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Historically, a number of technological ad-vances have been derived from the ability ofdevelopment of head-worn displays (HWDs). Ankind of arrangement (e.g., Melzer & MofÞtt, 1997;Patterson, Winterbottom, & Pierce, 2006; Peli,1999; Velger, 1998).When synthetic information is presented toallax (i.e., binocular disparity, the cue for stereo-eyes, it is called a binocular HWD. Various kindset al., 2006). Also, one can perform differentvisual targeting, passive entertainment, and activeBecause HWDs can offer advantages over tra-ease of mobility (Velger, 1998), attempts havedeveloped by Vision Systems International foruse by the U.S. Air Force in a variety of Þghteraircraft. The JHMCS involves a semitransparenttargeting symbology as well as other criticalßight information. This HWD is viewed by onepersonal use (Hakkinen, 2004; Velger, 1998), Binocular Rivalry and Head-Worn Displaysashington State University, Pullman, Washington, Marc WinterbottomByron Pierce,Air Force Research Laboratory,Mesa, Arizona, and anderbilt University, Nashville, TennesseeBackground:applied HWD literature in order to help provide insight for minimizing the effects ofof binocular rivalry, (b) stimulus factors affecting rivalry, (c) cognitive variablesaffecting rivalry, and (d) tasks affected by rivalry. This paper offers aset of recommendations for minimizing the effects of binocular rivalry when HWDsAddress correspondence to Robert Patterson, Department of Psychology, Washington State University, Pullman, WAACTORS001872007X249947. Copyright ©2007, Human Factors and Ergonomics Society. All rights reserved. Report Documentation PageForm ApprovedOMB No. 0704-0188Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Wash

ington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 1084December 2007 Ð al., 1998), command and control (Galster, Bolia,2005), maintenance training simulation (Wenzel,Castillo, & Baker, 2002), or high-performanceracing (Velger, 1998).Despite the potential advantages of HWDs, there1998). For example, Wenzel et al. (2002) founddizziness when an HWD was used for aircraftputer monitor. Morphew, Shively, and Caseywas performed (however, see Peli, 1998). Larameeand Ware (2002; see also Hakkinen, 2004) foundIn a study by Behar et al. (1990), a large num-ber of helicopter pilots reported at least one visuala monocular transparent HWD. About 70% ofpilots noted that vision occasionally and unin-eye either during or after ßight. Moreover, Rash,using the IHADSS reported difÞculty making thefactors, such as vestibular-visual cue conflict(Draper, Viirre, Furness, & Gawron, 2001; Draper,iirre, Furness, & Parker, 1997; Ehrlich, 1997).discussed by Patterson et al. (2006). In particular,eyes receive very different stimulation, resultinginterocular differences.interocular differences can occur with differenttypes of HWDs in different ways:the HWD information, interocular differences intypically present the sameinformation to the two eyes of a user, whereasfor stereoscopic depth perception. When the twoeyes view the same or similar information, thepotential for interocular differences to be createdeyesÕviews.with which to view complex imagery, such as acomputer interface or sensor imagery, whileimagery with the opposite eye. This scenariowould introduce significant interocular differ-size of the Þeld of view, only a central region ofwhich creates interocular differences in themonocular regions (Velger, 1998, p. 56).When the two eyes receive different st

imulationbinocular rivalry.& Rogers,1995; Levelt,1965). The visibility of theimages in the two eyes ßuctuates, with one eyeÕseyeÕs view are rendered invisible and suppressed, IVALRYANDMany authors (e.g., Blake, 2001, p. 5; Breese,the brain that two different objects exist at thesame time in the same location. Importantly, dur-ing binocular rivalry, portions of stimulation inHWD use. It is these authorsÕcontention that bin-rivalry when HWDs are used under real-worldtial effects of rivalry with the use of HWDs inaviation (e.g., Behar, et al., 1990; Rash et al.,1990). Another frequently used technique is ratingor tracking target stimuli visibility. For instance,Hershberger and Guerin (1975) used ratings ofvisibility to document the potential effects of 12different parameters (e.g., luminance, trans-parency, resolution, etc.) affecting binocularrivalry with monocular HWDs. The difficultyas rivalry, which may be intermittent and piece-subjective methods. However, it is not clear thatformance attributable to rivalry.of the userÕs direct line of sight (Hakkinen, 2004;Laramee & Ware, 2002; Peli, 1990). Hakkinenannoying and straining experience for the user.previously, associated with the U.S. ArmyÕsand to decrease the visibility of ßight symbologyin the U.S. Air Force JHMCS (Winterbottom,Patterson, Covas, Rogers, & Pierce, 2006; Winter-bottom, Patterson, Pierce, & Taylor, 2006).nocular HWD. These studies used ßight databaseing of U.S. Air Force pilots.er eye; and a binocular-fused condition wherebyby both eyes. Abrießy presented small target wasscene exerted a suppressive effect on the monoc-ular imagery but not vice versa. Winterbottom,occurrence of the rivalry.phenomenon labeled ÒluningÓ within theapplied literature on HWDs (e.g., Velger, 1998,there is partial overlap of the two eyesÕviews. Itrivalry, the same suppression processes are prob-ably operative. Grigsby and Tsou (1994) noted that 1086December 2007 Ð are viewed (e.g., Patterson et al., 2006; Velger,under real-world operational conditions. Theales & Fox, 1970), wherein the ability to detect,probe stimulus is

degraded, which is taken as evi-dence of suppression. The suppression is inducedthe stimuli that provoke the rivalry. However,detection or recognition of relatively large stim-impair the visibility of portions of symbology,cates which eyeÕs stimulus is visible at a givenÒexclusive visibilityÓ (i.e., one or the other eyeÕsportions of the two eyesÕimages is seen). Althoughof HWDs, they suffer from problems relating tothe response criterion of an observer, as to whenMoreover, the basic research on rivalry hastically. Although these types of stimuli providewould affect the kinds of tasks that would typi-cally be performed when HWDs are worn inThus, there remains a large amount of appliedlar rivalry affects the use of HWDs. Nonetheless,a key belief that undergirds this review is thatously, and that directions for future appliedresearch, as well as new insights for potentialbinocular rivalry. Thus, the purpose of this paperon binocular rivalry, in addition to many appliedfor future research. We begin with a discussion BINOCULAR RIVALRYlar rivalry can be provoked by interocular differ-polarity, form, size, and motion velocity and that2001, pp. 8Ð9). The inhibition or suppression thatarea of the retina, not upon the stimulus per se(e.g., Blake, 1989; Blake & Fox, 1974).Moreover, suppression takes time to develop.200 ms (Anderson, Bechtoldt, & Gregory, 1978;Blake, Westendorf, & Yang, 1991; Wolfe, 1986) or1984) do not provoke rivalry but, instead, may bethat are abruptly increased in luminance or contrast IVALRYAND& Check, 1968; Walker, 1975; Walker & Powell,tests of eye dominance (e.g., Washburn, Faison,dominance (Coren & Kaplan, 1973). The lack ofoperation of different underlying processes, withHowever, other researchers have reported thatsighting eye dominance affects rivalry domi-sighting dominance interacts with binocular ri-valry to affect a pilotÕs ability to attend to one orthe other eye. Thus, it remains to be determineddifferent types of HWDs.of the retina (Blake, 2001, p. 16; Fox, 1991). This1968; OÕShea, 1987) and can make large changes1968, Walker, 1975). The

loss of sensitivity canattention to targets in the visual field (Schall,Nawrot, Blake, & Yu, 1993).Bilotta, 2000; Ooi & Loop,1994; Smith, Levi, Har-werth,&White, 1982;Wales & Fox, 1970) indicatetivity is on the order of 0.5 log units (or, equiv-alently, 1.5 magnitude of inhibitory effects found with otherHerrmann,1967; Lehky, 1995), which is consistentport that they cannot predict suppression duration.The size and shape of the visual Þeld that isrivalry-inducing stimulus. When that stimulus issmall, the zone of suppression is larger than thestimulus, but when the stimulus is larger (e.g.,image of a face), only portions may be suppressedand suppression areas (Howard, 2002, p. 286).Generally, the size of the zones of suppressioncontaining high spatial frequency information (e.g.,6Ð10 cycles/¡; Blake, OÕShea, & Mueller, 1992;Kaufman, 1963; Liu & Schor, 1994), but thezones of suppression may be larger with images2002, pp. 286, 297), and with low levels of illu-mination (OÕShea, Blake, & Wolfe,1994) and highstimulus contrast (Liu & Schor, 1994). However,for individual suppression events, the size of asuppression area varies widely, in a haphazardpattern. Blake (2001, p. 11) noted that the zones ofsuppression within a given eye may show intraocu-lar and interocular grouping with synchronizedcommon hue (e.g., Alais & Blake, 1999; Kovacs,Papathomas, Yang, & FehŽr, 1996; Lee & Blake,There are reports that binocular fusion canminimize the occurrence of binocular rivalry. Forexample, in a basic study, Blake and BoothroydHowever, as we discussed previously, Winter-bottom, Patterson, Covas, et al. (2006) and Win- 1088December 2007 Ð viewed binocularly. This difference in resultsand the Winterbottom, Patterson, Covas, et al.(2006) and Winterbottom, Patterson, Pierce, et al.(2006) studies is likely attributable to differentwas described earlier. Whereas some authors ofable after about 30 min of use (Grigsby & Tsou,observer engages in a demanding task (J. Melzer,personal communication April 18, 2006), there is(Klymenko, Harding, Beasley, & Rash, 2001)Longridge, 1984) for targe

ts appearing close tominimize the effects of luning, Grigsby andof at least 40¡, whereas Melzer and MofÞtt (1997)and Klymenko, Verona, Martin, Beasley, andbinocular regions. However, although the intro-r factor that may influence the occurrenceis the use of a convergent or divergent design. Withconvergent binocular overlap, the left eye viewseye views the left monocular ßanking region. Withdivergent binocular overlap, the left eye views theMelzer and MofÞtt (1997) and Klymenko et al.with a convergent design relative to a divergentdesign. Klymenko, Harding, Beasley, Martin,and Rash (1999) tested the effect of Þeld-of-viewtarget acquisition task. These authors showed thatconvergent overlap resulted in better performancerelative to divergent overlap (although perfor-mance was still best for the full binocular-overlapdesign). Thus, a convergent design is better if aorder to increase the size of the Þeld of view.This distinction between convergent anddivergent overlap may be related to the idea thatlogically invalid monocular images. Shimojo andnaturally have slightly different views in a 3-Dright eye may see a portion of the background thatachi, 2004). Of course, this kind of viewingdifferences in luminance, contrast polarity, size,hue, and motion velocity. The suppression atten-dant to binocular rivalry takes about 200 ms toHWDs, for which both eyes view an outdoor scenethe HWD. Thus, although binocular fusion maysuppression still occurs when monocularly viewedviewed imagery.by Winterbottom, Patterson, Covas, et al. (2006) IVALRYANDand Winterbottom, Patterson, Pierce, et al. (2006)by Hakkinen (2004) and Peli (1990). Thus, sig-mance decrements owing to the occurrence ofmonocular HWDs. We strongly recommend thatsuch HWDs not be used.would be readily apprehended before the onset ofsuppression. An alternative method may be toaware that trading off binocular overlap againsttargets is important, full overlap may be a betteroverlap of at least 40¡is recommended, but eventhen suppression may occur in the periphery. Thefalse contour, may increase the ecological valid-ity of

an unnatural viewing condition. The win-dow frame or false contour may be interpreted bythe visual system as a valid foreground occluder,STIMULUS FACTORS AFFECTING RIVALRYproportion of time a stimulus is visible duringincrease with increased orientation differences be-tween stimuli in the two eyes (e.g., Abadi, 1976).Also, stimuli with a greater amount of contourIn basic research, interocular differences indominance; interocular differences in luminanceplay much less of a role (e.g., Levelt, 1965). StimuliRasche,1969; Hollins,1980; Levelt, 1965), and theare of high contrast (Alexander, 1951).) Ð and found that contrast affectedthe time course of rivalry. They found that alter-0.003 to 0.3 (see also Liu, Tyler, & Schor, 1992).difÞcult to interpret. Nonetheless, a few general-1980; OÕShea et al., 1997). Moreover, high spatialstimuli in the other eye (Yang, Rose, & Blake,When wavelengths are different enough toproduce percepts of different hues, such differ-ences can provoke rivalry. Moreover, when stimuli 1090December 2007 Ð stimuli of different hues. For example, Hollinswas less when both rival targets were blue (455 nm)difference in hue increased.(1899), and this effect of the potency of motionattention; see also Blake, Yu, Lokey, & Norman,ade, de Weert, and Swanston (1984) inves-tigated speed across the range of 0¡/s to 0.41¡/sslower stimuli when they differ in direction ofmotion. Blake, Zimba, and Williams (1985) usedrandom dot patterns and found that when the inter-ocular difference in motion direction exceeded30¡(at a speed of 1.5¡/s), rivalry occurred. Whenpiecemeal as interocular differences in velocityincreased. The smallest interocular difference inwhich provoked signiÞcant piecemeal rivalry.sity, higher contrast, or faster motion. Rivalryprovoked by interocular differences in contrastvision functions, up to bright photopic levels,which include cone functioning. Thus, rivalryspatial frequencies. This means that portions ofever, when the two eyes view a simulation of anoutdoor scene (e.g., a ßight simulator display),suppression (Winterbottom, Patterson,

Covas, etal., 2006; Winterbottom, Patterson, Pierce, et al.,2006), which would also be true when a real-world outdoor scene is viewed. The monocularlysuppression, however.HWD with high contrast (i.e., 0.5 log units greaterrivalry, especially for tasks used with HWDs underopaque HWDs. These latter displays present syn-synthetic information. Thus, a patent dichoptic-likely be signiÞcant interocular differences (incontrast, shape, hue, etc.) and therefore very robustrivalry, as shown by Peli (1990), Hakkinen(2004), Winterbottom, Patterson, Covas, et al.(2006), and Winterbottom, Patterson, Pierce, etal. (2006). Again, we recommend against the useFor partial-overlap binocular HWDs, targets or other kinds IVALRYANDunique dichoptic stimuli for inducing rivalry. It isCOGNITIVE VARIABLES AFFECTING RIVALRYfocused on the physical attributes of rivalry-and predominance. Some studies, however, exam-inedthe observer, which include familiar or meaningfulstimuli, or stimuli that induce affective states; andof the observer.good, 1966), but low-level stimulus features suchthese studies. Blake (2001, pp. 14Ð15) and Yucharacteristics and response bias. Yu and Blakefrequency, luminance, and contrast. Here, pre-reaction time technique, as well as subjectively.In a different line of research, Blake (1988) em-ployed a dichoptic reading paradigm in whichstimuli possessed no special status during rivalry.that attention can affect binocular rivalry goescentury later, Lack (1969) examined the effectrivalry alternations. In his study, the observershalf or double their alternation rate. In a differentstudy, Lack (1978, p. 61), using an objective letterIn a different basic research study, Collyer anded practice). They reported that with a 3-s ad-vance knowledge of target location and eye, thereWhen observers were instructed to hold one eyeÕssion, there was a 16% improvement. Thus, someMore recently, Ooi and He (1999) reported thatlus. Chong, Tadin, and Blake (2005) found thatcould be extended by about 50% by endogenouslus. However, this increase in duration may haveby the stringent requiremen

ts of the attention-rival stimulus via an increase in effective stimulus 1092December 2007 Ð pressed in synchrony, and this suppression maybe under attentional control (e.g., Alais & Blake,1999, Kovacs et al., 1996).However, in an applied study, recall that Win-Meng & Tong, 2004) found that voluntary atten-tion had no systematic effect on binocular rivalryviewed in a partially fused condition. Thus, al-rivalry suppression is not conclusive. Howard(2002, p. 309) has argued that the effects of high-level or top-down variables on binocular rivalry,factors, are small when compared with the effectscontrast, or color. Clearly, more research is need-it may be an effective strategy to mitigate theeffects of rivalry.From an applied standpoint, U.S. Army pilotsthe Apache helicopter using the IHADSS (Lip-although they regularly report visual difÞculties,may learn to manage the rivalry (e.g., by inßuenc-ingalternation rate) rather than truly eliminate it.a method of potentially reducing rivalry. For com-other methods of reducing rivalry may be required.method individuals could use to control rivalry.to create motion of images on the retina and,thereby, create luminance transients, which wouldcontours, would create such transients; but seevan Dam & van Ee, 2006). However, eye move-tive effects, especially with monocular HWDs: Iffrom the outside world, or vice versa. Afurthersors are slaved to the pilotÕs head movements andchange with the pilotÕs head position.The effects of cognitive variables are relative-their effect on the time course of rivalry. Lin-guistic information does not seem to affect binoc-meaning appears to have only a modest effect onsuppression or dominance duration and no effectamount. However, other recent studies, one ofch involved the simulation of a semitransparentmonocular HWD, have found no effect of atten-tion on rivalry. Practice may help individualseffects of cognitive variables on rivalry are con-differing methodology.nocular HWDs, it is not known whether extended IVALRYANDthat future research study the effects of practicewhen HWDs are used in real

-world situations.wanted effects from rivalry may be an option. Inform of luning may be affected by practice and/orwhether practice or task difÞculty affects targetrecognition when targets appear near the monoc-ASKS AFFECTED BY RIVALRYthe integration of vision with action (for review,binocular rivalry will affect the performance ofcertain tasks when HWDs are used will likelyshould still be visible some of the time. Tasks thatrecognition of stimuli should be less affected bytion must be immediate. Tasks that require con-tinuous responding, such as the control of headingusing optic flow information or engaging in visualby rivalry. It remains to be determined whetherThe effect of binocular rivalry on directingerance of attention to a given information channelat the expense of attending to a different, moreimportant, channel (Yeh & Wickens, 2001). How-ever, because binocular rivalry involves a processand cognitive tunneling are different phenomena.tion or recognition should be less affected bytion must be immediate. Tasks that require con-rivalry. Future research should explore exactlywhich types of tasks are affected by binocularGENERAL SUMMARY1. Binocular rivalry is provoked by interocular differ-as contrast polarity, size, hue, and motion velocity.except for brief exposures. There is evidence that2. Agiven stimulus viewed by one eye will typicallyformer possesses greater contour density, higher con-opaque monocular HWDs not be used.4. For binocular partial overlap HWDs the appearancethrough the use of false contours. Apartial overlaparea of at least 40¡and the use of a convergenteffects of suppression in the monocular flankingareas. However, the use of these techniques may noteliminate suppression of targets in the monocularßanking regions. The HWD designer will have totions in target visibility. 1094December 2007 Ð 5. Practice over a number of days (e.g., 10 days) maytions. When observers perform an attentionallydurations may be extended, which is an effectreduce effects of rivalry suppression can be mostquickly and effectively achieved and whether eyethe-window

view. This would be particularly use-6. Target detection/recognition has been shown to betasks relevant to HWDs. However, it remains to beThis work was supported by U.S. Air Force Con-tract FA8650-05-D-6502 to Link Simulation andraining (a division of L3 Communications Corp.).Abadi, R. (1976). Induction masking Ð Astudy of some inhibitory inter-ision Research, 16,Alais, D., & Blake, R. (1999). Grouping visual features during binocularrivalry. ision Research, 39, Alexander, L. (1951). The inßuence of Þgure-ground relationships onbinocular rivalry. Journal of Experimental Psychology, 41,Anderson, J., Bechtoldt, H., & Gregory, L. (1978). Binocular integrationin line rivalry. Bulletin of the Psychonomic Society, 11,Behar,I., Wiley,R., Levine,R., Rash,C., Walsh,D., & Cornum,R. (1990).sual survey of Apache aviators (VISAA)(Tech. Rep. USAARL90-15). Fort Rucker, AL: U.S. Army Aeromedical ResearchLaboratory.Blake, R. (1988). Dichoptic reading: Relative meaning in binocularrivalry. Perception and Psychophysics, 40,Blake, R. (1989). Aneural theory of binocular rivalry. Review, 96,Blake, R. (2001). Aprimer on binocular rivalry, including current con-Blake, R., & Boothroyd, K. (1985). The precedence of binocular fusionover binocular rivalry. Perception and Psychophysics, 37,Blake, R., & Camisa, J. (1979). On the inhibitory nature of binocularJournal of Experimental Psychology: HumanPerception and Performance, 5,Blake, R., & Fox, R. (1974). Adaptation to invisible gratings and theNature, 249,Blake, R., OÕShea, R., & Mueller, T. (1992). Spatial zones of binocularisual Neuroscience, 8,Blake, R., Westendorf, D., & Yang, Y. (1991). Discriminating binocularInvestigative Ophthalmology and VisualScience, 32,Blake, R., Yu, K., Lokey, M., & Norman, H. (1998). Binocular rivalryand visual motion. Journal of Cognitive Neuroscience, 10,Blake, R., Zimba, L., & Williams, D. (1985). Binocular correspondenceand visual motion. Breese, B. (1899). On inhibition. Psychological Monographs, 3,Chong, S., & Blake, R. (2006). Exogenous attention and endogenousattention inßuence initial dominance in b

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