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Low Vision Enhancement with Head-mounted Video DisplaySystems: Are We There Yet?

Ashley D. Deemer, OD, FAAO,1* Christopher K. Bradley, PhD,1 Nicole C. Ross, OD, FAAO,2 Danielle M. Natale, OD, FAAO,3

Rath Itthipanichpong, MD,1 Frank S. Werblin, PhD,4 and Robert W. Massof, PhD, FAAO1

SIGNIFICANCE: Head-mounted video display systems and image processing as a means of enhancing low visionare ideas that have been around for more than 20 years. Recent developments in virtual and augmented realitytechnology and software have opened up new research opportunities that will lead to benefits for low vision pa-tients. Since the Visionics low vision enhancement system (LVES), the first head-mounted video display LVES,was engineered 20 years ago, various other devices have come and gone with a recent resurgence of the technologyover the past few years. In this article, we discuss the history of the development of LVESs, describe the currentstate of available technology by outlining existing systems, and explore future innovation and research in thisarea. Although LVESs have now been around for more than two decades, there is still much that remains to be ex-plored. With the growing popularity and availability of virtual reality and augmented reality technologies, we cannow integrate these methods within low vision rehabilitation to conduct more research on customized contrast-enhancement strategies, image motion compensation, image-remapping strategies, and binocular disparity, allwhile incorporating eye-tracking capabilities. Future research should use this available technology and knowl-edge to learn more about the visual system in the low vision patient and extract this new information to createprescribable vision enhancement solutions for the visually impaired individual.

Optom Vis Sci 2018;95:694–703. doi:10.1097/OPX.0000000000001278Copyright © 2018 American Academy of Optometry

Author Affiliations:1Wilmer Eye Institute, Johns HopkinsUniversity School of Medicine,Baltimore, Maryland2New England College of Optometry,Boston, Massachusetts3LifeBridge Health Krieger Eye Institute,Baltimore, Maryland4University of California, Berkeley,Berkeley, California*[email protected]

Low vision refers to chronic disabling vision impairment causedby disorders of the visual system that cannot be corrected withglasses/contact lenses, medical treatment, or surgery. The typesof vision impairments that come under the rubric of low vision in-clude reductions in visual acuity, loss of contrast sensitivity, cen-tral scotomas, peripheral visual field loss, night blindness, slowglare recovery, photophobia, metamorphopsia, and oscillopsia.Often, low vision patients have combinations of these impairments.As a result of their vision impairments, low vision patients have dif-ficulty with or are unable to perform valued activities.1,2 Conse-quently, low vision can have a significant impact on patients'daily functioning, independence, social interactions, quality of life,and ultimately physical and mental health.3–5

Low vision rehabilitation focuses on maximizing visual functionthrough the use of adaptive strategies and accommodations andwith vision-enhancing assistive technology. Linear magnification(e.g., closed-circuit television and large print), relative distancemagnification (e.g., high add, microscope, and hand magnifier),and angular magnification (e.g., telescopes, binoculars, bioptics,and head-mounted video display systems) are the primary low visionenhancement strategies used to compensate for reduced visualacuity.6 Other common low vision enhancement strategies in-clude illumination control with filters (e.g., sunglasses andcolor-tinted lenses), task lighting (e.g., high-intensity light sources

and illuminated magnifiers), and head-mounted video displaysystems that use automatic gain control, which compensate forabnormal light and dark adaptation and glare recovery.6,7

Contrast enhancement (e.g., contrast stretching, contrast rever-sal, edge enhancement, and color and luminance contrast substi-tution), which is implemented primarily with closed-circuittelevision magnifiers, computer accommodation software, col-ored filters (to transform color contrast to luminance contrast),and head-mounted video display systems, is used to compensatefor reduced contrast sensitivity.8,9

Traditionally, magnification has been accomplished usingconventional optics; however, there are limitations includingfixed level of magnification and reduced field of view, as well asnarrow depth of field and close working distance for higher levelsof near magnification. Furthermore, other than color filters, opticaldevices cannot be used to enhance image contrast. Head-mountedvideo display systems equipped with optical or digital zoommagni-fication from system-mounted forward-looking video cameras, illu-mination control, and contrast-enhancement capabilities havebeen used in low vision rehabilitation for more than 20 years toovercome the limitations of conventional optical devices.7,10–12

These systems are intended to enable hands-free functioning, pro-vide binocular or biocular (same image presented to each eye with-out retinal disparity) viewing, and modify the image presented to

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the retina in real time to compensate for the patient's specificvisual limitations under changing viewing conditions.13,14 In prin-ciple, there is an image-processing strategy for each type of visionimpairment that will optimally enhance the patient's perception ofvisual information in the retinal image. The ideal head-mountedlow vision enhancement system would be able to compensate foreach type of vision impairment, without compromising field of view,binocularity, working distance, and resolution, andmore importantlyaid in the effective and efficient performance of the varied tasks ofdaily living without forcing the user to accept performance trade-offsto accommodate system limitations.15 In recent years, there havebeen remarkable advances in personal computing (e.g., smartphones),making customized real-time digital image processing to optimallyenhance low vision realizable and affordable. The big question nowis: Do we know enough about visual impairments and their relationto daily functioning to design customized low vision enhancementalgorithms that would be optimal for the individual patient?

Over the past 25 years, progress has been relatively slow indeveloping and implementing image-processing algorithms thatcan provide customizable enhancement of the patient's view ofthe environment. For example, customized contrast-enhancementand image-remapping strategies first demonstrated in the labora-tory more than three decades ago16,17 have not yet been imple-mented in commercial head-mounted low vision enhancementsystem systems. Although the past rate of progress has been lim-ited by the capabilities and cost of enabling technology, recentadvances in cameras, displays, and computing power now makeadoption of vision enhancement strategies demonstrated in thelaboratory feasible, if not imminent. The purposes of this articleare to provide a review of the current state of head-mounted lowvision enhancement system technology and to suggest areas ofresearch and development in personalized digital image process-ing that could be translated to practice in the future.

HEAD-MOUNTED LOW VISIONENHANCEMENT SYSTEMS

The Visionics Low Vision Enhancement System (Golden Val-ley, MN) was the first commercially available low vision en-hancement system based on a head-mounted video displaysystem.18 The displays were two 19-mm-diameter black-and-white cathode ray tubesmounted in the temple arms of the headsetand imaged at the user's far point for each eye through exit pupilsin the plane of the user's entrance pupils by field-correcting relayoptics and final aspheric mirrors. The display images were50 � 40 degrees with 40-degree binocular overlap, yielding a60-degree horizontal binocular field of view. Display resolutionwas 5 arcmin/pixel (equivalent to 20/100 visual acuity). A mono-chrome charge-coupled-device video camera in front of each eyeprovided unmagnified stereo video images of the environmentfor orientation. A single, tiltable (level to 45-degree down gaze),center-mounted, “cyclopean” video camera equipped with motor-driven optical zoom magnification (�1.5 to �12) and autofocus(with an auxiliary flip-up macro lens) provided the same magnifiedvideo image to both eyes. All video cameras had automatic gaincontrol to maintain constant average display luminance. TheVisionics low vision enhancement system was battery poweredwith user controls for switching between orientation and zoom cam-eras or external video and switching between manual focus control

and autofocus. The user also could control magnification, contraststretching, and contrast reversal.

Visionics low vision enhancement system competitors that cameon the market in the late 1990s and had similar features to theVisionics low vision enhancement system included the EnhancedVision Systems V-max (Enhanced Vision Systems, Inc., HuntingtonBeach, CA), the Keeler NuVision (Keeler Ophthalmic Instruments,Inc., Broomal, PA), the Innoventions Magnicam (Innoventions,Inc., Conifer, CA), and the Bartimaeus Clarity TravelViewer-to-go(Bartimaeus Group, McLean, VA), the latter two of which usedstand-mounted or handheld instead of head-mounted video cam-eras.13,14 The most enduring head-mounted video display low visionenhancement system was the Enhanced Vision Systems Jordy(Enhanced Vision Systems, Inc., Huntington Beach, CA), the successorto the V-max.

Several evaluative studies were conducted with these early lowvision enhancement systems. The use of these devices resulted insignificantly better distance and intermediate task performance thandid previously prescribed optical aids.19 The head-mounted lowvision enhancement system technology provided some improve-ment in home performance of activities of daily living, but opticalaids remained optimal for most of those tasks. Younger patientsperformed better overall.20 Newly diagnosed patients respondedmost positively to the technology; otherwise, preference could notbe predicted by age, sex, diagnosis, or previous electronicmagnifica-tion experience.21 No significant differences in outcomes betweenlow vision enhancement system devices were reported.20,21

All early head-mounted low vision enhancement systemseventually faded from the marketplace. To incorporate the com-puting power needed for image-enhancement strategies, largecomputer processing systems and hardware were needed. Atthe time, these were expensive components, and the companiesserving the boutique low vision market did not have the resourcesto advance the technology to the next level. However, recently,head-mounted low vision enhancement system technology has madea comeback. Products currently available include the eSight Eyewear(eSight Corp., Toronto, ON, Canada), NuEyes Pro Smartglasses(NuEyes USA, Newport Beach, CA), CyberTimez Cyber Eyez (CyberTimez, Winchester, VA), Evergaze seeBOOST (Evergaze, LLC,Richardson, TX), IrisVision (Visionize, L.L.C., Berkeley, CA), andthe return of a redesigned Enhanced Vision Systems Jordy. Thesenew systems usemoremodern-color microdisplay or cell phone dis-plays (liquid crystal display or organic light-emitting diode) andsmaller, higher-resolution color video cameras with cell phone cam-era optics. Table 1 compares the specifications of these new head-mounted low vision enhancement systems with each other andwith the specifications of the original Visionics low vision enhance-ment system. With the exception of the IrisVision, they all havesmaller fields of view than did the original Visionics low vision en-hancement system. Some devices such as the NuEyes, CyberEyez, and eSight also incorporate nonvision features such as opti-cal character recognition for text-to-speech and speech-output ar-tificial intelligence software for face and object recognition. Withmajor investments in technology development to serve the con-sumer virtual and augmented reality and personal theater markets,feature-rich enabling technology for head-mounted low vision en-hancement systems undoubtedly will continue to evolve and ad-vance to become more powerful and more attractive to wear atlower cost. To take advantage of these trends, attention now has tobe focused on developing and testing image-processing strategiesto optimize low vision enhancement.

Low Vision Enhancement with HMD Systems — Deemer et al.




CONTRAST-ENHANCEMENT STRATEGIES

Reduced contrast sensitivity is common among low vision pa-tients and is often cited as a major contributor to reductions inthe patient's ability to function visually.22,23 Patients with low vi-sion often report difficulty seeing facial features, interpreting facialexpressions, and recognizing familiar people. Significant contrastsensitivity loss can make seeing facial details, which are alreadylow in contrast, even more difficult.24–26 Difficulties with orienta-tion andmobility also are attributed frequently to reductions in con-trast sensitivity in this population.27,28 To generalize, one mightexpect that any activity that depends on recognition, identification,and interpretation of visual information depends heavily on theperson's ability to see varying levels of detail in the image.

Most vision scientists prefer to describe images and image pro-cessing in the spatial frequency domain (sums of sinusoidal spatialmodulations of luminance along each meridian with the amplitudeand phase varying as a function of spatial frequency and of orienta-tion). In this framework, contrast sensitivity at each spatial fre-quency can be interpreted as how much the amplitude at eachfrequency is reduced by the visual system. Thus, the visual systemis characterized as a linear filter for which the contrast sensitivityfunction is analogous to a modulation transfer function.

As shown with the y intercepts in the top panel and thescatterplot in the middle panel of Fig. 1, the maximum height ofthe contrast sensitivity function corresponds to contrast sensitivitymeasured using a Pelli-Robson, MARS, or other letter chart, andthe cutoff frequency (the spatial frequency that requires 100%contrast to be visible, as shown with the x intercepts in the toppanel of Fig. 1) corresponds to visual acuity measured using anETDRS or other high-contrast letter chart (shown in the bottom

panel of Fig. 1).30 As illustrated in the top panel of Fig. 1, Chungand Legge29 demonstrated that, to a good approximation, the con-trast sensitivity function has the same shape for people with low vi-sion as it does for normally sighted people when plotted on logcontrast sensitivity versus log spatial frequency coordinates.

The contrast threshold function is the inverse of the contrastsensitivity function (−log contrast sensitivity). As illustrated bythe area within the red and black curves in Fig. 2, only contrastsversus spatial frequency in the image that fall above the contrastthreshold function will be visible to the person. Formost natural im-ages, including images of faces, contrast decreases inversely pro-portional to spatial frequency (1/f contrast spectrum, which whenplotted on log contrast vs. log spatial frequency coordinates is a linewith negative slope).31,32 As shown by the solid green line in Fig. 2,a normal face at 10 ft has high contrast at low spatial frequenciesand 1/f drop in contrast with increasing spatial frequency. At ap-proximately 10 cycles/degree, contrast in the face image is toolow to be visible to the normally sighted person (solid line falls be-low the normal contrast threshold function). For the low vision pa-tient, all information in the face image greater than 3 cycles/degree is invisible. If the face image is magnified by �5, its con-trast threshold function will be shifted to the left by 0.7 log cy-cles/degree (dashed green line). However, because this patient'scontrast thresholds are elevated overall by 1 log unit, magnificationwould not improve the visibility of the part of the face contrast spec-trum that was visible to the normally sighted person but below thepatient's contrast threshold in the unmagnified image (falls withinthe black curve but not the red curve). Indeed, in this example,magnification could make the visibility of the face image evenworse for the patient. Contrast at spatial frequencies between 0.5and 1 log cycles/degree on the display has to be increased by atleast an amount ranging from 0.1 to 0.75 log unit over the

TABLE 1.Manufacture specifications of head-mounted video display LVESs

Visionics LVES Jordy SeeBOOST eSight 3 NuEyes Cyber Eyez App IrisVision

Price (U.S. $) $5000 (in 1994) $3620 $3500 $9995 $5995 $1997(software only)

$2500

Weight (g) 992.2 236 25 (plus theweight ofeyeglasses)

104 125 371.4 425.2

Binocularity Binocular Cyclopean* Monocular Cyclopean* Cyclopean* Monocular Cyclopean*

Display resolution 640 � 480 640 � 480 800 � 600 1024 � 728 1280 � 720 640 � 360 1210 � 9205 arcmin/pixel 2.8 arcmin/pixel 2.25 arcmin/pixel 2.2 arcmin/pixel 1.4 arcmin/pixel 1.9 arcmin/pixel 3.3 arcmin/pixel

20/100 Snellenequivalent





20/37.5 Snellenequivalent


Magnification (times) 1.5–12 1–14 1.4–7 1–24 0.6–12 (withadditionaloptical lens)

1–15 1–8

Field of view(horizontal �vertical; degrees)

50 � 40 30 � 17 30 � 22.5 37.5 � 28 30 � 17 20 70 � 50

Picture of device

Pictures are the authors' photographs unless otherwise referenced as follows: Visionics LVES, https://er.jsc.nasa.gov/seh/pg66s95.html; seeBOOST,seeBOOST product brochure; and Cyber Eyez App M300 package, https://cybertimez.com/?product=cyber-eyez-m300-complete-package. *Viewingthe same image with both eyes (no disparity). LVES = low vision enhancement system.




https://er.jsc.nasa.gov/seh/pg66s95.html

http://cybertimez.com/?product=cyber-eyez-m300-complete-package


frequencies of interest to optimize the visibility of the unmagnifiedface image. Peli et al.16,33 first demonstrated the feasibility ofthis frequency-selective contrast-enhancement strategy in 1984,but a considerable amount of work still needs to be done to developand test the optimal contrast-enhancement algorithm for low visionusers representing a wide variety of visual impairment.

As shown in Fig. 3, for a normally sighted person, both the max-imum contrast sensitivity and the cutoff frequency increase withmean luminance.34,35 The same luminance dependence of thecontrast sensitivity function has been shown for patients with retinaldiseases.36 The increase in these two parameters with increasingluminance explains why increasing ambient light can be helpfulto low vision patients, although manipulating the ambient lightlevel alone does nothing to change image contrast, which in theenvironment is determined by the ratio of reflectances. Althoughvisual acuity/cutoff frequency increases with luminance for bothnormally sighted and low vision observers, that occurs only overa limited range, after which it asymptotes at a maximum value.Therefore, contrast enhancement alone is not sufficient compen-sation for most patients, and it also is necessary to magnify theimage to compensate for the loss of resolution (no benefit isgained from enhancing contrast of spatial frequencies that ex-ceed the cutoff ). However, as illustrated in Fig. 2, middle spatialfrequency bands (e.g., 3 to 7 cycles/degree) are often above theresolution limit but still below contrast threshold for the patient.29

Using digital image processing, with current technology, we cannow enhance the contrast of selected spatial frequency bands inlive video images without significant frame delays.

It has been demonstrated in past studies that individuals withlow vision can recognize frequency-selective contrast-enhancedimages of faces better than faces that are unprocessed with the en-hancement system.16,33,37 In addition, removing the image detailmay actually improve recognition by reducing crowding in theimage for the low vision patient.9 Because of between-patientvariations in the contrast sensitivity function,29 it most likely willbe necessary to custom prescribe the optimal parameters forcontrast enhancement.

Most video magnifiers incorporate contrast stretching. As illus-trated in the middle panel of Fig. 4, contrast stretching consistsof mapping pixel intensities above a criterion value to themaximumintensity, mapping pixel intensities below another criterion to theminimum intensity, and linearly rescaling the pixel intensities thatfall between the two criteria. To minimize distorting the color or in-troducing color artifacts, contrast stretching should be limited tothe luminance component of the video signal (L channel in theLab color space), as was done in the middle panel of Fig. 4. How-ever, this type of contrast stretching at the pixel level does not takespatial frequency information into account.

Edge enhancement is another contrast-enhancement strategythat many of the current head-mounted low vision enhancementsystems use to compensate for reduced contrast sensitivity. Edgeenhancement selectively stretches the contrast at sharp luminancegradients in the image (at edges of objects and features).38,39 Usu-ally, edges are defined by high spatial frequencies. One very oldphotographic method that could be applied to enhancing contrastof digital video images in a defined frequency band is unsharpmasking. For this technique, the video image is masked with ablurred negative copy of the image (low frequencies only), whichleaves only frequencies that are higher than the cutoff frequencyof the mask. The contrast of the masked image is stretched andthen multiplied with the original image. The rightmost panel of

FIGURE 1. (Top) Comparison of log contrast sensitivity as a function oflog spatial frequency for the average of five normally sighted subjects(black points) and two low vision patients (green and red points) fromChung and Legge.29 The two parameters corresponding to Pelli-Robsoncontrast sensitivity and visual acuity, respectively, are the peaks of theCSF for normally sighted subjects (black) and low vision patients (greenand red), and x intercept values of the curves, which are the respectivecut-off frequencies. (Middle) Log contrast sensitivity at the peak of theCSF versus spatial frequency function versus log contrast sensitivitymeasured using the Pelli-Robson chart. (Bottom) Log cutoff frequencyfor the CSF versus log visual acuity measured using the ETDRS chartexpressed in units of log spatial frequency (unpublished data from 80low vision patients obtained in 1994). The solid line in the middleand bottom panels is the identity line (slope, 1; intercept, 0).





FIGURE 2. Log contrast threshold versus log spatial frequency for a normally sighted person (black curve) and a low vision patient (red curve). The areawithin the red curve represents contrasts that are visible to the patient. The area within the black curve represents contrasts visible to a normallysighted person.

FIGURE 3. Normal log contrast sensitivity for different average luminance levels (see legend). Cutoff frequency (corresponds to visual acuity) increaseswith luminance. Contrast sensitivity at the peak also increases with luminance.





Fig. 4 illustrates the result of unsharp masking applied only to theluminance channel of the original image in the leftmost panel.The technology now exists to implement these contrast-enhancementstrategies, but our knowledge of what constitutes optimal contrast en-hancement for different patients and how effective contrast en-hancement can be for different patients is still inadequate.

IMAGE-REMAPPING STRATEGIES

Typically, there is one-to-one, one-to-many (in the case of mag-nification), or many-to-one (in the case of minification) mapping ofcamera pixels to display pixels. Loshin and Juday17 suggested thatpixel mapping could be customized to distort images to prevent vi-sual information from falling in the patient's scotoma. As shown inFig. 5, the image can be torn and stretched around the scotoma.This form of local image remapping produces distortions in the im-age. Some demonstration studies have been reported that suggestthat such remapping could be beneficial to the patient, especiallyfor reading.40–42 However, to properly implement this remappingstrategy, the tear and distortion of the image on the display wouldhave to be stabilized on the retina to keep it registered with thepatient's scotoma regardless of the direction of gaze. This approachrequires eye tracking built into the head-mounted system with real-time image remapping occurring at video frame rates.

We developed an image remapping method for magnification,which could best be described as a virtual bioptic telescope. Thisnew magnification strategy, which is now incorporated in theIrisVision, consists of a magnified region of interest, called the“magnification bubble” that is embedded in a larger unmagnifiedfield of view. Our approach borrows from some of the earlier ideasof Loshin and Juday,17 but like a bioptic telescope, head move-ments rather than eye movements are used to relocate the bubbleto a new location. This approach avoids inherent problems associ-ated with current eye-tracking systems such as poor accuracy andprecision, poor reliability, time lags due to frame delays, and diffi-culty with calibrations for users who cannot fixate reliably. The sizeand shape of the magnification bubble can be manipulated by theuser to accommodate individual preferences and the requirements

of specific tasks. For example, as shown in Figs. 6A, B, a rectangu-lar view might be optimal for reading and a circular view for seeingfacial expressions. The amount of magnification within the bubblealso can be controlled by the user. Unlike a conventional bioptictelescope, which overlays a magnified image on the unmagnifiedfield of view and creates an artifactual ring scotoma, themagnifica-tion bubble distorts the image by remapping the transition fromthe magnified image in the bubble to the unmagnified surround-ing field so as not to overlay and lose any visual information.

Presumably, low vision patients fixate the bubble with themacula, or with a preferred retinal locus if the macula is impaired,to look at themagnified area of interest.43–45 Experiments simulat-ing scotomas in normally sighted subjects imply that training caninfluence the development and location of the preferred retinal lo-cus.46,47 Because there are currently no eye-tracking capabilitiesincorporated into the system, head movements alone change thevisual information that falls in the bubble. Users can accomplishthis movement of the bubble to a new region of interest quite

FIGURE5. Image remapped around scotoma (black area) with a tear inthe image and distortions to prevent any information from being lost.

FIGURE 4. (Left picture) Original unprocessed image. (Middle picture) Contrast stretched in the luminance (L) channel of the original image and com-bined with unaltered color channel images (a, b). (Right picture) Unsharp mask applied to luminance (L) channel of original picture and combined withunaltered color channel images (a, b).





comfortably and with little training. In addition to being able toma-nipulate magnification within the bubble and to change the sizeand shape of the bubble, the user has the option of manuallyrelocating the bubble to any part of the video display. With an inte-grated eye-tracking system in the future, it might be feasible to useeye movements to relocate the bubble on the display so that it al-ways stays superposed on the part of the retina used for fixation (as-suming that the preferred retinal locus is consistent under differentviewing conditions and directions of gaze). Saunders and Woods48

have discussed system requirements that must be met to makesuch gaze-controlled image processing acceptable to the user.The development of accurate, precise, and reliable gaze-con-trolled real-time image processing in head-mounted displays,which is being pursed for many different applications, creates re-search opportunities for problems that have received relatively lit-tle attention in the low vision rehabilitation field.

One recent study completed by Aguilar and Castet49 tested agaze-controlled system that magnified a portion of text whilemaintaining global viewing of the rest of the text. Their resultssuggest that user preference and reading speed were greater forthe gaze-controlled condition when compared with uniformly ap-plied magnification without any specificity to region of interest(mimicking commercial closed-circuit televisions), but therewas no significant difference between the gaze-controlled sys-tem and a system with zoom-induced text reformatting. We knowthat limitations exist in eye pointing and visual search with thistype of gaze-contingent system from experiments described byAshmore et al.50 using a fisheye magnification system in nor-mally sighted observers. Based on information they gathered,hiding this magnification bubble during visual search leads toimprovement in speed and accuracy over eye pointing with nobubble or with a bubble that is continually attached to the user'sgaze. We also know that fixation stability in patients with central vi-sion impairment is generally poor,51,52 which may add complication

to eye-tracking systems during calibration and subsequently duringimage remapping. More research is needed to determine the mosteffective way to incorporate eye tracking and gaze control with mag-nification within low vision enhancement systemswhen performingvarious tasks.

Conceptually, one could extend the image-remapping strategyto produce custom distortions in images that are designed to undothe effects of, and thereby correct, metamorphopsia. This approachalso would require precise eye tracking to register the compensat-ing image distortions with the part of the retina experiencingmetamorphopsia. Virtual and augmented reality systems arecreating a demand for high-performance eye-tracking systemsin head-mounted displays to increase computing efficiency withgaze-referenced high level of detail rendering of graphical ob-jects, so ongoing research and development are soon likely to pro-duce the enabling eye-tracking technology at a price consumerscan afford.

MOTION COMPENSATION STRATEGIES

The visual vestibulo-ocular reflex produces eye movements thatcompensate for head movements to keep the image stationary onthe retina (i.e., “doll's eye” phenomenon). When viewing an an-gularly magnified image, the velocity of image motion relative tohead motion is magnified by the same factor.53 When there is amismatch between velocities recorded by the visual system andthose recorded by the vestibular system, the individual canadapt to a limited extent with neural changes in the gain of thevestibulo-ocular reflex. However, the range of physiological gainchange is too small to be useful for the levels of magnificationused for low vision enhancement, and this results in image slipon the retina.53,54 With increased levels of magnification, theimage presented to the retina spans a larger field of view, and

FIGURE 6. Two magnification bubbles of different shape. (A) A rectangular bubble potentially more helpful for reading tasks. (B) A circular bubblethat may be used for object and facial recognition. Note the distortion aroundmargins of the bubble to prevent information from being lost because ofimage overlap.





the magnitude of vestibulo-ocular reflex eye movements needed toproperly compensate for image motion eventually falls outside therange of gain control of the reflex. Image motion velocities greaterthan 20 degrees/s (which, e.g., are commonly experienced whenwalking) progressively decrease contrast sensitivity and visual acu-ity with increasing velocity,55 which often is described clinically asdynamic visual acuity.7,14 Because the camera incorporated in ahead-mounted low vision enhancement system moves with theusers' head movements, magnified image motion not only de-creases image resolution but also increases the risk of motion sick-ness and other symptoms of visual discomfort as the userintentionally or inadvertently moves his/her head.56 Embeddingthe magnification bubble in a bioptic design allows for the visualinformation outside the bubble to be presented with no addedmagnification, which minimizes the overall image motion experi-enced with increased levels of magnification. Although this strategyreduces the susceptibility to motion sickness, magnified image mo-tion within the bubble still imposes limits on the user's performancebecause of dynamic visual acuity limitations on resolution.14

Mismatches between head and image motion beyond the rangeof vestibulo-ocular reflex adaptation are a problem that has plaguedvirtual and augmented reality systems for decades.57–59 However,advances in angular and linear motion sensor technology, nowroutinely incorporated in smartphones and modern virtual andaugmented reality systems, and increases in computer graphicsspeed and power have made it possible to inexpensively matchimage motion to head motion plus the vestibulo-ocular reflex. Im-provements in the accuracy and precision of MicroElectroMechanicalsystems accelerometers and gyroscopes aid in better calculation oflinear and angular motion needed to compensate for magnifiedimage motion. For example, with the current capabilities of asmartphone, it is now possible to convert angular magnificationto linear magnification in head-mounted low vision enhancementsystems. This strategy has been implemented in the IrisVision bytexture mapping a high-resolution image from the camera, or fromstored or streamedmedia content, onto a virtual screen that movesin virtual reality at the negative of the velocity of natural head move-ments. This strategy eliminates artifactual imagemotion from the vi-sual vestibulo-ocular reflex when viewing snapshot images andmedia content. With further development, this approach has the po-tential of eliminating magnified image motion from head-mountedvideo camera movements. This strategy makes any amount of mag-nification practical in an arbitrarily large field of view that can be ex-plored with head movements.

When coupled with eye tracking, image motion compensationstrategies could be used to neutralize oscillopsia in conditions suchas nystagmus or bilateral vestibular loss. One could also considerextending motion compensation strategies to enhancement of vi-sual flow fields, which are important to perceiving self-motion inthe environment, maintaining balance and preventing falls, andjudging closing velocities to prevent collisions. Such an extensionis likely to press the state of current technology. With the constantlyaccelerating rate of technology development, it is not too early tobegin investigating these possibilities.

AUGMENTED REALITY STRATEGIES

Unlike virtual reality, which refers to a computer-generated en-vironment in which the user is immersed and with which the usercan interact, augmented reality refers to graphic overlays on, or

graphic objects inserted in, live images of the real environment.Peli's60 strategy of vision multiplexing, which can be implementedoptically, digitally, or with hybrid technology, is a pioneering appli-cation of augmented reality to low vision enhancement. The term“multiplexing” is used when multiple streams of information sharea single mode of transmission. In the case of vision, multiplexingrefers to a controlled form of spatial diplopia (superposed semi-transparent images that are visible simultaneously) or temporaldiplopia (alternate presentation or alternate suppression of super-posed images).60,61 Two examples of optical strategies for imple-menting vision multiplexing are Peli prisms,62 which in the case ofhemianopic visual fields superimpose images from unseen portionsof the peripheral field onto nonfixating seeing areas of remainingfield, and the intraocular telescope, which magnifies a wide-field,centrally viewed image in one eye only. Both methods can be imple-mented in augmented reality with a head-mounted (e.g., seeBOOST)or heads-up (e.g., Google Glass and Cyber Eyez) display.60

Considerable basic research has been conducted on perceptual“filling-in” phenomena with artificial scotomas and the physiolog-ical blind spot.63,64 Most low vision patients with central scotomasare unaware of blind areas in their vision because the visual systemcovers them over with images that blend in with the background.65,66

This filling-in phenomenon causes objects, words, facial features,and other visual information to unexpectedly vanish or be replacedwith incomprehensible patterns manufactured by the visual sys-tem. Consequently, the scotoma interferes with reading, visualsearch, face recognition, reaching for and grasping objects, anddetecting obstacles. Making the scotoma visible by controlling thefilling-in (e.g., with stabilized graphic annuli that are distinct fromthe rest of the background)67 may prove to be helpful in that at leastthe person would know where the blind spot is and how it has tobe moved to look behind it.68 Currently, we do not know what kindof filling-in will occur with augmented reality graphic images (or im-age remapping around scotomas as discussed earlier) and whetherit will cause confusion. There is much research that still needs tobe done to further understand the filling-in process and to deter-mine if and how we can control the image that fills in the blind spot.

Combined with object and face recognition artificial intelli-gence software (e.g., OrCam MyEye [OrCam Technologies Ltd,Jerusalem, Israel] and Microsoft Seeing AI application [MicrosoftCorp., Redmond, WA]), augmented reality strategies could be usedto assist low vision patients by augmenting visible but uninterpret-able visual information (e.g., highlighting obstacles to assist mobil-ity, highlighting scan paths or fixation history to assist visualsearch, and tagging or captioning objects and faces to assist withrecognition). Combined with Global Positioning System–based nav-igation software, augmented reality strategies also could be used toassist low vision patients with wayfinding. Many such systems un-doubtedly will be developed for the normally sighted consumer, sothe implementation of such hybrid augmented reality strategiesmight bemore of an issue of adaptation or accommodation ratherthan an independent effort to develop a dedicated low vision en-hancement system product.

FUTURE INNOVATION

The low vision field needs to prepare to realize the promise ofhead-mounted low vision enhancement systems as virtual realityand augmented reality technology development expands. This





preparation can be accomplished only with innovative low vision re-search. We need to learn more about eyemovements in the visuallyimpaired population, how the neural visual system adapts to visualimpairments, and how people with different types and degrees ofvisual impairment respond to various image-processing strategiesthat are now possible to implement at video frame rates withhead-mounted computer technology. Using these strategies tosimplify and create improved fixation patterns may, for example,enhance reading performance in patients with central vision im-pairment.69 Future advances in head-mounted display technologythat can be used as a platform for the ultimate head-mounted lowvision enhancement system still need to address the issues of size,weight, field of view, resolution, battery life, user interfacing, andattractive design while also having the computer power to integratecustomizable low vision enhancement operations. The develop-ment of display technology with improved diffractive and lightweightoptics has helped in the advancement of these systems, but to in-crease both resolution and field of view, even better high-density dis-plays must be developed along with high-density drivers. Fortunately,the requirements for low vision applications are less demanding in thisregard than are the requirements of the general augmented reality andvirtual reality consumer market, so the burden of enabling hardwaredevelopment does not fall on the low vision industry.

Arguably, the single most important required innovation stilloutstanding is to provide binocular disparity information tothe patient under conditions of magnification. Because scotomasare rarely binocularly symmetric, binocular viewing, with its in-creased field of view and accompanying depth cues, offers poten-tial advantages to the patient. However, diplopia and rivalry havenegative consequences, and constant suppression would defeat at-tempts to gain a binocular advantage.70,71 Also, scotomas andmonocularly measured preferred retinal locations are not likely tofall on corresponding points.72 We do not know how patients re-spond when this happens, but it is certainly an area of needed re-search and exploration. Binocular disparity of objects in differentdepth planes drives vergence, which will alter the overlap of

scotomas in the two eyes resulting in a change in the binocular sco-toma.73 This result has consequences for avoiding diplopia inremapped images. Diplopia also is a concern for magnificationwithin a region of interest centered on the preferred retinal locusbecause angular magnification (pixel magnification) is used, whichalso magnifies binocular disparity. Thus, the unmagnified part ofthe scene might be fused but not necessarily zero disparity be-cause of Panum's area, and angular magnification in the regionof interest then magnifies the disparity angle beyond the Panumtolerance limit causing diplopia.

The next generation of low vision enhancement will use image-processing algorithms individually prescribed to optimize eachpatient's vision. Although there is some existing research highlight-ing the benefits of image-remapping and contrast-enhancementimage-processing techniques,9 additional research on optimizingalgorithm parameters and their implementation in commerciallyavailable technology must expand to increase the potential benefitof low vision enhancement systems for individual patients. Thesenew devices and low vision enhancement strategies require skilledrehabilitation services, and the field therefore has to be prepared totrain the patient.

Eye care professionals will likely have to soon learn newmethods of vision rehabilitation. A similar situation was createdon a small scale with the introduction of the implantable miniaturetelescope74 and with various competing versions of phosphene-based prosthetic vision systems.75,76 Although optics and closed-circuit televisions are still the convention, with new head-mountedlow vision enhancement system and artificial intelligence prod-ucts, we are experiencing the vanguard of a new wave of technologythat soon will demand significant changes in how low vision pa-tients are evaluated and the rehabilitation services that areprovided to them. The limitations of optical aids point out thepotential gaps electronic systems may fill across all individualusers within the low vision population. With growing technology,we move closer to truly customizing products for each individualvisual impairment.

ARTICLE INFORMATION

Submitted: February 1, 2018

Accepted: June 28, 2018

Funding/Support: Reader's Digest Partners for SightFoundation (to RWM); National Eye Institute (R01EY026617;to RWM); and National Eye Institute (grant R44EY028077;to FSW).

Conflict of Interest Disclosure: ADD, CKB, NCR, DMN,and RI – no financial conflicts of interest. FSW –

shareholder and the chief executive officer of VisionizeLLC; the author was responsible for the preparation ofthis manuscript. RWM – scientific advisory board ofEvergaze LLC; the author was responsible for thepreparation of this manuscript and the decision tosubmit this article for publication.

Author Contributions: Conceptualization: CKB, NCR, DMN,FSW, RWM; Investigation: RI; Methodology: RWM;Resources: NCR, RI, FSW, RWM; Software: CKB, RI, FSW;Supervision: RWM; Visualization: ADD, RWM; Writing –

Original Draft: ADD; Writing – Review & Editing: ADD,CKB, NCR, RI, FSW, RWM.

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