Microfluctuations in accommodation: an update on their characteristics and possible role.

PURPOSE: Following the recognition, some 80 years ago, that the accommodation of the eye is not stable but fluctuates over a range of about ±0.5 D, mainly at frequencies of up to a few Hz, there has been a continuing interest in the characteristics of these microfluctuations (MFs) and their possible role in the control of accommodation. This paper reviews relevant work carried out since 1988, when we previously reviewed the same topic (Charman WN, Heron G. Fluctuations in accommodation: a review. Ophthalmic Physiol Opt 1988; 8: 153-164). RECENT FINDINGS: Studies relating to the effects on the MFs of stimulus form, luminance and vergence are described, together with those of pupil diameter, age and ametropia. Important advances in the understanding of the links between the characteristics of the MFs and those of the cardiopulmonary system, higher-order aberrations and ocular depth-of-focus are outlined. Only limited progress has been made in understanding the role of MFs in accommodation control. SUMMARY: While the dependence of the characteristics of the MFs in relation to observing conditions is now reasonably well understood, their involvement in accommodation control still needs clarification. The current consensus appears to be that any role is more likely to be concerned with maintaining an appropriate response, rather than in initiating responses to abrupt changes in stimulus vergence. Fluctuations at lower temporal frequencies (<0.6 Hz) are probably important to the control process, which may make use of the associated changes in the contrast and spatial frequency spectrum of the retinal image.

The Pulfrich phenomenon; clumsiness and collisions which can be ameliorated.

The Pulfrich phenomenon has been described many times but usually just as an interesting academic issue with only diagnostic significance. The phenomenon is due to the fact that patients with unilateral optic neuropathy due to ischaemia, facial injury or multiple sclerosis may have delayed transmission of retinal responses to light from one eye to the brain. This leads to an altered and incorrect interpretation of the pathway and direction of moving targets. Clinicians should be aware of the unique symptomatology of the Pulfrich phenomenon because it is often difficult for patients to describe their symptoms which can be both unpleasant and troublesome. An appropriate tint in either a spectacle or contact lens over the normal eye delays retinal conduction, so re-establishing visual processing symmetry and ameliorating the symptoms.

Pulfrich's phenomenon in optic nerve hypoplasia.

BACKGROUND: Optic nerve hypoplasia (ONH) is a congenital deficiency of retinal ganglion cells and their axons that form the optic nerve. The condition is associated with visual deficits ranging from no light perception in severe cases, to vision within normal range in mild cases of ONH. We report here a case of mild ONH with normal vision and with visual symptoms relating to a spontaneously occurring Pulfrich effect. METHODS: A twelve-year-old girl presented with typical visual symptoms (veering to the right when walking, bumping into things) associated with the spontaneous Pulfrich effect. When asked to follow a bob suspended from a simple rigid pendulum swinging in the frontal plane, a clockwise ellipse was reported, indicative of a left-sided Pulfrich effect. This effect was neutralised by an 85% transmission ophthalmic tint placed before the right eye. The inter-ocular latency difference responsible for the illusion was calculated to be 0.88 +/- 0.55 ms. RESULTS: Fundus biometry showed an asymmetry in R and L optic nerve dimensions, the left being smaller than the right. Vision was normal in each eye, as were visual fields, colour vision and stereopsis. The affected eye showed normal flash and pattern VEPs and ERGs, except for a borderline reduction of the pattern ERG (PERG) to very small checks. CONCLUSIONS: A case of mild ONH with normal visual function in each eye resulted in a small inter-ocular delay associated with errors in visual perception consistent with a spontaneous Pulfrich effect. Wearing a pair of glasses with the right lens tinted eliminated these difficulties.

Visual adaptation to interocular brightness differences induced by neutral-density filters.

PURPOSE: Interocular brightness differences such as those caused by asymmetrical cataract have been found to have a minimal effect on interocular brightness matches. In the present study, the measured binocular visual response to interocular differences in retinal illuminance was measured over time. METHODS: Interocular differences in retinal illuminance of magnitudes 0.3, 0.6, and 0.9 log units were induced using neutral density (ND) filters under two conditions: (1) naturally mobile pupils and (2) with fixed artificial pupils (3 mm). Interocular brightness differences were quantified by measuring interocular brightness matches using the simultaneous interocular brightness sense test every 15 minutes over a 2-hour period in eight visually normal subjects. RESULTS: Initial interocular brightness matches were as predicted by the induced interocular differences in retinal illuminance (P > 0.05). A significant reduction in the interocular difference in brightness was observed over time (P < 0.01). These reductions in the interocular difference in brightness over time followed a logarithmic progression reaching asymptotic values equal to the reciprocal of the square root of the interocular retinal illuminance ratio. This value is equal to the midpoint of the induced interocular difference in retinal illuminance at time 0 and that found without the introduction of the ND filters. Binocular visual adaptation to interocular brightness differences occurred with both mobile and fixed pupils. CONCLUSIONS: Visual adaptation occurs in response to interocular brightness differences induced by asymmetrical ND filters. The level of visual adaptation can be predicted by Fechner's Paradox and is independent of interocular differences in pupil diameter.

The effect of monocular and binocular viewing on the accommodation response to real targets in emmetropia and myopia.

PURPOSE: Decreased blur-sensitivity found in myopia has been linked with reduced accommodation responses and myopigenesis. Although the mechanism for myopia progression remains unclear, it is commonly known that myopic patients rarely report near visual symptoms and are generally very sensitive to small changes in their distance prescription. This experiment investigated the effect of monocular and binocular viewing on static and dynamic accommodation in emmetropes and myopes for real targets to monitor whether inaccuracies in the myopic accommodation response are maintained when a full set of visual cues, including size and disparity, is available. METHODS: Monocular and binocular steady-state accommodation responses were measured with a Canon R1 autorefractor for target vergences ranging from 0-5 D in emmetropes (EMM), late-onset myopes (LOM), and early-onset myopes (EOM). Dynamic closed-loop accommodation responses for a stationary target at 0.25 m and step stimuli of two different magnitudes were recorded for both monocular and binocular viewing. RESULTS: All refractive groups showed similar accommodation stimulus response curves consistent with previously published data. Viewing a stationary near target monocularly, LOMs demonstrated slightly larger accommodation microfluctuations compared with EMMs and EOMs; however, this difference was absent under binocular viewing conditions. Dynamic accommodation step responses revealed significantly (p < 0.05) longer response times for the myopic subject groups for a number of step stimuli. No significant difference in either reaction time or the number of correct responses for a given number of step-vergence changes was found between the myopic groups and EMMs. CONCLUSION: When viewing real targets with size and disparity cues available, no significant differences in the accuracy of static and dynamic accommodation responses were found among EMM, EOM, and LOM. The results suggest that corrected myopes do not experience dioptric blur levels that are substantially different from emmetropes when they view free space targets.

Retinotopic accommodation responses in myopia.

PURPOSE: A reduced sensitivity to retinal image blur has been reported in myopes. Diminished blur detection reduces the error signal to the retinotopic (blur-induced) accommodation system and results in impaired accommodation responses under retinotopic conditions. This study was conducted to investigate retinotopic accommodation responses in emmetropia and myopia under dynamic conditions. METHODS: Static accommodation responses to a blur-only target with vergences of 0 to 4.5 D were measured with an optometer. Microfluctuations of accommodation were recorded with the subject viewing the target at a vergence of 4 D, and dynamic step responses were measured for step stimuli from 2.5 to 3.5 D and 2.0 to 4.0 D, with the optometer in dynamic recording mode. Measurements were obtained from a group of 32 visually normal emmetropes (EMMs) and subjects with progressing myopia. RESULTS: Stimulus-response curves were not significantly different between the refractive groups. Subjects with late-onset myopia (LOMs) demonstrated significantly larger accommodation microfluctuations compared with emmetropes and subjects with early-onset myopia (EOMs). Fourier analysis revealed that the increase in the magnitude of the fluctuations was mediated by the low-frequency components. Accommodation step responses revealed longer reaction times in LOMs. Further analysis showed that LOMs responded to accommodation step stimuli only between 43% and 64% of the time. In contrast, the other groups showed a response rate of almost 100%. CONCLUSIONS: The experiments demonstrate a reduction in retinotopic processing in LOMs, which results in an increased variability in their dynamic accommodation response to stationary near targets and reduced performance for dynamic step tasks. The results demonstrate a reduced blur appreciation under dynamic conditions in these refractive groups that may lead to periods of retinal image blur of varying magnitude during near work.

Visual latency in the spontaneous Pulfrich effect.

BACKGROUND: Interocular visual latency differences in patients with traumatic optic neuropathy due to midface injury were compared using both measurements of the delay from the spontaneous Pulfrich effect and pattern visual evoked potentials (VEPs). METHODS: Six patients with a spontaneous Pulfrich effect following midfacial injury observed a target which oscillated sinusoidally with an elliptical path in the frontal plane. The spontaneous Pulfrich delay was calculated from the size of the minor axis of the target ellipse on an XY plotter adjusted until the patient judged that no depth (i.e. no ellipse) was seen. Six separate pattern-reversal VEPs were recorded monocularly, with an artificial pupil, for a 4 cycle/degree grating. The mean P100 peak latency for each eye was used to calculate interocular latency differences. For both the Pulfrich effect and VEP measures normal subjects were included for comparison. RESULTS: Patients had P100 delays ranging from 2.8 to 17.8 ms, whereas Pulfrich delays were much shorter, ranging from 0.14 to 1.3 ms. A significant positive correlation was found between the two measures of delay, but the magnitude of the interocular difference in VEP was much greater than the delay calculated for the spontaneous Pulfrich effect. Tints used to correct the spontaneous Pulfrich effect in the patients were generally of high transmission (75-85%) and not dense enough to provoke an effect in normal observers. CONCLUSION: We have described a technique to quantify interocular delay associated with a spontaneous Pulfrich effect in patients with midfacial trauma. Interocular delays measured from pattern-reversal VEPs are similar in direction and relative severity but are larger in absolute terms.

Correction of the Pulfrich phenomenon by surgery and laser.

We describe 2 patients who presented with asymmetrical media changes and a spontaneous Pulfrich phenomenon. In the first case, the phenomenon was due to cataract formation and an interocular latency difference of 1.49 ms +/-0.07 (SD) was found. After cataract surgery, the symptoms resolved and no interocular latency difference could be measured. In the second case, the phenomenon was caused by posterior capsule opacification and an interocular latency difference of 1.16 +/- 0.08 ms was found. The symptoms were alleviated and no interocular latency difference could be measured after a neodymium:YAG laser capsulotomy was performed. The symptomatology of the Pulfrich phenomenon is often difficult for patients to explain. Clinicians should be aware of the phenomenon with cataract patients.