Biomedical subjects

H Kergoat

Publications and source records attributed to H Kergoat.

31 records · Page 2Linked to original sources

PubMed1991

Influence of transiently altered retinal vascular perfusion pressure on rod/cone contributions to scotopic oscillatory potentials.

The oscillatory potentials (OPs) are sensitive to vascular disturbances in the retina. Rods in man are more susceptible than cones to ischaemia. The findings in a recent study showed that the white flash OP-5 had a heightened sensitivity to altered retinal vascular perfusion pressure (RVPP), compared with earlier OPs and the b-wave. In the present study, a comparison was made of the sensitivity of scotopically matched blue and red flash OPs to dim white flash OPs, to transient stepwise changes in the RVPP, in 10 healthy young adults. After 30 minutes of dark adaptation, two consecutive 0.3 Hz flash groups (n = 20) were filtered and averaged to obtain 100 ms flash ERGs and OPs simultaneously via DTL fibre electrodes for white, blue, and red flashes. To minimize any carry-over effects across flash colour, or repeated testing, the retinal responses to each of these stimuli were measured in separate test sessions several hours to days apart. The RVPP was increased or decreased by 20% and 40% non-invasively by body inversion and scleral suction respectively. For the flash intensities used, blue and red flash OPs appeared more variable than white flash OPs. The OP index increased or decreased across all flash colours when the RVPP was increased, with blue OPs showing the greatest spread of data. Retesting in five subjects using white and red flashes against a blue background disclosed the input of both rods and cones to the white flash scotopic OPs. The provocative test conditions described here offer a potentially profitable approach for studies attempting to differentiate the effects of vascular disorders on rod versus cone function.

Action Potentials↗

PubMed1990

The effects of altered retinal vascular perfusion pressure on the white flash scotopic ERG and oscillatory potentials in man.

The oscillatory potentials (OPs) of the flash-elicited electroretinogram b-wave have been identified as sensitive indices of abnormalities within the retinal circulation. A recent study by Lovasik and Kothe has identified the rod system as being more vulnerable than the cone system to transient alterations of the retinal vascular perfusion pressure (RVPP). In view of these previous findings, we investigated the susceptibility of the scotopic OPs to altered retinal perfusion, in 10 normotensive paid volunteers between 21 and 31 years of age. A transient increase in RVPP was effected by body inversion, while a transient decrease in RVPP was obtained by suction ophthalmodynamometry. Group averaged data showed that OP-1 to OP-4 decreased in amplitude with either an increase or a decrease in RVPP. In contrast to all other OPs, OP-5 was reduced to a greater degree when RVPP was decreased and showed a pronounced gain in amplitude as RVPP was increased. Our study has isolated a component-specific vulnerability to altered retinal perfusion. This finding may be interpreted as indicating a different retinal site of origin for the generators of OP-5. The heightened sensitivity of OP-5 to alteration of the RVPP may offer diagnostic advantage for investigating chronic diseases causing retinal ischemia.

Adult↗

PubMed1990

Time course of cycloplegia induced by a new phenylephrine-tropicamide combination drug.

A motorized and computer-interfaced phoropter was used to track the development of cycloplegia and recovery of accommodation over a 60-min period, after the topical application of a phenylephrine 5%-tropicamide 0.8% drug combination (Phenyltrope). Phenyltrope was introduced into the Canadian market about 2 years ago (Compendium of Pharmaceuticals and Specialties, 1987), and advertised as a fast acting cycloplegic and mydriatic drug. Here we report the results of our investigation of the depth of action and the temporal aspects of cycloplegia for this drug combination as a function of iris color. We also compare the action spectrum of Phenyltrope to that of tropicamide 1% under similar test conditions. Our results indicate that the latency for cycloplegia was shorter for tropicamide, the maximum rate of accommodative loss similar for both drugs, and the resultant cycloplegia at 20 min deeper for Phenyltrope. Recovery from induced cycloplegia was greater for tropicamide 60 min after drug administration. For both Phenyltrope and tropicamide, no significant differences in any of the parameters investigated were observed as a function of iris color. We conclude that even though Phenyltrope induced a measurably deeper cycloplegia than did tropicamide, the amount of residual accommodation present at 20 min (about 38%) is insufficient for most refractive purposes.

Accommodation, Ocular↗

PubMed1990

Visual neural performance for chromatic displays.

Various subjective procedures have been used in the past to examine the visual disorders and ocular symptoms often associated with prolonged usage of Video Display Units (VDU's). We examined visual neural performance for VDU stimuli which differed in size (14, 21, or 28 min arc), chromaticity (white, red, green, or blue), and retinal clarity, by transient pattern visually evoked potentials (t-p VEP's). Such information could prove useful in the design of electro-optical display systems that optimize visual neural performance and minimize ocular fatigue. Stimuli consisted of "monochromatic" (W, R, G, or Blue on black) and "multichromatic" (Blue/R, Blue/G, and R/G) checkerboards with brightness-matched chromatic elements displayed on a high resolution RGB monitor at 40 and 80 cm. The ambient lighting level was 54 lux. Group-averaged amplitudes and implicit times of t-p VEP's from 20 visually normal subjects indicated a differential neural response across target colors and three experimentally induced levels of blur. For monochromatic stimuli, the Blue/Black targets elicited t-p VEP's with the lowest amplitude, longest implicit time, and greatest sensitivity to optical defocus. Increasing the target element size reduced the VEP sensitivity to defocus across all colors except red. For multichromatic targets, Red/Green targets elicited the most vulnerable t-p VEP's. Several optical and neural explanations are given to explain these results. Implications for VDU designs are presented.

Accommodation, Ocular↗

PubMed1990

Influence of target color and vergence of light on ocular accommodation during binocular fixation.

In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.

Accommodation, Ocular↗

PubMed1989

A pupillographic evaluation of a phenylephrine HCl 5%-tropicamide 0.8% combination mydriatic.

A continuously recording high-resolution pupillometer was used to measure changes in pupil size and pupil reactivity to temporally modulated light, following topical application of a phenylephrine HCl 5%-tropicamide 0.8% combination mydriatic (Phenyltrope). A controlled light stimulus induced pupillary changes in the left eye while changes in the consensual pupillary responses were measured in the mydriatic-treated right eye. The effects on pupil light reactivity of a single drop of the combination mydriatic were also compared to those caused by one drop of tropicamide 1% alone. Phenyltrope was introduced onto the ophthalmic market ostensibly offering faster mydriasis because of its combined effect of both the parasympathetic and sympathetic innervation to the pupil. In the present study, we looked for any differences in the pupil reaction times and amplitudes as a function of time after drug administration, iris coloration and type of mydriatic instilled. Our results show that while the combination is a fast acting mydriatic providing a large and stable pupillary dilation in subjects with either blue or brown irides, it does not differ appreciably in efficacy from the mydriatic effect of tropicamide 1% alone in healthy subjects between 20 to 36 years of age. Applications of our findings to the clinical environment are presented.

Administration, Topical↗

PubMed1989

Neural, optical, and search performance in prolonged viewing of chromatic displays.

The effect of prolonged cathode ray tube (CRT) work on the afferent visual neural and focusing system of the human eye was examined by visually evoked potentials (VEPs) and laser optometer techniques, respectively. These measures were compared with parallel measurements of search performance. Ten volunteers performed three 4-hr sessions of a search/decision task on a high-resolution-red/green/blue (RGB) monitor. Stimuli were numbers and symbols presented in a blue/black, red/black, or red/green foreground/background combination. Results showed minimal changes in the neural and focusing responses to each foreground/background combination over time on task. Weak correlations were found between neural-optical and search performance measurements. These data suggest that the use of chromatic stimuli in the color combinations examined do not have a detrimental effect on visual performance.

Adult↗

PubMed1988

Accommodative performance for chromatic displays.

Over the past few years, video display units (VDUs) have been incorporated into many varieties of workplaces and occupational demands. The success of electro-optical displays in facilitating and improving job performance has spawned interest in extracting further advantage from VDUs by incorporating colour coding into such communication systems. However, concerns have been raised about the effect of chromatic stimuli on the visual comfort and task efficiency, because of the chromatic aberration inherent in the optics of the human eye. In this study, we used a computer aided laser speckle optometer system to measure the accommodative responses to brightness-matched chromatic letters displayed on a high-resolution RGB monitor. Twenty, visually normal, paid volunteers in a 22-35 year age category served as subjects. Stimuli were 14, 21, 28 minutes of arc letters presented in a 'monochromatic' (white, red, green or blue, on a black background) or 'multichromatic' (blue-red, blue-green, red-green, foreground-background combinations) mode at 40 and 80 cm viewing distances. The results demonstrated that while the accommodative responses were strongly influenced by the foreground-background colour combination, the group-averaged dioptric difference across colours was relatively small. Further, accommodative responses were not guided in any systematic fashion by the size of letters presented for fixation. Implications of these findings for display designs are discussed.

Accommodation, Ocular↗

PubMed1988

The effect of optical defocus on the accommodative accuracy for chromatic displays.

Blur, probably arising from focusing inaccuracies, has been cited in ergonomic and vision literature as one of the causes of visual stress and decreased task efficiency in operation of video display units (VDUs). With the advent of colour coding in electro-optical displays, the need for a detailed quantification of focusing responses to chromatic stimuli is particularly important because of the influence of the chromatic aberration present in ocular optics on the focusing response of the eye. In this regard, we used a computer-aided laser speckle optometer system to measure the accommodative responses of 20 visually normal subjects, to brightness-matched monochromatic and multichromatic stimuli displayed on a high-resolution RGB monitor. We also investigated the effect of target size, target clarity, and viewing distance on the response accuracy of the focusing system of the eye. Our results show that while the accommodative responses are systematically influenced by target chromaticity, they are relatively independent of target size. There was no evidence of any anomalous focusing responses resulting from either target chromaticity or defocus that could account for the asthenopia frequently reported by VDU users. Furthermore, moderate levels of optical defocus did not drive accommodation into any visible hunting pattern for optimal focus or towards its tonic resting position. Implications of these findings in display designs are presented.

Accommodation, Ocular↗

PubMed1987

The influence of letter size on the focusing response of the eye.

The monocular and binocular steady state accommodation responses in 10 normal subjects were measured for different letter sizes presented at 6m and 0.4m observation distances. A computer-aided Badal laser speckle optometer system was used to measure accommodative responses. For each subject, and group-averaged data, accommodative responses were independent of letter limb subtense. Binocular and monocular responses averaged across subjects did not differ significantly. The magnitude of accommodative responses was related to the resting focus of accommodation and was independent of subjects' accommodative amplitude, refractive error and pupil size. The implications of these findings for clinical and human factor methods for examining visual performance are discussed.

Accommodation, Ocular↗

PubMedDate not supplied

Correlation of an exercise-induced increase in systemic circulation with neural retinal function in humans.

Although the effects of physical exertion on intraocular pressure and systemic blood pressure are well established, the retinal response to such physiologic stress has not been examined. We studied the effect of short-term intense exercise on the principal waves in the scotopic and photopic flash electroretinograms, as well as the lower-amplitude oscillatory potentials. Sixteen healthy volunteers between 20 and 30 years of age participated in this experiment. The electroretinograms and oscillatory potentials were recorded with a Nicolet CA-1000 clinical averager, using DTL-type fiber electrodes. All retinal potentials were taken immediately before and after a minimum 20-min period of stationary bicycling that increased the heart rate to about 140 beats per minute. The electroretinograms were recorded from eyes with dilated pupils, 10 min after white-light adaptation of the right eye, and 30 min after dark adaptation of the left eye. Red flashes and dim white flashes were used to elicit photopic and scotopic electroretinograms, respectively. While no changes were recorded for any of the electroretinogram components recorded under photopic conditions, the amplitude of OP5 was decreased and the implicit time of OP4 was delayed after exercise for scotopic conditions. We concluded that exercise caused component-specific changes in the scotopic oscillatory potentials. Since it is well known that oscillatory potentials are vulnerable to ischemia, scotopic oscillatory potentials may be used as simple noninvasive indices of the reactivity of the retinal vascular autoregulatory system during exercise.

Adaptation, Ocular↗

PubMedDate not supplied

Neuroretinal function is normal in early dementia of the Alzheimer type.

We recently demonstrated that retinal ganglion cell function, optic nerve head parameters and the retinal nerve fiber layer thickness are not altered in early dementia of the Alzheimer type (DAT). Our current objective was to assess whether the function of cells located more distally in the retina is also unaffected by the disease. We evaluated 23 individuals with early to moderate DAT and 23 healthy age-matched subjects, all displaying clinically normal visual function. Scotopic and photopic flash electroretinograms (fERGs) and oscillatory potentials (OPs) were recorded. The amplitude and latency of the retinal potentials did not differ between DAT and control subjects. Our current results showing normal fERGs and OPs in early DAT indicate that the underlying neurons giving rise to these signals are not impaired by the disease process. These data support and extend our recent findings suggesting that visual deficits in DAT do not stem from neuroretinal dysfunction.

Aged↗

PubMedDate not supplied

Normal optic nerve head topography in the early stages of dementia of the Alzheimer type.

In view of the existing controversy as to whether or not the optic nerve head (ONH) is altered in Alzheimer disease, we used modern imaging technology to evaluate the ONH structure in individuals with dementia of the Alzheimer type (DAT). Real-time topographical images of the ONH were obtained with a Heidelberg retina tomograph from individuals in the early stages of DAT and age-matched controls. The various ONH parameters examined in this study did not differ significantly between DAT and age-matched subjects. These results suggest that the deficits in visual function that are known to occur in DAT are not related to ONH structural anomalies, at least in the earlier stages of the disease.

Aged↗