John V. Lovasik

Pharmacokinetics of topically applied cyclopentolate HCl and tropicamide

ABSTRACT The time course of accommodative loss after the topical application of 0.5% and 1.0% concentrations of cyclopentolate HCI and tropicamide was measured over a 20‐min interval in 50 age‐ and sex‐matched subjects between 20 and 30 years of age. Computer‐assisted measures of residual accommodation provided detailed data on the temporal aspects of the cycloplegia induced by these commonly utilized drugs when used alone or with the topical anesthetic proparacaine HCI. The pattern of recovery of ocular accommodation from cycloplegia was also measured over a 5‐h period, starting 2 h after drug application. The results show that latency, depth of cycloplegia, and rate of accommodative loss are related to drug type and concentration, and are influenced by the iris coloration of the test eye. The rate of onset of cycloplegia was not accelerated in blue or brown irides by the preadministration of proparacaine. Regardless of iris pigmentation, recovery from tropicamide cycloplegia was much faster than recovery from cyclopentolate cycloplegia. In contrast, the depth of cyclopentolate cycloplegia present in brown irides during the recovery phase was much greater than in blue irides. Mechanisms to explain these observations are proposed and clinical implications of these findings are presented.

Visual performance and subjective discomfort in prolonged viewing of chromatic displays

Visual search and decision-making performance together with subjective fatigue were investigated over a 4-hr time block as a function of display foreground and background chromaticity using colors matched for brightness. Although some small differences in performance related to chromaticity were observed, these were not exacerbated over time. On the basis of the performance data obtained and the subjective reports, there was no support for the general recommendation to avoid the use of red and blue stimuli, either alone or in combination, in CRT displays.

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.

A parametric evaluation of retinal vascular perfusion pressure and visual neural function in man.

Body inversion and ophthalmodynamometry were used to alter the intraocular pressure/retinal vascular perfusion pressure (IOP/RVPP) relationship in 10 normotensive adults. Using scotopic and photopic flash electroretinograms (fERGs), a heightened susceptibility of the rod system relative to the cone system was identified for transient alterations of the RVPP. The scotopic fERG b-wave decreased by about 40% and the photopic fERG was relatively unchanged when the RVPP was reduced by 51%. When the RVPP was increased by some 90%, the scotopic b-wave decreased by about 11%, and the photopic fERG by 8%. The greater vulnerability of the rod system than the cone system to decreased RVPP was tentatively attributed to structural differences between the photoreceptor types, their retinal distribution, and the effectiveness of vascular autoregulatory mechanisms. The neural generators of the pattern evoked retinal potential (pERG) showed a large impairment in function with both an increase and a decrease in the RVPP. In contrast, pattern evoked cortical potentials (pVEPs) showed a significant reduction in amplitude only when the RVPP was increased. These latter results suggest that the generators of pERGs are dependent on some ideal and narrow range of RVPP for normal activity whereas the generators of pVEPs are more vulnerable to the direct pressure effects of vascular engorgement within the eye and intracranial structures.

Clinical evaluation of ocular and visual functions in insulin-dependent juvenile diabetics.

ABSTRACT Ocular and visual functions were evaluated in 30 insulin‐dependent diabetics, aged 12 to 20 years, and 30 age‐ and sex‐matched nondiabetics. The test and control groups were compared in terms of visual resolution, accommodative ability, color discrimination, functional retinal reserve, and afferent optic nerve function. These results were considered along with the blood‐glucose level, crystalline lens, fundus appearance, level of diabetic control, and disease duration. No widespread differences in ocular or visual function were found between the test and control groups. Measures of accommodative ability, functional retinal reserve (photostress recovery time), and blood‐glucose level best differentiated the two groups. This study suggests that in‐office evaluation of accommodation and functional retinal reserve may be sensitive indicators of early visual functional deficits in the presence of minimal anatomical changes associated with diabetes.

Comparison of noninvasive methods to derive the mean central retinal artery pressure in man.

Purpose. The pressure within the ophthalmic artery can be estimated by several noninvasive procedures based on measurements of the pressure either within the central retinal artery (CRA) or the brachial artery. In this study we compared 5 methods of deriving the mean pressure within the ophthalmic artery in 10 healthy volunteers 21 to 31 years of age. Methods. The pressure within the ophthalmic artery was calculated from estimates of the systolic and diastolic pressures within the CRA derived by suction ophthalmodynamometry (s- ODM), compression ophthalmodynamometry (c-ODM), interpolation from scleral compression/intraocular pressure (IOP) conversion tables, and measurements of the brachial blood pressure (BP). Results. Group average CRA pressure values varied significantly across techniques, with the largest difference among methods being about 15 mm Hg. CRA pressures derived by s-ODM or c-ODM and direct measurements of the IOP yielded statistically identical values. These latter values were significantly lower than CRA pressures estimated by either scleral compression/lOP conversion tables, or those predicted from brachial BP measurements with the arm held up alongside the head, both of which produced equivalent values. The highest estimates of CRA pressures were obtained when brachial BP values were derived with the arm in its normal anatomical position. Conclusion. The patency of the vascular network to the eye and subsequent perfusion of intraocular neural tissue essential to normal visual function can be evaluated by simple clinical procedures. Although all techniques to estimate the pressure in the ophthalmic artery are relatively simple to use, they do not all yield the same absolute values and consequently should be interpreted in this light when used for either clinical or research purposes.

Variability in clinically measured photopic oscillatory potentials

Oscillatory potentials found on the ascending phase of the electroretinogram b-wave probably originate in some element(s) of the inner plexiform layer. As oscillatory potentials are particularly sensitive to changes in retinal, and possibly choroidal, blood flow, they have been used extensively to provide clinical measures of the degree of retinal ischemia during the progression of diabetic retinopathy. Recent studies in our laboratories have disclosed previously unreported significant variability in the photopic oscillatory potentials on repeated measures even in tightly controlled conditions. The amplitude of five recordable light-adapted wavelets exhibited considerable intra- and inter-subject variability. Until further investigation can determine factors affecting standardization of testing, it appears that changes in oscillatory potential implicit times rather than in amplitudes are a better measurement in clinical neurophysiology.

Accommodative performance for chromatic displays

Over the past few years. video display units (VDUs0 have been incorporated into many varieties of workplaces and ocupational 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 eficiency‐because of the chromatic aberration inherent in the opties of the human eys. 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‐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 are 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 nfluenced by the foreground ‐back ground 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 latters presented for fixation. Imlications of these findings for display designs are discussed.

VARIATION OF DARK FOCUS OF ACCOMMODATION WITH LASER SPECKLE EXPOSURE DURATION

Abstract Dark focus of accommodation (DFA) was measured in 10 subjects using a computer‐aided He‐Ne Badal laser optometer having high temporal and amplitude resolution. Accommodative responses were tracked during 5 min sessions for 10 speckle exposure durations ranging from 100 to 1000 ms. DFA‐speckle exposure functions fell into four distinct groups. This identifies the laser speckle exposure time as a possible contaminant in measures of DFA made by laser optometers if individual variability across exposure times is ignored. Group averaging of accommodative responses for laser exposure times obfuscates the reported individual variability.

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 performancé.