Stereoacuity is affected by inducedphoria but returns toward baseline duringvergence adaptationSarah Spencer, BMedSci, and Alison Y. Firth, MSc, DBO(T)

PURPOSE In normal binocular single vision adaptation to an induced deviation occurs over a periodof several minutes. This study investigates the effect on stereoacuity during vergenceadaptation.

METHODS Stereoacuity, using the Frisby near stereotest, was measured in 20 participants aged 20.2 � 1.9years with normal binocular single vision. Measurements were taken immediately oninducing a phoria with 12� base-out prisms (split), and after 3, 6, and 9 minutes ofadaptation. A measure of stereoacuity was also taken with the same size prisms that wereused concomitantly to control for the reduced visual acuity.

RESULTS Stereoacuity was found to decrease following introduction of the 12� base-out prismaticglasses (9.4 � 2.5 arcsec compared with 24.4 � 21.4 arcsec) and then increase over the 9minute period of adaptation (ANOVA, p � 0.0002). Using post-hoc tests with Bonferronicorrection, the decrease of stereoacuity on introduction of the prism was significant ( p �0.0039), and although an increase in stereoacuity appeared to occur after 3 and 6 minutesof wearing the 12� glasses (14.9 � 4.3 arcsec, 12.3 � 4.5 arcsec), this did not return to thebaseline value until 9 minutes, when the stereoacuity had increased to 12.6 � 10.6 arcsec( p � 0.1982).

CONCLUSIONS In these participants, inducing a deviation with base-out prisms negatively affected nearstereoacuity. However, as adaptation occurred, the level of stereoacuity was seen toincrease back toward the baseline measurement. ( J AAPOS 2007;11:465-468)

W hen a prism is placed in front of one of the eyes,the induced phoria initially changes by thatamount. As binocular vision is regained, the

phoria measurement declines gradually in an exponentialmanner toward the baseline value. This ability to adapt tothe prism is known as prism or vergence adaptation.1

This phenomenon is reported to consist of twocomponents2—the fast component, which responds to theretinal image and quickly realigns the eyes to removeretinal disparity and therefore prevent diplopia, and theslow fusional vergence, which is responsible for maintain-ing binocular alignment. The slow fusional vergence sys-tem receives its input from the output of the fast fusionalvergence system.

Several authors have studied the time course for prismadaptation3-6 and it is generally believed that most of the

Author affiliations: Academic Unit of Ophthalmology and Orthoptics, University ofSheffield, Sheffield, United Kingdom

This study was conducted at University of Sheffield.Submitted January 11, 2007.Revision accepted March 22, 2007.Published online May 30, 2007.Reprint requests: Sarah Spencer, Ophthalmic Department, Warrington Hospital, Lovely

Lane, Warrington, Cheshire, WA5 1QG, UK (email: sarahspencer28@hotmail.com).Copyright © 2007 by the American Association for Pediatric Ophthalmology and

Strabismus.

1091-8531/2007/$35.00 � 0doi:10.1016/j.jaapos.2007.03.014

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adaptation occurs soon after placement of the prism andreaches saturation by 10 minutes. Following this length oftime for adaptation, the prism fusion range appears rela-tively unchanged.7,8 Literature examining stereoacuitychanges throughout the adaptation period currently ap-pears lacking.

The aim of the present study was to investigate whethernear stereoacuity is affected during the adaptation pro-cess using the Frisby stereotest (Frisby stereotest, Shef-field, UK).

Materials and MethodsA repeated measures design was used. Participants were recruitedfrom the student population of the University of Sheffield viaverbal or email request. The study was approved by the Unit’sEthics committee. All participants were given an informationsheet and signed consent was obtained.

The inclusion criteria were as follows: 6/6 visual acuity eacheye for near (1/3 m) and distance (6 m), using reduced Snellen/Snellen, with contact lenses or glasses if worn; prism fusionranges of 25� base-out to 10� base-in for near and 10� base-outto 5� base-in for distance; stereoacuity of at least 55 arcsec on thenear Frisby test.

A measurement was taken using a modification of the Maddoxrod and tangent scale at 1 m. The participant held the Maddoxrod over one of their eyes horizontally and viewed a horizontal

scale with a penlight in the center (at 0) held at 1 m. He/she was

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asked on which side the line was seen in relation to the light, andthe size of the deviation was measured in prism diopters by thedisplacement of the line.

Participants underwent a short training period with the Frisbystereotest in the near position. This involved conducting theFrisby test as it would be carried out in the experiment. Thisensured the test was familiar to the participant and it could beconducted easily, without delay, during the experiment whentime was crucial.

Stereoacuity was recorded to threshold. The Frisby plate washeld in the crossed position (circle appearing to jump outward)each time. The test was brought forward slowly from where the1.5 mm plate could not be seen and moved directly toward theparticipant, keeping the plate perpendicular to the visual axis,until the 3D circle could be identified and this distance noted.The situation was the same for each time measured and everyparticipant tested. As the test was taken to threshold, it was arequirement that the Frisby test be carried out beyond 80 cm (20arcsec). This was achieved by using a measuring tape to measurethe distance at which the Frisby circle was identified, thus en-abling the authors to produce disparities including 20, 15, 10, and5 arcsec (80, 95, 115, and 165 cm). The stereoacuity values(arcsec) were calculated using the Frisby equation. This equationis as follows: Disparity � [206264.81( pdl )]/[1.49(l2)], where p isthe interpupillary distance, l is the viewing distance, and dl is theplate thickness as measured with a micrometer. The number 1.49refers to the refractive index of the plastic. The response wasrecorded when the circle was seen at a distance two of threetimes.9 The Frisby plate was moved forward each time towardthe participant until the circle was identified.

The participants were required to wear plano glasses withFresnel prisms fitted to them. Those already wearing spectaclesfor a refractive error wore the plano glasses over the top of theirexisting spectacles. One pair of the plano glasses containedFresnel prisms of 6� base-out prism on one lens and a 6� base-inprism on the other (control glasses), and the other pair of planoglasses contained 6� base-out prisms on each lens (12� glasses).The order of the incomitant (12� glasses) and concomitant (con-trol glasses) prism conditions was alternated for each consecutiveparticipant. Conducting a stereoacuity test with the controlglasses was useful to account for blur caused by the Fresnelprisms and the effect this may have on stereoacuity in the 12�

glasses condition. This stereoacuity value is used as the preprismvalue throughout the results. The stereoacuity testing took placeas previously reported with the 12� glasses.

When the 12� glasses were introduced, the Maddox rod wassimultaneously held horizontally over one eye and a measure-ment taken (as above). This enabled the full induced phoria to bemeasured before binocular viewing, and therefore, adaptationtook place.

The Maddox rod was then removed and the Frisby test carriedout immediately as above. This stereoacuity measurement wasrecorded at “0 minutes.” The participant then fixated on a targetat approximately 1 m or closer (ie, a magazine) to ensure thatthey were continuing to adapt to around the distance that thenear Frisby test was being carried out as opposed to a greater

distance (eg, 6 m), if the participant was able to look around the

room. At 3 minutes, the participant carried out the Frisby stereotest again, to threshold. They then continued to look at the targetat less than 1 m away. The same procedure was carried out whenthe participant had been wearing the glasses for 6 and 9 minutes.When the participant had completed the Frisby test at 9 minutes,the Maddox rod was again introduced horizontally to one of theeyes and the horizontal measurement was read from the chart.Once the phoria measurement had been found, the participantcould then remove the glasses and relax their eyes.

If stereoacuity measures with the control glasses took placeafter the 12� glasses, the participant had to relax their eyes for aperiod of 5 minutes after the 12� glasses were removed to allowrecovery from the adaptation. This was checked by a Maddox rodmeasurement after 5 minutes of rest. If the measurement wasback to the pre-12� glasses value, recovery had occurred. If themeasurement was not back to the preprism value, the participantwas required to rest their eyes for a further 3 minutes to ensurethat full recovery had occurred. After this time period, the mea-surement was taken again.

Statistical analysis was performed using analysis of variance(ANOVA; Statview) and post-hoc t-tests with Bonferroni cor-rection.

ResultsTwenty-three participants volunteered to take part in theexperiment. Three did not fulfill the criteria due to areduced prism fusion range and therefore were excluded.The remaining 20 participants (3 males, 17 females) com-pleted the experiment. The average age of the participantwas 20.2 � 1.9 years old (range, 18-28 years). Of the 20participants, 6 were wearing glasses, 5 were wearing con-tact lenses, and 9 had no significant refractive error.

The mean stereoacuity preprism (with control glasses)was 9.4 � 2.5 arcsec, and with 12� glasses at 0 minutes,this decreased to a mean of 24.4 � 21.4 arcsec. Thestereoacuity increased after 3 minutes of wearing the 12�

glasses and then again after 6 minutes. At 9 minutes afterinitial introduction of the 12� glasses, the stereoacuitydecreased slightly to a mean of 12.6 � 10.6 arcsec. Theresults are shown graphically in Figure 1.

ANOVA showed a statistically significant change instereoacuity measurement. Post-hoc t-tests with Bonfer-roni correction showed that the difference in stereoacuitybetween the measurement with the control glasses ( pre-prism) and 0 minutes after the initial presentation of the12� glasses was significant ( p � 0.0039). The differencebetween the stereoacuity at 0 and 3 minutes followingintroduction of the 12� glasses did seem to increase; how-ever, it was found not to be statistically significant ( p �0.0428). The difference between the stereoacuity after 3and 6 minutes was significant ( p � 0.0032). However, thedifference between the stereoacuity between the 6- and9-minute intervals was not significant ( p � 0.8640). Thedifference between the stereoacuity preprism and at 6minutes following introduction of the 12� glasses was

significant ( p � 0.0092), whereas the difference between

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Volume 11 Number 5 / October 2007 Spencer and Firth 467

the stereoacuity between the preprism and 9-minute valuewas not significant ( p � 0.1982).

The mean heterophoria before introduction of theprism was 0.25� exophoria. This increased to 11.10� exo-phoria on introduction of the 12� glasses and reduced to3.20� exophoria at the end of the 9-minute period. Aphoria measurement was not obtained at 3 and 6 minutes.

DiscussionThe results from the study have found that there is asignificant reduction in stereoacuity when a deviation isinduced by prisms. The stereoacuity improves and returnsto the baseline value following adaptation.

It has been shown that when the 12� base-out prism wasfirst introduced to the eyes, the stereoacuity immediatelydropped. The stereoacuity then began to gradually in-crease again. The stereoacuity had not reached the base-line value by 6 minutes and a statistical test still showed asignificant difference between the values. The final ste-reoacuity measurement at 9 minutes and the preprism(control glasses) value was not statistically significant, sug-gesting that stereoacuity had returned to the baseline valueat the end of the 9 minute period. However, it is possiblethat a small effect failed to be detected due to a low powerwith this number of participants. The increase of thestereoacuity appears to increase in a similar manner to thatreported for prism adaptation.1,10

The near Frisby test was performed differently than rec-ommended by the manufacturer. Ordinarily, all three plateswould be used and moved to the set distances recommendedby Frisby. In order for the test to be conducted quickly in thisstudy, only the 1.5 mm plate was used. As it was necessary forthe test to be performed to threshold, the test had to beperformed from a position beyond the 80 cm distance set byFrisby, using the 1.5 mm plate and a tape measure as previ-ously performed by Davis et al.9 The plate was moved froma distance where the three-dimensional circle was not seen

0

5

10

15

20

25

30

35

40

45

50

Pre- prism 0 minutes 3 minutes 6 minutes 9 minutes

Time intervals following prism introduction

Ste

reo

acu

ity a

rcse

c

FIG 1. Stereoacuity measurements at various time intervals after theintroduction of the 12� glasses, including SD bars.

and moved gradually directly toward the participant’s eyes

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while keeping the plate perpendicular to the line of sight.This technique was performed exactly the same for eachparticipant on each test; therefore, the relative differencebetween each participant and each time interval should notchange.

The Maddox rod test was chosen as the method formeasuring the phoria. This test was thought to be the mostappropriate as it is less dissociative and quicker to carry outthan other methods of phoria measurement.

There was an anomalous result in one participant.The reason for this anomaly was that the participantbecame nervous at the 9 minute interval and panicked atnot being able to identify the Frisby circle. This resultedin the participant losing some concentration, whichresulted in an inaccurate result. Removing this partici-pant from the analysis, there was a further increase instereoacuity at 9 minutes, with the mean stereoacuity at6 minutes measuring 11.8 � 4.2 arcsec, and the meanstereoacuity at 9 minutes measuring 10.4 � 3.5 arcsec.The agreed adaptation time was unchanged for thisparticipant and after the 9-minute measurement, testingwas terminated.

Fresnel prisms reduce visual acuity11 and stereoacuity isreduced where visual acuity is bilaterally reduced.12 Thusthe purpose of the control glasses was to enable a com-parison of stereoacuity without the need to consider areduction in stereoacuity due to reduced acuity.

A total of 12� base-out in Fresnel prisms were used toinduce the exophoria. This value was chosen as it neces-sitated a change in vergence, which can be comfortablyachieved, the level of visual acuity would not be excessivelyreduced, and the same size prism could be placed in frontof each eye.

We chose to measure stereoacuity after a period of 3minutes as North and Henson13 found that on intro-duction of a 6� base-out prism, adaptation was ap-proaching saturation. Carter6 found that most of theadaptation occurred within the first 5 minutes (althoughhe allowed a 15 minute period for complete adaptation),and Stephens and Jones14 allowed 10 minutes for adap-tation prior to finding a return to baseline for fusionalvergences. It seemed sensible to keep the time intervalsconstant and so measured the stereoacuity also at 6 and9 minutes.

Although the stereoacuity was back to the preprism(control glasses) value statistically, the adaptation may nothave been fully completed in 9 minutes as the phoriameasurement still shows some residual induced exophoria.Our design did not include a measurement of the amountof adaptation during the 9-minute period (ie, at 3 or 6minutes) due to the dissociation and disruption to adapta-tion it would have caused. Adaptation to the prism mayhave occurred within an earlier time frame than 9 minutes,and this would be expected from some of the literaturediscussed above, without recovery of stereoacuity. This

question would require further investigation to answer but

Volume 11 Number 5 / October 2007468 Spencer and Firth

may relate to the two stages of adaptation as suggested bySethi and Henson.15

It has been suggested that an increase in fixationdisparity can decrease the stereoacuity of the individual.Ukwade et al16 used a mirror haploscope with a movablescanning mirror incorporated into each eye channel toinduce fixation disparity and found that stereothresh-olds increased significantly when the amplitude of dis-conjugate motion increased beyond 0.0625° per eye.This suggests that if the vergence error reaches a criticalvalue, the stereoacuity is significantly reduced.Kromeier et al17 found that the stereoscopic thresholdincreased before double vision occurred when examinedunder increasing base-in and base-out prisms. The ste-reoacuity is found to be highest in the absence of apedestal disparity.16 This could give an explanation forthe initial decrease in stereoacuity on the introductionof the prisms in this study, and the gradual recovery ofstereoacuity up to 6 minutes, as the induced fixationdisparity reduced during prism adaptation.

This study has investigated another aspect of visualfunction during prism adaptation. Fresnel prisms are givenout frequently to patients in clinic to join diplopia or to aidthe control of large deviations. When the prisms are ini-tially given, often up to high values as large as 20�, ste-reotests are used to confirm binocularity. Further researchis necessary in this area, but our results would suggest thata higher level of stereoacuity may be reached if the patientis allowed time to adapt to the prism. However, the cur-rent study was performed on volunteers with normal bin-ocular single vision and cannot be extrapolated to a patientgroup.

AcknowledgmentsThe authors thank David Buckley for help with the statistical analysis

involved with this project.

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