Time for a PEP Talk: The Perceptual Eye Position

Thanks to Dr. Charles Boulet for bringing this news item from the Review of Optometry to our attention:

Let’s talk about these two parameters independently. You’re likely very familiar with stereoacuity. Bob Sanet, Pilo Vergara, and I discussed this at length in our article on stereoacuity as an index of binocular treatment success in anisometropic amblyopia in our article in VDR on the subject several years ago. Chances are, however, that you’re as unfamiliar with perceptual eye position, or PEP, as I was before reading this. So let’s dig into it a little bit.

The article that the Review of Optometry cites as the source of the stereoacuity and PEP data, published in bmc ophthalmology, is: Ocular and visual perceptive factors associated with treatment outcomes in patients with anisometropic amblyopia. The stereoacuity test used was a Randot test. The test used to establish the PEP was the following:

figure 1

Patients performed task while wearing polarized glasses, which allowed right eye to see a circle and left eye to see a cross (A). They were instructed to place the cross into the circle’s center by using a computer mouse (B). The horizontal and vertical deviation were automatically recorded by the system, basically a pixellated version of fixation disparity (1 pixel equals 0.04 prism – see the discussion further below). The nice thing about this is that is capturing vertical and horizontal disparity in the same task where disparity the way we usually do it clinically is one or the other but not both simultaneously.

As an aside, the PEP task seems analogous to the binocular measurement approach associated with the Neurolens measurement device. It would be particularly intriguing to think about prescribing prism to re-balance or re-calibrate the PEP system.

The study concluded that larger vertical PEP deviation (disparity) and worse stereoacuity were the most influential risk factors for treatment failure. It should be noted that their treatment consisted solely of having the patient wear the anisometropic Rx for a period of 4.5 months, followed by occlusion as needed if amblyopia remained severe. Interestingly, although interocular difference in power and interocular suppression were factors in treatment success, they were not as influential as PEP and stereoacuity.

The original paper on which the PEP concept was based is this 2017 article from the online journal Medicine, titled Comparison of perceptual eye positions among patients with different degrees of anisometropia. That article contains the following pertinent information:

“Perceptual eye position (PEP) is a new concept describing binocular alignment raised by Zhao et al in 2014.  It is one of the indicators used to evaluate fixation disparity and binocular function measured by a computer-controlled perceptual examination evaluation system … Different from the traditional definition of eye position using the Hirschberg test, cover test, or synoptophore, PEP is measured under a dichoptic vision condition, and the PEP pixels are recorded by a computer so that binocular misalignment can be quantified precise … The minimum unit of binocular misalignment checked by computer-controlled ocular misalignment software is 1 pixel, whereas the synoptophore is 1 prism, equaling 25 pixels … In children with normal visual acuity and eye position, the mean vertical PEP is 1 to 3 pixels and the mean horizontal PEP 4 to 8 pixels. The deviation pixels of PEP in amblyopic children are much higher compared with normal children. The more severe the degree of amblyopia is, the more serious the abnormality becomes … The deviation of vertical PEP in patients with interocular SE difference ≥2.50D was much higher than that in patients with interocular SE difference <2.50D, indicating that the instability of vertical PEP may be associated with the development of severe anisometropia.”

The PEP has more recently been used to study other conditions of binocular disruption or imbalance, such as intermittent exotropia, in this study which was also based in China and published in a very interesting journal:

View details for Cyberpsychology, Behavior, and Social Networking cover image

All of this adds to the evolving paradigm of amblyopia as a binocular problem overtly seen as a reduction in monocular visual acuity through one eye as compared to the other eye, but covertly manifest in deficits that compromise the binocular system that in fact have more impact on visual function than visual acuity.

3 thoughts on “Time for a PEP Talk: The Perceptual Eye Position

  1. If the patient were corrected with spectacles, the results of such testing would critically depend on monitoring the exact head position and subsequent eye position compared to the ocular center of the lens. The same would be true for measuring stereopsis. I would imagine, however, that this was considered by those in the study. If not, then how well patients did with treatment would depend heavily on the fitting of the glasses and if the patient used them primarily for distance or near (downward) seeing. A contact lens correction would, of course, likely eliminate these considerations as well as reducing magnification differences and the subsequent problem of having to adapt to move one eye further than the other.

    As your note, however, the study is important as further evidence for viewing refractive amblyopic acuity loss as part of a binocular problem.

  2. As always, thanks for the article Len. This sounds very similar to a diagnostic activity within VTS4, Motor Field, that also has a dichoptic setup in 9 POG.

  3. My pleasure, Tanner, and thanks as always for reading. Agreed – anything within the dichoptic family apparently has a salient effect on amblyopia. Our traditional “criteria for success” in amblyopia have been far too narrow, hanging our collective hats on endpoint acuity. I think I feel another blog coming on. 😉

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