Unusual visual symptoms

Small-aperture monovision and the Pulfrich experience: absence of neural adaptation effects

PURPOSE

To explore whether adaptation reduces the interocular visual latency differences and the induced Pulfrich effect caused by the anisocoria implicit in small-aperture monovision.

METHODS

Anisocoric vision was simulated in two adults by wearing in the non-dominant eye for 7 successive days, while awake, an opaque soft contact lens (CL) with a small, central, circular aperture. This was repeated with aperture diameters of 1.5 and 2.5 mm. Each day, monocular and binocular pattern-reversal Visual Evoked Potentials (VEP) were recorded. Additionally, the Pulfrich effect was measured: the task of the subject was to state whether a a 2-deg spot appeared in front or behind the plane of a central cross when moved left-to-right or right-to-left on a display screen. The retinal illuminance of the dominant eye was varied using neutral density (ND) filters to establish the ND value which eliminated the Pulfrich effect for each lens. All experiments were performed at luminance levels of 5 and 30 cd/m(2).

RESULTS

Interocular differences in monocular VEP latency (at 30 cd/m(2)) rose to about 12-15 ms and 20-25 ms when the CL aperture was 2.5 and 1.5 mm, respectively. The effect was more pronounced at 5 cd/m(2) (i.e. with larger natural pupils). A strong Pulfrich effect was observed under all conditions, with the effect being less striking for the 2.5 mm aperture. No neural adaptation appeared to occur: neither the interocular differences in VEP latency nor the ND value required to null the Pulfrich effect reduced over each 7-day period of anisocoric vision.

CONCLUSIONS

Small-aperture monovision produced marked interocular differences in visual latency and a Pulfrich experience. These were not reduced by adaptation, perhaps because the natural pupil diameter of the dominant eye was continually changing throughout the day due to varying illumination and other factors, making adaptation difficult.

Interocular differences in visual latency induced by reduced-aperture monovision

PURPOSE

To explore the interocular differences in the temporal responses of the eyes induced by the monocular use of small-aperture optics designed to aid presbyopes by increasing their depth-of-focus.

METHODS

Monocular and binocular pattern-reversal visual evoked potentials (VEPs) were measured at a mean photopic field luminance of 30 cd/m(2) in seven normal subjects with either natural pupils or when the non-dominant eye wore a small-aperture contact lens (aperture diameter 1.5, 2.5 or 3.5 mm, or an annular opaque stop of inner and outer diameters 1.5 and 4.0 mm respectively). Responses were also measured with varying stimulus luminance (5, 13.9, 27.2 and 45 cd/m(2)) and a fixed 3.0 mm artificial pupil.

RESULTS

Mean natural pupil diameters were 4.7 and 4.4 mm under monocular and binocular conditions respectively. The small-aperture contact lenses reduced the amplitude of the P100 component of the VEP and increased its latency. Inter-ocular differences in latency rose to about 20-25 ms when the pupil diameter of the non-dominant eye was reduced to 1.5 mm. The measurements with fixed pupil and varying luminance suggested that the observed effects were explicable in terms of the changes in retinal illuminance produced by the restrictions in pupil area.

CONCLUSIONS

The anisocoria induced by small-aperture approaches to aid presbyopes produces marked interocular differences in visual latency. The literature of the Pulfrich effect suggests that such differences can lead to distortions in the perception of relative movement and, in some cases, to possible hazard.

Reduced-aperture monovision for presbyopia and the Pulfrich effect

Purpose

Monocular use of reduced-aperture optics in the form of a corneal inlay or contact lens may improve near vision of presbyopes by increasing their depth-of-focus (DOF). However, the associated induced interocular differences in retinal illuminance may cause distortion in spatial perception due to the Pulfrich effect.

Methods

Three young subjects were used to explore the Pulfrich effect during reduced-aperture monovision using afocal contact lenses (in the non-dominant eye) which were either opaque with a central clear aperture of 1.5, 2.5 or 3.5 mm diameter, or had an annular opaque stop of inner and outer diameters 1.5 and 4.0 mm, respectively. The two-alternative forced choice (2AFC) task of the subject was to state whether a 2° circular spot appeared in front or behind the plane of a central cross when moved left-to-right or right-to-left. The retinal illuminance of the dominant eye was varied using neutral density (ND) filters to establish the ND value which eliminated the Pulfrich effect for each lens.

Results

The Pulfrich effect was observed with all the lenses. The ND value required to null the effect decreased as the diameter of the aperture of the lenses increased. A reasonably good agreement was found between observed ND values for the different lenses and those predicted from the relative areas of the effective pupils of the two eyes. Minor discrepancies were attributed to decentration of the contact lenses with respect to the natural pupils.

Conclusions

Reduced-aperture monovision generates marked Pulfrich-type distortions in spatial perception under conditions where the visual world is changing dynamically as a result of movement.

The Pulfrich effect in the clinic

First described in 1922 by Carl Pulfrich, the Pulfrich effect is a stereo-illusion thought to be caused by an inter-ocular signal latency difference stimulating neurons jointly tuned to disparity and motion. Clinically, this can be a spontaneous manifestation due to various ocular and central visual pathway pathologies, and cause symptoms independent of a range of routine visual parameters which may seem bizarre to both the patient and the clinician. Eliciting such symptoms of difficulties with motion and depth perception in a clinical history should direct the clinician to the possibility of the presence of the spontaneous Pulfrich effect, and to proceed to test for it. A spontaneous Pulfrich effect may be the first presentation of a previously undiagnosed condition, and warrants further investigation to identify the underlying cause. Alternatively, a spontaneous Pulfrich effect may be the unrecognized complication of a previously diagnosed condition. The symptoms associated with a spontaneous Pulfrich effect can be managed by treating the underlying cause in reversible conditions, or by prescribing appropriate monocular tinted lenses in front of the unaffected or less affected eye in persistent conditions.

Influence of the Pulfrich phenomenon on driving performance

BACKGROUND

The Pulfrich stereoillusion occurs spontaneously in diseases inducing asymmetric visual pathway delays. Its influence on driving performance has never been investigated and was, therefore, assessed using a three-dimensional (3D) computer driving simulation.

METHODS

A 3D driving scenery of a road with obstacles was visualised on an autostereoscopic 3D display. Seven normal subjects drove at a speed of 6 m/s using a steering wheel and three angles of view of the scenery (0 degrees, 45 degrees to left, and 90 degrees to left) with different interocular delays (25 ms on the right, 25 ms on the left, and no delay). One subject drove the scenery at an angle of 90 degrees without delay and with a delay of 8 ms, 16 ms, and 25 ms on the right and left, respectively, at speeds of 6 m/s, 12 m/s and 18 m/s.

RESULTS

Stereo-illusion only influenced car position if the angle of view was 90 degrees (p<0.05). At this angle, increasing car speeds were associated with larger car displacements (delay on right p<0.001, on left p<0.01) and smaller delays with smaller car displacements (p<0.001).

CONCLUSIONS

This study showed that Pulfrich phenomenon has an influence on car position only if the viewing angle is 90 degrees. No influence could be found if the driving direction corresponded to the visual axis of the driver. These findings are in agreement with reports of patients with spontaneous Pulfrich phenomenon who indicate that while driving, distances are only misjudged when looking sideways.

Decision-making in second-eye cataract surgery: can presence of Pulfrich phenomenon help?

PURPOSE

To determine whether testing for the Pulfrich phenomenon (PP) can be used as a tool to assess the need for and optimal timing of second-eye cataract surgery.

METHODS

A total of 61 patients with logMAR 0 visual acuity (VA) after cataract surgery in one eye and logMAR 0.2-0.7 VA in the other eye were tested for PP using a computer-generated oscillating target at Baskent University Hospital. Only patients who had no ophthalmologic or systemic problem that could cause PP were included. In all, 15 normal patients with logMAR 0 VA in both eyes served as controls. The main outcome measures were presence and magnitude (measured by neutral density filters) of PP and presence of complaints related to binocular vision.

RESULTS

Of the 61 patients, 36 (59%) and none of the controls were PP (+) (P<0.001). A total of 27 (75%) of the PP (+) patients had logMAR 0.7-0.4 VA, and nine (25%) had logMAR 0.3-0.2 VA (P=0.01). In all, 16 patients (all PP (+)) had developed binocular vision-related complaints since cataract surgery. The mean PP magnitude in these cases was significantly greater than the mean for the 20 patients without complaints (1.2+/-0.5 vs0.6+/-0.4 log units, respectively; P<0.001). There was no significant difference between the mean VA in the complaint (+) and complaint (-) subgroups (P=0.213).

CONCLUSION

PP testing may detect binocular visual dysfunction after first-eye cataract surgery; thus, it could help assess the need for second-eye cataract surgery on this basis.

A bedside test to determine motion stereopsis using the Pulfrich phenomenon

OBJECTIVE

Many diseases induce asymmetric delays in the visual pathway, resulting in a spontaneous Pulfrich phenomenon (PP). The PP is a visual stereoillusion that may cause difficulties in persons when traveling in cars, crossing the road, or playing ball games. The authors developed and tested a simple new bedside procedure to detect PP.

DESIGN

A case series.

PARTICIPANTS

Disease simulation in 2 normal subjects and 18 patients with optic neuritis (ON) was examined. Ninety normal subjects were studied to determine normal range of PP.

INTERVENTION

The new test, called swinging pen test (SPT), is performed by oscillating a pen by hand. The SPT was compared to a gold standard, a mechanical pendulum (MP).

MAIN OUTCOME MEASURES

The authors measured simulated PP in two normal subjects and PP in 18 patients with ON and 90 normal control subjects. The Pearson product-moment correlation (r) and the Spearman rank correlation (rs) between SPT and MP were calculated.

RESULTS

The magnitudes of simulated PP determined with the SPT and the MP correlated well (r = 0.92, P < 0.005, and r = 0.96, P < 0.001). Correlation also was good in patients with ON (rs = 0.90, P < 0.05). The positive predictive value of the SPT was 100%, and the negative predictive value was 92%. The PP was absent in all control subjects testing with either pendulum. The normal range for PP varied from -1.40 to 1.52 msec. For the SPT, the intraobserver variability coefficient was 8.2%, and the interobserver variability coefficient was 10.5%.

CONCLUSIONS

The authors believe that SPT will be of value to clinicians on bedside evaluation of motion stereopsis dysfunctions. The normal range of PP was approximately +/- -1.5 msec (approximately +/- -1.5 cm), corresponding to a 0.3-log unit neutral density filter).

Pulfrich revisited

The Pulfrich phenomenon is a stereo-illusion resulting from latency disparities in the visual pathways. It is common after optic neuritis, but is also to be found with other conditions. The symptoms are often difficult for the patient to explain and for the physician to understand. Symptoms may be sufficiently disturbing to significantly interfere with a patient's life (e.g., prevention of driving). Treatment with the use of monocular tints is simple and effective.

The Pulfrich phenomenon and its alleviation with a neutral density filter

A case is described in which a presumed vascular accident resulted in long-standing visual difficulties in both reading and the analysis of vectors of moving objects. Clinical examination revealed minimal right optic atrophy with a relative superior altitudinal visual field defect associated with a positive Pulfrich effect. A partial head turn to the right in association with paresis of saccades and pursuit eye movements to the right was also evident. Spectacles for distance which incorporated a neutral density filter before the left eye were prescribed. These practically eliminated the Pulfrich effect and alleviated the problems of vector analysis. A near correction was provided which incorporated prisms with bases to the right. This eliminated the reading difficulties. The visual problems experienced by patients suffering from a positive Pulfrich effect are described and discussed.