions: evidence for separate retinal (temporal) and extraretinal (spatial) mechanisms in achromatic vision.

We investigated the influence of selective rod light and dark adaptation on cone-mediated sensitivity to monocular displays modulated sinusoidally in both spatial and temporal domains. Rod light adaptation (1) increased sensitivity to high spatial frequencies [> or = 8 cycles per degree (cpd)] flickered slowly (< or = 2 Hz), an effect that we refer to as grating suppressive rod-cone interaction (gSRCI); (2) increased sensitivity to low spatial frequencies (< or = 2 cpd) flickered rapidly (> or = 8 Hz), an effect that we refer to as flicker suppressive rod-cone interaction (fSRCI); and (3) had relatively little influence on intermediate temporal-spatial-frequency combinations. The magnitudes of both gSRCI and fSRCI increased as the retinal position of the test display was increasingly displaced parafoveally. In parafoveal retina, both forms of suppressive rod-cone interaction increased as the overall dimension of the test stimulus decreased. However, sensitivity to high spatial frequencies is equally well influenced by adaptation of the viewing and the contralateral eye, while the adapted state of the nonviewing eye negligibly influences sensitivity to rapid flicker. Moreover, gSRCI cannot be observed with a small (30-arcmin) grating restricted to the fovea, while fSRCI is a prominent effect with small foveal test stimuli. Collectively, these results and neurobiological evidence suggest that fSRCI reflects a mechanism restricted to distal retinal, while gSRCI involves extraretinal neural circuitry.

Adaptation, Ocular↗
PubMed1993

Tonic interocular suppression, binocular summation, and the visual evoked potential.

PURPOSE: Psychophysical studies have shown that a dark-adapted eye exerts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstrate TIS by means of visual evoked potential (VEP) procedures. METHODS: Evoked cortical potentials were obtained in response to reversing checkerboard patterns with fundamental Fourier frequencies between 3 and 12 cycles per degree. Responses were obtained under monocular viewing conditions when the contralateral "adapting" eye was dark adapted, under monocular viewing conditions when the adapted state of the adapting eye was experimentally manipulated, or under binocular viewing conditions. Data were collected from three healthy young men, two native regarding purpose of experimentation. RESULTS: Regardless of spatial frequency, monocular responses evoked by stimulating a "test eye" were always smaller in amplitude when the contralateral adapting eye was dark adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting field was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of the monocular evoked response decreased: the time course of this decline follows that of psychophysically measured rod thresholds in the directly adapted eye. CONCLUSIONS: TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in VEP amplitude resulting from binocular viewing may be attributable to the removal of TIS rather than to "physiologic, binocular summation."

Adult↗
PubMed1992

Influence of rod adaptation upon cone responses to light offset in humans: I. Results in normal observers.

Dark-adapted rods exert a tonic suppressive influence upon cone-mediated sensitivity to rapid flicker, a phenomenon called suppressive rod-cone interaction (SRCI). However, rod dark adaptation has negligible influence upon cone-mediated thresholds measured with more usual psychophysical procedures. The present study separately examined the influences of rod light and dark adaptation upon cone-mediated sensitivity to transient increases or decreases in illumination using sawtooth flicker with rapid-on (ramp-off) or rapid-off (ramp-on) waveforms. In the parafoveal retina, cones alone were stimulated with flicker by spatially superimposing long- and short-wavelength stimuli presented in counterphase and matched in scotopic illuminance. Several different adaptation procedures were used. For higher (greater than 4 Hz) frequencies, sensitivity of cones to both waveforms is nearly identical under any condition of adaptation; sensitivity decreases as rods progressively dark adapt. A considerably different situation exists for slower frequencies (1-4 Hz). Sensitivity of cones to rapid-off flicker is appreciably greater under light-adapted conditions confirming recent observations by Bowen et al. (1989). But as rods progressively dark adapt, sensitivity of cones to rapid-off waveforms decreases considerably while sensitivity to rapid-on waveforms is much less affected; in the totally dark-adapted eye, sensitivity to both waveforms is identical. These results confirm and extend recent physiological observations in amphibian retina (Frumkes & Wu, 1990) suggesting that SRCI specifically involves responses to transient decreases in illumination.

Adaptation, Ocular↗
PubMed1991

Tonic interocular suppression and binocular summation in human vision.

1. Spatial sensitivity of human foveal vision was examined using sinusoidally modulated gratings. Our primary concern was the influence of interocular light adaptation upon monocular visibility. 2. Interocular adapting influences depend upon spatial frequency and adapting luminance. Interocular adaptation has a negligible influence upon the sensitivity to 1 cycle/deg gratings. Any visible interocular adapting field improves the sensitivity to intermediate spatial frequencies (2-5 cycles/deg). 3. Brighter interocular backgrounds (greater than 0.1 cd/m2) improve sensitivity to higher spatial frequencies (10-20 cycles/deg). 4. The interocular adapting influences summarized in (2) and (3) above cannot be duplicated by monocular or binocular adaptation. Similarly, monocular or binocular adaptation have negligible influences upon binocular visibility. 5. The interocular adapting effect summarized in (3) above can be duplicated by pressure blinding the contralateral eye. We conclude that monocular spatial sensitivity is subject to a tonic interocular suppression (TIS) from the dark-adapted eye. 6. The spatial sensitivity resulting from binocular viewing is nearly identical to that observed by combining monocular viewing with interocular light adaptation. We suggest that the improvement in sensitivity resulting from two-eyed viewing may be attributable to the removal of TIS instead of to binocular physiological summation.

Adaptation, Ocular↗
PubMed1988

Rod light and dark adaptation influence cone-mediated spatial acuity.

The influence of rod light and dark adaptation upon cone mediated spatial acuity was studied in the near parafoveal retina of normal human observers. The luminance just necessary to detect squarewave test gratings of variable frequency provided an index of spatial acuity. Such thresholds were determined in the presence of background fields which were varied in luminance, shape, and size, or throughout the time period of dark adaptation. Spectral controls determined the type of photoreceptors influenced by all stimuli. Cone mediated spatial acuity is improved by presenting background fields too dim to directly affect cones, and is increasingly suppressed during the rod recovery stage of dark adaptation. These effects are small with spatial frequencies less than 4 c/deg but increase with spatial frequency to greater than 1 log10 unit with the highest spatial frequency examined, 21 c/deg. These influences upon cone mediated spatial vision reflect the state of long-term adaptation of rods in a large annular area surrounding the locus to which the test grating is presented. Our results emphasize the differing influences of long-term dark adaptation and prevailing luminance level upon visual acuity. Ironically, spatial acuity is optimized under dim light conditions by selectively light adapting the receptors most sensitive to feeble stimuli, the rods.

Adaptation, Ocular↗
PubMed1987

Conducting applied research on Vietnam combat-related post-traumatic stress disorder.

A paradigmatic shift in post-traumatic stress disorder (PTSD) research is underway. Formistic and mechanistic research designs, characterized by single-category, single-cause, single-effect models, gradually are being replaced by contextual and organistic research designs that feature multi-category, multi-cause, and multi-effect interactional models. Such changes in diagnostic and treatment outcome research require solving many methodological issues in such areas as: measuring types of traumas and stressors; measuring PTSD symptoms and subtypes; measuring subject dispositional characteristics (such as ethnic differences); assessing concurrent and/or pre-existing psychiatric (Axis I) disorders; classifying personality styles and concurrent and/or pre-existing personality (Axis II) disorders; evaluating phase in the development of PTSD as a disorder; measuring current environmental stresses and interpersonal interactions; and assessing secondary gains and readiness for treatment. These and other methodological problems must be addressed as research on PTSD shifts to longitudinal measurement of subjects randomly assigned to treatment conditions.

Humans↗
PubMed1984

Factor-implied scales of ward atmosphere.

Evaluation of environments in hospital settings with regard to their therapeutic qualities has been a problem for clinicians and evaluation specialists for sometime. The present project examined the Ward Atmosphere Scale (WAS) factor-analytically and derived second-order factors inconsistent with those proposed by the authors of the scale. Items of the WAS were arranged psychometrically into three scales as implied by the factor analysis. Results suggest that interpretation of the WAS can be simplified considerably if these three super-ordinate dimensions are used to organize item-level data.

Health Facilities↗