Although several different contact-lens approaches have been devised to provide reasonable standards of both distance and near vision for presbyopes, including spectacle overwear, monovision, simultaneous image, and alternating image methods, these have not succeeded in meeting the visual needs of all patients. It has, however, long been known that the DOF of the eye may be increased by reducing the pupil diameter. Many years ago it was therefore suggested that contact lenses, which were opaque except for a small central aperture might, by increasing the DOF, provide enhanced near vision for presbyopes while still allowing good distance vision.1 Efron2 suggests that the lens be worn monocularly in the non-dominant eye. The disadvantage of such lenses is that less light contributes to the retinal image, so that the wearer finds vision more difficult under mesopic or scotopic conditions. There may also be a reduction in visual field,2 leading to problems with mobility. As a result of these major limitations, reduced-aperture contact lenses have found little application, except as artificial pupils for albinos.3

Recently, however, this concept has re-emerged in the form of a monocularly implanted corneal inlay with a small, central, clear aperture, the other eye having a natural pupil4: the optical principles involved seem to be related to an earlier design of contact lens.5 The inlay is normally implanted in the non-dominant eye of the presbyopic emmetrope and the approach might be termed reduced-aperture monovision. The inlay which is currently commercially available is the Kamra (originally Acufocus) inlay (Acufocus Inc., Irvine, USA). This consists of a thin, quasi-opaque disc with an outer diameter of 3.8mm and a central clear aperture 1.6mm in diameter.6 The inner diameter represents a compromise between improved DOF, light loss and optical quality, since diffraction with smaller diameters degrades retinal image quality and acuity.7 The outer diameter was presumably selected on the basis of minimizing any obstruction of nutrients and waste products through the cornea, allied to the desire to increase retinal illuminance under dim lighting conditions when the natural pupil dilates behind the inlay: the normal appearance of the iris is largely preserved. The inlay is surgically inserted at about halfway through the thickness of the cornea. Clinical reports suggest good levels of patient satisfaction and useful improvements in binocular intermediate vision, near vision and reading performance, the effects being stable for up to 4 years.8–12 A laboratory study confirms that binocular acuity at near is similar to that achieved monocularly by the eye with the inlay.13 It is claimed that inlays have the advantage of being minimally invasive and easily reversible. Obviously the same basic approach could be implemented with a well-centred contact lens giving similar pupil geometry, with the advantage that the contact lens could be powered to correct any ametropia.

Although the reduced-aperture monovision approach may yield adequate distance and near static acuity, particularly if some small residual myopia is left in the reduced-aperture eye,14 several problems arise with this attempted solution for the difficulties of presbyopes, including reduced stereopsis and losses in binocular acuity and contrast sensitivity.15 We address here the problems that may arise in spatial vision when viewing a highly dynamic scene, as when the individual is walking or driving. These occur because, with monocular wear of a reduced-aperture contact lens or inlay, a substantial difference can exist between the retinal illuminances in the two eyes. As is well known, such a situation leads to the induced Pulfrich effect and distortion in the apparent path of objects which are moving with respect to the observer. The resultant misinterpretations of the relative positions, sizes and speeds of objects may cause significant problems with mobility and driving. Here we discuss first the characteristics of the Pulfrich and related effects, and then some experiments in which the Pulfrich effect is demonstrated to occur during reduced-aperture monovision using contact lenses. It is confirmed that the effects are largely explicable in terms of the relative retinal illuminances in the two eyes.

Preliminary experiments showed that, when no contact lenses or artificial pupils were used, a filter with a ND of at least 0.2 was required to reliably elicit an obvious Pulfrich effect. A marked Pulfrich effect was observed with all the lenses and subjects.

Fig. 2 shows the raw psychometric plots for the 3 CLs with circular central apertures. It will be noted that for all subjects the value of ND required to null the Pulfrich effect reduces as the diameter of the clear aperture of the contact lenses increases. Note additionally that the null point is generally fairly precisely defined, with a reliability ranging between ±0.04 and ±0.19 in ND.

Figure 2.

Psychometric plots for 3 subjects of the probability of making the response “front” as a function of the value of the neutral density filter placed before the dominant eye, when the non-dominant eye wears a reduced-aperture contact lens with a central clear diameter as indicated. The values correspond to the neutral density required to null the Pulfrich effect (response probability, P=0.50). Reliability was calculated for the probability range between 0.25 and 0.75.

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The results obtained for P=0.50 in the 3 cases when the “annular” contact lens was used are shown in Fig. 3. The filter value required to null the Pulfrich effect reduces as the amount of light which can enter the non-dominant eye from the region outside the CL's opaque annulus increases. Table 1 summarises the data for all lens conditions.

Figure 3.

Psychometric plots for 3 subjects of the probability of making the response “front” as a function of the value of the neutral density filter placed before the dominant eye, when the non-dominant eye wears the annular contact lens (with inner and outer diameters of 1.5 and 4.0mm, respectively) with a natural pupil diameter (5.0mm) or an artificial pupil (of 4.5 and 4.0mm) placed in the spectacle plane. The values correspond to the neutral density required to null the Pulfrich effect (response probability, P=0.50). Reliability was calculated for the probability range between 0.25 and 0.75.

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The basic hypothesis was that the value of the ND filter required to null the Pulfrich effect under any set of conditions was that required to equalize the retinal illuminances in the two eyes, where the retinal illuminance in each eye was directly proportional to the clear areas of its effective pupil.

Fig. 4 shows the data for the cases in which the contact lens had a small central clear aperture. The full and dashed curves show theoretical predictions, respectively with and without allowance for the Stiles–Crawford effect, and the symbols the results obtained with the 3 subjects.

Figure 4.

Values of ND filter required to null the Pulfrich effect when placed in front of the dominant eye, as a function of the diameter of the central clear aperture in the CL worn by the non-dominant eye. Both eyes have a fixed artificial pupil 4.5mm in diameter placed in their spectacle plane. The continuous and dashed curves are theoretical predictions based on the relative pupil areas, respectively, with and without correction for the Stiles–Crawford effect, and the assumption that all the pupils are ideally centred.

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It can be seen that, while there is reasonably good agreement between theory and observation for the smallest, 1.5mm pupil, for 2 subjects and the 2.5 and 3.5mm pupils, a higher ND value is required than that predicted. We attribute this to the problems of centration with the contact lenses. Relative decentration of the clear aperture of the contact lens with respect to the 4.5mm artificial pupil may result in a reduction in effective pupil area for the non-dominant eye, due to the artificial pupil vignetting the aperture of the contact lens. This will create a requirement for increased filter density in front of the dominant eye.

Fig. 5 presents the results for the “annular” contact lens. In this case the observed filter values for the three configurations are plotted against those calculated on the basis that all pupils are exactly concentric. It can be seen that when the predicted values approach unity, the observed values are markedly lower. The highest value of ND corresponds to the case where the outer diameter of the opaque annulus matches the diameter of the 4.0mm diameter artificial pupil, so that ideally no light should enter the non-dominant eye from outside the annulus. It is clear that relative decentration between the two pupils must be increasing the light reaching the retina of the non-dominant eye and hence reducing the density required in front of the dominant eye to null the Pulfrich effect.

Figure 5.

Results for the 3 observers and 3 conditions studied when the non-dominant eye wore the “annular” CL, as a function of the predicted values of ND. Filled symbols relate to predictions based simply on relative pupil areas, open symbols predictions where allowance is made for the Stiles–Crawford effect. The dashed line represents exact agreement with predictions.

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In spite of these discrepancies occasioned by CL decentration, there is no doubt that small-aperture monovision generates the expected, marked Pulfrich-type distortions in spatial perception under conditions where the visual world is changing dynamically as a result of movement. The exact effects observed under real-world conditions will depend not only on the geometry of the small-aperture optics but also on the scene luminance and consequent natural pupil diameter (Fig. 1).

Experience with patients suffering from both the induced and spontaneous Pulfrich effects suggests that the spatial distortions experienced can adversely affect personal mobility and ability to cope with road traffic conditions, with the consequent risk of accident and injury.22,24–26,29 Although exact equivalence between the induced and spontaneous Pulfrich effects cannot be assumed, we note that the monocular filters required to null the Pulfrich effect and alleviate the symptoms in clinical patients typically lie in the range 0.1–0.5 ND.17,23 Under the various small-aperture monovision conditions used in the present study, filters with densities 0.2<ND<1.0 were required to null the induced Pulfrich effect. It might be expected, then, that in the absence of a correcting filter, under some conditions small-aperture monovision might lead to symptoms similar to those of the clinical patients, in at least some individuals. This is supported by the results of a recent study in which a 0.9 ND filter was worn in front of one eye, leading to a marked impact on the judgement of speed during driving.29 Although there is evidence that adaptation effects may reduce the Pulfrich-type distortions experienced as a result of interocular differences in retinal illuminance, the distortions persist at a reduced level.32–34 Adaptation is less likely to occur with small-aperture contact lenses than with permanent corneal implants, since the former are likely to be removed fairly regularly for at least part of the waking day.

Overall, the results suggest that reduced-aperture monovision can cause adverse distortions in spatial perception when there is relative movement with respect to the environment. Further limitations of this approach to presbyopic correction may include field restriction,2 and reduced stereopsis, contrast sensitivity and acuity.15 As a result of these deficiencies, we believe that small-aperture monovision is an unsatisfactory solution for the needs of the presbyope.

The authors did not receive any financial support from any public or private sources.

IG Pallikaris forms the medical director of the Flexivue Corneal Inlay (Presbia). Sotiris Plainis, Dionysia Petratou, Trisevgeni Giannakopoulou, Hema Radhakrishnan and W Neil Charman have no proprietary or commercial interest in any material discussed.