Effect of an intervening screen on accommodation to a distant object

Historical, Diagnostic, and Chromatic Treatment in Visual Snow Syndrome: A Retrospective Analysis

PURPOSE

This study aimed to determine the historical, diagnostic, and treatment aspects in patients with documented visual snow syndrome/visual snow in an academic, optometric setting.

METHODS

A retrospective analysis was performed in patients (N = 40, aged 12 to 55 years) with documented visual snow syndrome/visual snow examined over a 4-year period. Information was collected by a detailed case history and the Visual Snow Syndrome Symptom Survey. Treatment assessment was performed using the Intuitive Colorimeter, and a wide selection of chromatic tints was assessed under the most provocative/exacerbating and other conditions.

RESULTS

Visual snow was typically constant and monochromatic, with it being present on average 6.43 years. Bright and dark surfaces were the most provocative/exacerbating/revealing conditions, along with the viewing of computer screens. The most common etiology was mild traumatic brain injury. The most common primary and secondary symptoms were photosensitivity and tinnitus, respectively. There was a high frequency of occurrence of oculomotor deficits, especially accommodative and vergence insufficiency (~40 to 50%). Eighty percent of the patients were prescribed a chromatic tint with subjective visual reduction of visual snow ranging from 15 to 100% (mean, 45%).

CONCLUSIONS

The present information will help in understanding this unusual medicoperceptual condition, especially with respect to simple treatment frequently using readily available chromatic tints.

Surround Propinquity and Tonic Accommodation

PURPOSE

Previous studies of dark focus have found evidence of a propinquity response, in which subjects accommodate to the perceived distance of their surroundings despite being in absolute darkness. Their distance perception in a lighted room allows a spatial representation to persist within the darkened room. This study sought to establish the significance and magnitude of the propinquity response in a large sample of young adults, and to determine whether the propinquity response can be predicted by a subject's initial dark focus in an unknown dark room.

METHODS

Dark focus was measured with a dynamic infrared optometer (Plusoptix PowerRef 3) in 30 young adult subjects, 26 of whom were naive to the testing room and its dimensions. Dark focus was then measured at varying dioptric wall distances of 0.25-4D. The subject viewed the lighted room before each measurement. For each individual, the dark focus was plotted as a function of dioptric wall distance. The slope of this function was used as a measure of the propinquity response.

RESULTS

The mean initial dark focus value was 1.05D (SD 1.04D) for the 26 naive subjects. The mean slope of the plot of dark focus as a function of dioptric wall distance was small (+0.097), yet highly statistically significant (P = .0002). The propinquity response function showed no statistically significant quadratic or cubic trends, and so may be considered linear. No statistically significant correlation was found between naive dark focus and propinquity response (r = +0.246, P = .226).

CONCLUSIONS

Propinquity seems to be a small but statistically significant factor in dark focus measurements. Though it is unlikely to contaminate tonic accommodation measurements in large samples under normal laboratory conditions, a minority of individuals exhibit large propinquity responses equal to that of proximal accommodation in lighted conditions.

Dynamic accommodation responses to stationary colored targets

PURPOSE

When the targets or the background in a display are different colors, longitudinal chromatic aberration ensures that there is no single correct accommodative response. The purpose of the present study was to determine whether the response becomes more variable when viewing certain multicolor displays.

METHODS

Accommodative responses of five young participants were measured with a dynamic infrared optometer while they viewed steady targets at a nominal stimulus level of 3 D. Target-on-background color combinations were black on white, black on blue, black on red, blue on red, red on blue, dark blue on red, and dark red on blue.

RESULTS

When compared with the standard black-on-white target, responses to targets with reduced spectral bandwidth were not significantly more variable. In most participants, responses to near-isoluminant targets (e.g., red on blue and blue on red) were not more variable than to the standard target. However, calculated confidence intervals cannot rule out moderate to large changes in variability near isoluminance. Responses to these multicolor targets tended to favor the blue focus.

CONCLUSIONS

In most individuals, viewing multichromatic targets does not increase significantly the variation in accommodative response as compared with broadband black-and-white targets.

Dynamic accommodation response in the presence of astigmatism

It has been suggested that in the presence of astigmatism some individuals make cyclic changes in focus over the astigmatic interval to obtain better visual performance. The aim in the present study was to identify such cyclic accommodative behavior and to characterize the variability of the response in the presence of astigmatism. The dynamic accommodation response in the presence of induced astigmatism was recorded objectively with an infrared optometer in seven young adults. Astigmatism led directly to increased accommodative variability in certain individuals. In two of seven participants there was evidence for aperiodic cyclic accommodative responses between different portions of the astigmatic interval. However, the amplitude of these tracking responses was much smaller than the astigmatic interval.

Monocular accommodation response for totally occluded objects

PURPOSE

To investigate the accuracy and stability of the monocular accommodation response when attention is directed toward a totally occluded distant object.

METHODS

A static infrared optometer was used to measure the monocular accommodative response in 16 participants while they viewed a distant letter target in the presence or absence of an intervening object that totally occluded the distant target from view. Participants were instructed to attend to and focus for the distant target.

RESULTS

Participants differed widely in their accommodative responses for a totally occluded distant object, focusing close to the distant object, close to the intervening object, in between the two objects, and beyond the distant object. Some of this variability could be attributed to individual differences in the dark focus. Accommodation was more variable when the distant target was totally occluded by an obstruction at an intermediate distance (50 cm).

CONCLUSIONS

Many individuals focus poorly in the presence of a visual obstruction that totally occludes the object of regard.

Emmetropization in chicks uses optical vergence and relative distance cues to decode defocus

When visual information is confined to one object plane, the emmetropization end-point is adjusted in accord with the corresponding incident optical vergence at the eye [Proceedings of the 7th International Conference on Myopia (2000) 113]. We now report the effect of adding extra visual information beyond the target plane. Visual conditions were controlled using a cone-lens system: black Maltese cross targets on white opaque backgrounds (OMX) were attached to the open faces of 2.5 cm translucent cones fitted with either 0, +25 or +40 D imaging lenses. An alternative target (TMX) was made by substituting the opaque target background for a transparent background, which allowed access to visual information beyond the target plane. The imaging devices were applied to 7-day-old chicks and worn for 4 days. Prior to this treatment, on day 2, some chicks underwent ciliary nerve section (CNS) to preclude accommodation. All treatments were monocular. Refractive errors and axial ocular dimensions were measured using retinoscopy and A-scan ultrasonography under halothane anesthesia. Treatment effects were specified as mean ( +/-S.D.) interocular differences. Eyes with the OMX/+40 D lens combination remained emmetropic (+0.73 +/-3.57 D), consistent with the target plane being approximately conjugate with the retina. Switching to the TMX caused a hyperopic shift in refractive error (+3.78 +/-3.41 D). This relative shift towards hyperopia in switching from the OMX to the TMX target also occurred for the other two lens powers. Thus, the OMX/+25 D lens induced myopia (-7.00 +/-5.88 D), corresponding to the imposed hyperopic defocus (target plane now imaged behind the retina), and switching to the TMX resulted in a reduction in myopia (-1.73 +/-5.36 D). The OMX/0 D lens combination produced the largest myopic shift, and here, switching to the TMX condition almost eliminated the myopic response (-15.50 +/-6.62 D cf. -0.56 +/-1.24 D). This relative hyperopic shift associated with switching from the OMX to the TMX target was eliminated by CNS surgery. Thus, the two CNS/TMX groups were both more myopic than the equivalent no CNS/TMX groups (+40 D lens: -2.66 +/-2.34 D; +25 D lens: -7.97 +/-6.87 D). When the visual information is restricted to one plane, incident optical vergence appears to direct emmetropization. Adding visual information at other distances produces a shift in the end-point of emmetropization in the direction of the added information. That these effects are dependent on the integrity of the accommodation system implies that accommodation plays a role in emmetropization and represents the first reported evidence of this kind.