Predicting optimal accommodative performance from measures of the dark focus of accommodation.

Leibowitz and his colleagues found that accommodation rests at an intermediate distance that shows wide interindividual variation. They proposed that this intermediate dark focus is useful for correcting anomalous refractive errors, but this proposal was later questioned when different measurement techniques yielded discrepant dark focus values. The present study measured dark focus under two levels of visual attentiveness: (a) when performing an open-loop, active viewing task (aDF); and (b) when looking passively into darkness (pDF). These dark focus measures were then compared with an optimal accommodation distance that was derived from accommodative response functions in bright and dim luminance. The aDF measures were found to be more myopic (nearer) than the pDF measures and highly correlated with the optical accommodation distance. No significant relationship was found between pDF and optical accommodation distance. These findings confirm that measures of dark focus are affected by nonoptical aspects of the measurement technique; they also suggest that techniques that demand visual attention (aDF) yield dark focus values that are more useful for optimizing accommodation and potentially reducing fatigue in difficult situations.

Dark focus values measured by retinoscopy: a clinical chart review.

The present study investigated the utility of dark retinoscopy as a technique to measure the dark focus of accommodation during routine clinical examinations. Using data from clinical examinations collected during a period of 6 months, a clinical chart review quantified dark focus and mean spherical equivalent refractive error for 480 patients ranging in age from 6 to 55 years. Results showed significant variations in mean dark focus values over the age groups. All groups showed wide interpatient differences, with mean dark focus values that were significantly nearer than the farpoint. Individual dark focus values ranged from 2.25 to -1.00 D (hyperopic), with an overall mean of 0.53 D. Potential clinical applications of dark focus-based correction are discussed.

The effect of tilted stripes in an optokinetic drum on gastric myoelectric activity and subjective reports of motion sickness.

BACKGROUND: The effects of tilted and vertical optokinetic drum stripes on self-reports of motion sickness and electrogastrograms (EGG's) were measured to investigate possible differences in the effects of stripe orientation. The EGG is a non-invasive measure of gastric myoelectric activity. The normal frequency is 3 cpm, and dysrhythmic 4-9 cpm activity, gastric tachyarrhythmia, has been found to be associated with motion sickness symptoms, particularly nausea. It is theorized that these symptoms are related to a sensory mismatch between the visual and vestibular systems elicited by the drum's rotation. HYPOTHESIS: We hypothesized that tilted stripes would elicit more symptoms than vertical stripes due to an additional sensory mismatch between the visual and vestibular verticals. METHODS: We divided 45 subjects into 2 stripe orientation groups: Vertical, and Tilted 15 degrees in the direction of drum movement. After the subject sat inside the stationary drum for a 6-min baseline, the drum was rotated (10 rpm) for a maximum of 16 min, followed by a 6-min recovery period. Throughout the experiment we measured the subjects' EGG's and subjective symptoms of motion sickness (SSMS). RESULTS: During rotation and recovery, gastric tachyarrhythmic activity was higher for subjects in the Tilted condition; the difference between stripe conditions was significant during rotation (p < 0.05). There were, however, no differences between SSMS scores. CONCLUSIONS: Off-vertical stripes increase the dysrhythmic activity of the stomach that often precedes or accompanies nausea and other symptoms of motion sickness.

Effects of moderate dose alcohol on visual contrast sensitivity for stationary and moving targets.

OBJECTIVE: Contrast sensitivity involves distinguishing threshold luminance differences and is usually assessed using static sine-wave gratings over a range of different spatial frequencies. The purpose of this study was to determine the effects of various levels of acute alcohol intoxication on contrast sensitivity to stationary and moving sine-wave gratings. Moving gratings required the subjects to make pursuit eye movements. A secondary goal was to investigate whether any alcohol-related effects were associated with any measures of intoxication. METHOD: Male volunteers (N = 8) participated in three counterbalanced, double-blind, testing sessions (low alcohol, moderate alcohol and placebo) plus a control session with no beverage. Breath alcohol concentration and two subjective measures of intoxication were measured for each subject. Static and dynamic contrast sensitivity were determined for electronically generated sine-wave gratings that were either stationary or traveled in a circular path with a diameter of 9 cm (3.7 degrees) at 51.7 rpm, thus requiring the subject to make smooth pursuit eye movements. RESULTS: The mean blood alcohol concentration measured in the moderate alcohol condition was 0.043% and in the low alcohol condition 0.011%. Moderate dose alcohol consumption significantly impaired both static and dynamic contrast with a greater effect for moving targets. CONCLUSIONS: Objective and subjective measures of intoxication were unrelated to the alcohol-related losses in contrast sensitivity. Although most states currently prohibit driving with BACs of 0.08-0.10%, the present data indicate reliable visual impairment at approximately half of that level (.44%).

Measuring and predicting the effects of alcohol consumption on contrast sensitivity for stationary and moving gratings.

Contrast sensitivity was measured for 12 healthy young males while sober, after ingestion of an alcohol placebo, and after ingestion of alcohol (95% grain alcohol; mean estimated blood alcohol level = .088%). Observations were made for both stationary gratings and gratings that traveled through a circular path and required pursuit eye movements. The significant alcohol-related reduction in contrast sensitivity was 2.6 times greater for moving (.29-log-unit reduction) than for stationary gratings (.11-log-unit reduction). The loss in contrast sensitivity for the moving gratings of high spatial frequency (12 cpd) was particularly severe (.37 log unit). Estimated blood alcohol level was correlated with the loss in contrast sensitivity for moving gratings (r = .61), but not with the loss for stationary gratings. Estimated blood alcohol level was strongly correlated with the difference between the loss in contrast sensitivity to moving and stationary gratings (r = .75). These results are consistent with reports that alcohol consumption degrades the ability to make pursuit eye movements. Subjects' perceived intoxication level was not a reliable predictor of any index of visual performance.