R. J. Miller

Mood changes and the dark focus of accommodation.

The consistency of dark focus of accommodation (the refractive power of the eye in total darkness) and its relationship to mood were examined. Twenty-one subjects were tested over a 2- to 3-week period using a laser optometer and several standard mood measures (including the Multiple Affect Adjective Check List). It was found that dark focus was reasonably consistent in most subjects over that period. Furthermore, correlations between dark focus and mood were related to dark focus variability; that is, higher dark focus-mood correlations and higher scores on several mood measures were most apt to be found among subjects with higher session-to-session dark focus variability. It was concluded that dark focus is a fairly stable phenomenon. It may also prove to be a promising technique for investigating mood changes, particularly for subjects whose dark focus variability is high, although more research is required.

Ocular vergence-induced accommodation and its relation to dark focus

Two experiments were conducted in an attempt to develop an open-loop technique for assessing the influence of ocular vergence on accommodation changes. Past research had shown that a small point of light (1.2 mm in diameter) induces relatively small changes in monocular accommodation at viewing distances greater than 50 cm. However, Experiment 1 (using seven male and six female undergraduates as subjects) demonstrated that, even with a smaller light spot (.29 mm), subjects could accommodate to the stimulus at distances closer than 50 cm. It was only with a very impoverished target (.08 mm diameter), as used in Experiment 2 (in which five male and five female undergraduates served as subjects), that it was possible to present a stimulus that could induce ocular vergence without being a direct stimulus in itself for accommodation. It was also shown that ocular vergence can drive accommodation and that ocular vergence can vary systematically under conditions in which illumination is greatly reduced. In addition, both experiments showed significant relationships among dark focus and both monocular and binocular accommodation.

Fatigue of accommodation: changes in accommodation after visual work.

ABSTRACT The present study was designed to determine whether classical accommodation theory or dual‐innervation accommodation theory best explains the visual fatigue effects of prolonged visual work. The accommodation responses of 20 young male adults were measured, using a laser optometer, while they viewed a near target (30 cm), a far target (6 m), or in total darkness. Measurements were made before and after 1‐h fatigue sessions, during which subjects either read monocularly presented slides of text at near or far distances, or remained in darkness. There were no significant changes in near or far target static accommodation as a result of any of the fatigue sessions. Dark focus accommodation shifted inward after the near target viewing task and after prolonged dark viewing. The results are discussed in terms of their implications for research in accommodative fatigue, and their relation to other current research.

Accommodation fatigue and dark focus: The effects of accommodation-free visual work as assessed by two psychophysical methods

Past studies of accommodation fatigue have yielded inconsistent results, partly because they have not used direct measures of accommodation, and partly because they may have been based on a misleading conception of the nature of accommodation. The dual-innervation theory of accommodation suggests that the resting position of accommodation may be neuromuscular rather than just muscular, and that it lies not at optical infinity, as assumed by older conceptions, but at some intermediate position (dark focus). Among the predictions that may be deduced from this theory is that long-term visual work not requiring active accommodation will not induce fatigue. The present study involved continuous measurements of dark focus for 10 young adults over a 3-h period, using the laser optometer with two psychophysical procedures Ibracketing and staircase. Consistent with the prediction, no changes in dark focus were found, in spite of the demanding visual task. Furthermore, it was found that both psychophysical methods yielded essentially identical results. The practical and theoretical implications of these results are discussed, and recommendations are given regarding situations in which each of the psychophysical methods is likely to be most useful.

The effects of ingested alcohol on accommodative, fusional, and dark vergence

To examine the effects of ingested alcohol on vergence, we measured vergence in 8 male emmetropes, aged 21–23 years, under two alcohol dosage conditions (placebo and 1.5 ml/kg of 95% ethanol). For each of these conditions, after consumption of the appropriate drink, dark vergence and fusional and accommodative vergence to near and far targets (viewing distance = 30 cm and 6 cm) were assessed every 30 min for a total of 6 h. Intoxication produced significant increases in convergence for far fusional and far accommodative targets. Intoxication also led to a tendency toward decreased convergence for near fusional and near accommodative targets, although the results were less clear than in the case of the far viewing conditions. Dark vergence did not change with intoxication; it also did not appear to be significantly related to other alcohol-induced vergence changes, although the size of the sample made it impossible to reach meaningful conclusions regarding this last point.

The effect of ingested alcohol on fusion latency at various viewing distances

Although diplopia is a frequently reported symptom of severe alcohol intoxication, there have been few past attempts to describe in any systematic fashion the specific impact of alcohol on fusion. In the present experiment, fusion latency was assessed for 8 male emmetropes. At the beginning of the alcohol condition, they consumed a drink containing 1.4 mllkg of 95% ethanol, plus diluent. For the placebo condition, the drink consisted only of diluent. In each condition, fusion latency (the time required to fuse a binocularly visible target) was recorded for targets requiring a magnitude of vergence that varied from +10 through −2 meter angles, in 1-meterangle steps. It was found that the far point offusion approached the observer as a direct function of intoxication level. Furthermore, fusion latency increased for optically far and near targets as a systematic function of intoxication, with no effect of alcohol on intermediate targets. The results were consistent with previous findings of alcohol-induced narrowing of vergence range, suggesting that alcohol causes the farthest and nearest distances at which fusion latency is affected to approach an intermediate value. Measurable effects on fusion latency were observed with blood alcohol levels as low as 0.05%–0.06%, considerably below the legal limit for operating a motor vehicle in most states.

The effects of ingested alcohol on accommodation

To determine the effects of ingested alcohol on accommodation, seven males, aged 21–22 years, had their accommodation assessed under three alcohol dosage conditions (placebo, 0.8 ml/kg, and 1.3 ml/kg of 95% ethanol). For each of these conditions, following drinking, static accommodation to near and far targets (viewing distance = 30 cm and 6 m) and dark focus accommodation were measured every 15 min for 3 h in the placebo and medium-dosage conditions and for 6 h in the high-dosage condition. Results showed that dark, near-target, and far-target accommodation all increased with intoxication. Observations regarding the relationship between dark focus and alcohol-induced static accommodation changes were inconclusive, although there was some indication that this relationship increased at the highest intoxication levels. It was concluded that moderate amounts of ingested alcohol produce small, but significant, increases in accommodation.

Overaccommodation, underaccommodation and the clinical phoropter: a study of instrument myopia and related effects

ABSTRACT To explore the degree to which a phoropter can induce overaccommodation (instrument myopia) and/or underaccommodation, 19 college‐age subjects participated in a series of measurements of accommodation response. Accommodation responses were assessed while subjects looked through a phoropter (both with standard 19‐mm viewing apertures and through 2‐mm artificial pupils) and when no phoropter was present. Subjects were tested both monocularly and binocularly, and under three stimulus conditions (near target, far target, and total darkness). Even without the phoropter, subjects tended to overaccommodate for the far target. The 19‐mm phoropter produced no significant overaccommodation or underaccommodation effects over and above those observed without the presence of the phoropter. The 2‐mm artificial pupils produced significantly more underaccommodation for the near target. Results indicate a tendency for accommodation response to approach an intermediate resting position as the viewing situation becomes impoverished.

Influence of flicker on perceived size and depth

Previous research (e.g., Wong & Weisstein, 1984a, 1985) has shown that flickering stimuli appear to be more distant than nonflickering stimuli at the same physical distance. Given this relation between flicker and perceived depth, inappropriate constancy scaling theories predict that flickering stimuli should be perceived as larger than nonflickering ones. In contrast, links between flicker and motion perception suggest that flickering stimuli should be perceived as smaller than nonflickering ones. Two experiments tested these contrasting predictions. In Experiment 1, 22 subjects compared flickering and nonflickering vertical lines and reported that the flickering stimulus appeared significantly smaller than the nonflickering one. In Experiment 2, 21 subjects reported that the stimuli used in Experiment 1 produced depth effects similar to those reported in previous experiments: flickering stimuli were perceived as more distant than nonflickering ones. The observed effect of flicker on perceived size was contrary to predictions from inappropriate constancy scaling theory, but consistent with views that motion and flicker are processed by the same pathway.

Effect of pupil size on dark focus accommodation

A potential concern in some studies of dark focus accommodation is the role of pupil size (and related spherical aberration) in laser optometer measurements. Dark focus measurements were obtained for 25 young adult subjects while they viewed the laser speckle pattern through artificial pupils of 3, 4, 5, 6, 7 and 8 mm diameter. Analysis showed no significant effect of pupil size on dark focus. It would appear that pupil size variations should not be a source of concern in laser optometer dark focus assessment.