Monovision in name only

Pseudophakic monovision is in fact anisometropic rivalry, which the brain is hard-wired to handle.

William F. Maloney

During a recent discussion of pseudophakic presbyopia correction, one colleague, a strong advocate for one of the multifocal lens implants, voiced a misgiving regarding pseudophakic monovision, saying, “I just don’t have enough conviction that the patient will accept it.”

I was struck by the irony, as this is exactly how I feel each time I describe the 9-month period of neuroadaptation each multifocal candidate should be aware of. Nine months is a long time, and I always worry that any but the most tolerant could lose confidence and give up before the process is complete.

Conversely, I am now sufficiently confident that genuine pseudophakic monovision requires no such adaptation that I no longer include the possibility in my preoperative guidance. This insight came slowly over the years and mostly from listening to patients.

Our postoperative questionnaire asks each presbyopia correction patient how long it took for the result to feel “completely natural.” For those with pseudophakic monovision, the typical answer is “1 or 2 days.” Even this brief interval likely reflects clearance of any corneal edema, not neuroadaptation. My confidence was bolstered several years ago when I finally recognized the two key things that pseudophakic monovision is not.

In the September 1 issue (See “Surgical monovision is not optometric monovision” ), we read that surgical monovision is not merely a permanent version of contact lens monovision. Rather, it methodically employs a comprehensive series of quantitative assessments to precisely determine each candidate’s anisometropic sweet spot (Figure 1).

In this column, we address the essential fact that pseudophakic monovision is not monovision at all, but a specific class of binocular rivalry induced by an exact amount of anisometropia. Resolving just this sort of interocular image disparity is inherent in the neural circuitry of the visual cortex. That’s right; our brains are hard-wired for pseudophakic monovision.

Figure 1: Anisometropic rivalry: Pseudophakic monovision assessment
Dominance Sighting Sensory: Near and distance Oculomotor Anisometropia Focus zone chart Interocular defocus tolerance Suppression capacity Pupillometry Stereopsis Vergence, Version, Other	Source: Maloney WF

‘Fuse or choose:’ binocular vision in action

Approximately 20 million years ago, primates evolved two defining characteristics: the opposable thumb and binocular vision. Rather than just the “either-or” alternating monocular sighting of, say, the frog, primates could now fuse corresponding images. Stereopsis and depth perception — important evolutionary upgrades — became possible.

However, Panum’s area quickly reminds us that strict retinal correspondence and “flat” fusion occur only on the razor-thin line of the horopter. Fusion with stereopsis — a process that uses binocular image disparity to reconstruct depth — comes about only in a narrow adjacent area. All else consists of modulating greater differences between interocular images in search of a coherent percept.

Two eyes vying for the mind’s eye

In fact, according to recent comprehensive theories of physiologic binocular vision, all three aspects of binocularity — fusion, stereopsis or rivalry — share the same neural process. Which one of the three comes about is a function of the degree of disparity. So while we have been able to “fuse or choose” for 20 million years, choosing still concerns a large part of our elaborate visual circuitry.

We are always already choosing — by way of selective regard and suppression — which visual stimulus to attend. When we choose an object along a line of sight, we simultaneously suppress any other by denying it access to awareness, thereby averting physiologic diplopia.

Similarly, when each eye reports different image size and clarity, as with sufficient anisometropia, we systematically attend one and suppress the other. Determining which image is permitted to pass the full length of the visual pathway into awareness is the work of our physiologic binocular vision (Figure 2).

In this manner, the visual cortex mediates each interocular contest for the mind’s eye. This selective regard and suppression is the routine traffic of the neural circuitry of the visual pathway, which must always either “fuse or choose.”

Figure 2: Anisometropic rivalry: Fuse or choose
Physiologic binocular vision hardwired for anisometropic inter- ocular disparity, not intra-ocular disparity. Panum’s anisometropic fusion limits	Source: Maloney WF

Hidden in plain sight: anisometropia’s mission

The pseudophakic monovision approach to presbyopia correction makes strategic use of anisometropia as a ticket to ride these rails. Each interocular defocus limit is precisely calculated to generate a specific range of uncorrected vision, according to a candidate’s ocular dominance, suppression capacity, defocus tolerance and particular reading goals. Each unique anisometropic alliance then jumps aboard established neural circuitry to realize the intended result.

Neuronal gates evaluate and instantaneously select the better image, or in some cases, elements of each image, to obtain the most effective single percept in accordance with the task at hand. Suddenly, one begins to appreciate how misleading the misnomer “monovision” has been — and not only for patients. It obscures the reality that pseudophakic monovision is binocular vision, operating just as it has been for 20 million years.

The precise mechanism that alternates awareness between right eye and left eye has been the subject of unusually lively research in the past decade. These reports are shaping a broad new understanding of how and where the neuronal gating of awareness and suppression occurs. Although these findings are far from final, they are widely seen as groundbreaking.

The new neurophysiology of binocular rivalry

The point of access to visual awareness has fascinated mankind for centuries. Thanks to new pinpoint electrophysiological techniques in animals and functional MRI studies in humans, we are closer to identifying the precise neural processes involved. Fortunately for us, binocular rivalry is the subject of these studies.

Recall each leg of the hierarchical visual pathway. Retinal photoreceptors, bipolar neurons, then ganglion cells and the nerve fiber layer extend to the lateral genticulate nucleus with its alternating monocular layers. Axons extend to the primary visual cortex where they synapse with higher monocular neurons segregated into the recently discovered “ocular dominance columns” (Figure 3).

Figure 3: Anisometropic rivalry

Figure 4: Anisometropic rivalry

Source: Maloney WF

Increasingly complex neurons distinguish more nuanced detail as they ascend. Not until the sixth-order do we encounter horizontal binocular interaction between adjacent ocular dominance columns. Here, for the first time in the visual pathway, images from each eye meet up.

Although certainly oversimplified (Figure 4), new evidence indicates that right-eye and left-eye images face off within these complex binocular “mediating” neurons. With likely input from higher cortical areas, the outcome of this contest for awareness is determined by an instantaneous, “winner-take-all” decision in favor of the contender with the following characteristics (among others):

Focus: sharper focus less suppressible

Stimulus strength: high luminance, high contrast, regular contour less suppressible

Dominance: dominant eye for the task at hand less suppressible

Context: image more useful to the task at hand less suppressible

By means of the same familiar excite/inhibit binary gating common to sensory neurons elsewhere, the winning stimulus gains immediate access to awareness, while the loser is suppressed and remains unnoticed. The moment the task at hand changes — from reading the Sunday newspaper to a glancing up at the game on TV — a rematch is instantly under way, this time with the opposite outcome.

Multifocal’s greatest feat

Returning to, “I just don’t have enough conviction that the patient will accept it,” the irony is now vivid. Multifocal stimuli clearly cannot draw upon neural circuits segregated into adjacent ocular dominance columns aligned to modulate inter-ocular image disparity.

Multifocality’s intra-ocular image competition has no physiologic precedent. Without a neural template to single out and convey the winning percept into awareness, a prolonged neuroadaptation period is needed for the brain to put in place the necessary neural tracks. As that work goes on, the haloed vision emblematic of truncated image distinction slowly moderates and often — but not always — gradually disappears. It is a testament to the astonishing plasticity of the visual cortex that this is ever accomplished at all, let alone within 9 months.

In the November 1 issue:

What if the primary “task at hand” is not to read, but to prove the surgeon wrong? We will discuss the pseudophakic presbyopia correction patient personality profile.

For more information:

• William F. Maloney, MD, is head of Maloney Eye Center of Vista, Calif., and a well-known teacher of cataract and lens-based refractive surgery techniques. He can be reached at 2023 West Vista Way, Suite A, Vista, CA 92083; e-mail: maloneyeye@yahoo.com. In the interest of objectivity, Dr. Maloney has no financial interest in any ophthalmic product and has no financial relationship with any ophthalmic company.

For further reading:

• Search words: binocular rivalry, cortical suppression, ocular dominance column, visual awareness, visual pathway.
• Blake R. A neural theory of binocular rivalry. Psychological Review. 1989;96:145-167.
• Blake R, Logothetis N. Visual competition. Nature Reviews/Neuroscience. 2002;31:1-11.
• Blake R, Overton R. The site of binocular rivalry suppression. Perception. 1979;8:143-152.
• Lehky S, Blake R. Organization of binocular pathways: Modeling and data related to rivalry. Neural Computation. 1991;3:44-53.
• Ooi TL, He ZJ. Binocular rivalry and visual awareness: The role of attention. Perception. 1999;25:551-574.
• Polonski A, Blake R, et al. Neuronal activity in human primary visual cortex correlates with perception during binocular rivalry. Nature Reviews/Neuroscience. 2000;3:1153-1159.
• Sengpiel F, Blakemore C, Harrad R. Interocular suppression in the primary visual cortex: A possible neural basis of binocular rivalry. Vision Research. 1995;35:179-195.
• Tong F, Nakayama K, et al. Binocular rivalry and visual awareness in human extra striate cortex. Neuron. 1999;21:753-759.

• Lens-based Refractive Surgery Column Mission Statement: To educate readers on all aspects of lens implant refractive surgery including presbyopia correction, refractive cataract surgery, refractive lens exchange and phakic IOLs.