Normal correspondence of tectal maps for saccadic eye movementsin strabismus
John R. Economides,1 Daniel L. Adams,1,2 and Jonathan C. Horton1
1Beckman Vision Center, Program in Neuroscience, University of California, San Francisco, California; and 2Center forMind/Brain Sciences, The University of Trento, Trento, Italy
Submitted 7 July 2016; accepted in final form 6 September 2016
Economides JR, Adams DL, Horton JC. Normal correspondenceof tectal maps for saccadic eye movements in strabismus. J Neuro-physiol 116: 2541–2549, 2016. First published September 7, 2016;doi:10.1152/jn.00553.2016.—The superior colliculus is a major brainstem structure for the production of saccadic eye movements. Electricalstimulation at any given point in the motor map generates saccades ofdefined amplitude and direction. It is unknown how this saccade map isaffected by strabismus. Three macaques were raised with exotropia, anoutwards ocular deviation, by detaching the medial rectus tendon in eacheye at age 1 mo. The animals were able to make saccades to targets witheither eye and appeared to alternate fixation freely. To probe the organi-zation of the superior colliculus, microstimulation was applied at multiplesites, with the animals either free-viewing or fixating a target. On average,microstimulation drove nearly conjugate saccades, similar in both ampli-tude and direction but separated by the ocular deviation. Two monkeysshowed a pattern deviation, characterized by a systematic change in therelative position of the two eyes with certain changes in gaze angle. Theseanimals’ saccades were slightly different for the right eye and left eye intheir amplitude or direction. The differences were consistent with theanimals’ underlying pattern deviation, measured during static fixation andsmooth pursuit. The tectal map for saccade generation appears to benormal in strabismus, but saccades may be affected by changes in thestrabismic deviation that occur with different gaze angles.
Electrical stimulation of the superior colliculus drives arapid eye movement, or saccade, that is nearly identical ineach eye. In strabismus, the eyes are offset, but microstimu-lation still generates similar saccades. Minor discrepan-cies in saccade amplitude and direction are sometimespresent, which are likely due to altered downstream ocularmotor pathways that also mediate smooth pursuit andsteady fixation.
MOST PATIENTS WITH DIVERGENT strabismus (exotropia) havenormal visual acuity in each eye. They usually have a dominanteye but can freely alternate fixation on visual targets. Objectsare localized with surprising precision, by either eye or hand,despite the fact that images falling on each retina fail to land onlocations that normally correspond (Agaoglu et al. 2014; Bucciet al. 2009; Das 2009; Das et al. 2004; Griffiths et al. 2011;Niechwiej-Szwedo et al. 2014). Diplopia is prevented by sup-pression of the peripheral temporal retina in each eye (Cooper
and Record 1986; Economides et al. 2012; Herzau 1980;Joosse et al. 1997). Confusion is avoided by shifting theperceived location of objects sensed via the deviating eye(Cooper and Feldman 1979). Typically, the magnitude of theshift is equal to the ocular deviation, effectively cancelling it(Economides et al. 2012). The fovea of the deviating eye, bybecoming perceptually realigned, acquires a common visualdirection with a point in the temporal retina of the fixating eye(Herzau 1996). Consequently, the center of gaze of the devi-ating eye in retinotopic coordinates is mapped anomalously toa peripheral location in a body-centered frame of reference.Where this transformation is represented in the brain is un-known.
In normal monkeys, the superior colliculus contains binoc-ular cells organized in a topographic map, with sensory cells inthe superficial layers and sensory/motor cells in the deeperlayers (Schiller 1984; Wurtz and Albano 1980). The latter cellshave receptive fields activated by visual stimuli and movementfields encoding a saccade of corresponding size and direction.Electrical activation of a locus in the tectal map generatesconjugate saccades (Robinson 1972). The superior colliculusplays a critical role in the guidance of eye-hand movements, aswell as in the selection of targets (Gandhi and Katnani 2011;Glimcher and Sparks 1992; Horwitz and Newsome 1999;Schall 2001; Song and McPeek 2015).
Few studies have been undertaken of the superior colliculusin strabismus, although it is a logical place to begin the effortto understand how the brain achieves accurate spatial localiza-tion despite ocular misalignment. In cats, after early transectionof eye muscles, neurons in the superior colliculus have beenreported to remain binocularly driven (Gordon and Gummow1975; Gordon and Presson 1977). This result is surprisingbecause strabismus has the opposite effect in striate cortex.Lack of concordant visual stimulation during early life leads toenhanced segregation of geniculate inputs within ocular dom-inance columns and to a striking loss of binocular neurons(Wiesel 1982). In the superior colliculus, eye input to thesuperficial layers is organized into separate zones that areanalogous to the ocular dominance columns in striate cortex(Hubel et al. 1975). One would expect strabismus to increasethe segregation of this direct retinal input. The indirect visualinput that comes from striate cortex should also show reducedbinocularity.
If neurons in the superior colliculus remain binocular despitestrabismus, then the visual system faces a challenge when itcomes to controlling which eye will acquire a visual target. Forexample, a stimulus situated between the fixation points in anexotropic subject falls on the temporal retina in the left eye and
Address for reprint requests and other correspondence: J. C. Horton, Beck-man Vision Center, Univ. of California, San Francisco, 10 Koret Way, SanFrancisco, CA 94143-0730 (e-mail: [email protected]).
J Neurophysiol 116: 2541–2549, 2016.First published September 7, 2016; doi:10.1152/jn.00553.2016.
the nasal retina in the right eye (Fig. 1). The stimulus evokessensory responses at two different loci within the superiorcolliculi (Fig. 1B). Given that neurons are binocular, theresponses cannot be coded by eye, yet only one eye is destinedto acquire the target. Somehow, the ocular motor system mustdisregard or suppress sensory responses at one locus and allowneurons at the other locus to generate an appropriate saccade.To explore this phenomenon, it would be interesting to com-pare neuronal activity at a given site during trials when a targetwas presented and then subsequently fixated by either the lefteye or the right eye.
We have recorded from single units in the superior colliculus ofthree macaques raised with alternating exotropia while they wereengaged in alternating fixation onto targets. To localize the supe-rior colliculus for recordings, electrical microstimulation wasapplied. This report describes the impact of strabismus on the eyemovements obtained from electrical stimulation.
METHODS
Animals. Three male macaques (Macaca mulatta) were reared withstrabismus at the California National Primate Research Center (Davis,CA) by performing a tenotomy of the medial rectus muscle in eacheye at age 4 wk. The muscle eventually reattaches to the sclera, butfusion is disrupted during the critical period for binocular vision. Thisleads to an alternating exotropia without amblyopia, which sharesmany features of decompensated intermittent exotropia in humans(Economides et al. 2007). It differs, however, in some importantrespects. First, adduction remains reduced compared with normalanimals, in part because the muscle insertion site may be abnormal.Second, the onset of exotropia is sudden and irrevocable, rather thanoccurring via a process of gradual decompensation. Other models ofstrabismus have been developed successfully, each with advantagesand disadvantages (Crawford and von Noorden 1980; Das et al. 2005;Kiorpes 1992; Tychsen and Burkhalter 1997).
After the monkeys reached age 3 yr, they were transferred to ourlaboratory at the University of California (UC) San Francisco. Atitanium headpost and recording chamber were implanted, as describedpreviously (Adams et al. 2007, 2011). The chamber was situated on theright side over medial parietal cortex, just anterior to the lunate sulcus, toavoid making penetrations through striate cortex. The right superiorcolliculus was stimulated in most experiments, but the left superiorcolliculus was stimulated in two monkeys by making penetrations nearthe medial edge of the chamber (Fig. 1B). All procedures were approvedby the Institutional Animal Care and Use Committee at UC Davis or UCSan Francisco. Monkey 1 was referred to as “monkey 2” in an earlierstudy (Economides et al. 2007); monkeys 2 and 3 have not been previ-ously reported.
Video eye tracking and target presentation. Each monkey’s eyemovements were recorded while the animal was head-restrained in a
primate chair. Computer-generated targets (Cambridge Research Sys-tems, Rochester, UK) were rear-projected onto a tangent screen by aDLP projector (Hewlett Packard, Palo Alto, CA) with a 60 -Hz refreshrate. The tangent screen, subtending �45° horizontally and vertically,was placed 57 cm in front of the monkey. Eye movements weremonitored by two independent eye trackers, operating at 60 Hz, eachusing an infrared video camera (SensoMotoric Instruments, Teltow,Germany). The comparatively slow sampling rate limited the preci-sion of saccade measurements but should not affect comparisonsbetween the eyes. The cameras were mounted overhead; a dielectricmirror that reflected infrared light was oriented obliquely to obtainvideo images of the animal’s eyes. Each eye was illuminated by aseparate infrared light source, positioned laterally. This arrangementenabled us to track each eye over a wide range, from 20° nasally toalmost 80° temporally. This was an important advantage, given thelarge exotropia present in the strabismic monkeys. The position ofeach eye and the location of visual stimuli on the tangent screen weresampled at 120 Hz by a Power1401 data acquisition and controlsystem (Cambridge Electronics Design, Cambridge, UK). To calibratethe eye trackers, the digital gain and offset were adjusted online tomatch eye and target locations while the monkey tracked a spotoscillating sinusoidally in a horizontal and then vertical direction.Each eye was calibrated independently, with the other eye covered.The monkey was rewarded with food puree for accurate fixationwithin an adjustable window.
Superior colliculus stimulation. A plastic grid with holes 1 mmapart was placed inside the recording chamber. A 30-gauge guide tubewas lowered to �10 mm above the superior colliculus. Recordingswere made with quartz-platinum/tungsten tetrodes (Thomas Record-ing, Giessen, Germany) having an impedance of 0.5–1.0 M�. As thetetrode was advanced, electrical stimulation was employed to locatethe surface of the superior colliculus (STG1001; MultiChannel Sys-tems, Reutlingen, Germany). Typically, trains of biphasic square-wave pulses, 500 �s at 500 Hz, 20–400 �A, were applied for500–1,000 ms to generate repetitive saccades (Katnani and Gandhi2012). After staircase saccades were observed, the tetrode was oftenadvanced another 500–1,500 �m to reduce the current required toelicit eye movements (20–200 �A). Sites stimulated in each monkeyare shown in Fig. 1B. The average number of stimulation trials at eachsite was 30.
For each monkey, saccade size and direction elicited by electricalstimulation were compared for the right eye and the left eye. Eachstaircase eye movement comprised saccades that were similar, butthere was a tendency for successive saccades to diminish in amplitude(Breznen et al. 1996; Stryker and Schiller 1975). Successive saccadesalso tended to be more variable than initial saccades. This was due toseveral factors, including intrusion of volitional saccades, limits inocular excursion (a major issue in strabismic animals), and eye trackerinaccuracy at extreme gaze angles. For this reason, only the firstsaccade in each staircase was analyzed to compare right eye and lefteye movements.
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Fig. 1. Electrical stimulation of the superiorcolliculus of strabismic monkeys. A: diagramshowing right eye (RE) fixating at the originand exotropic left eye (LE). A target (blackcircle) presented between the eyes’ fixationpoints could be acquired by the left eye with a30° saccade or by the right eye with a 20°saccade. B: schematic map of the superiorcolliculus (SC), dorsal view, showing wheresensory responses would be evoked by thetarget in A for the left eye (blue circle) and theright eye (red circle). Sensory responses atonly one site would be followed by a motordischarge to bring an eye onto the target.Small circles denote sites where microstimu-lation was delivered in each monkey.
To obtain eye velocities, horizontal and vertical position signals foreach eye were differentiated using a three-point central differencealgorithm (Spike2; Cambridge Electronics Design). After stimulusonset, each saccade duration was defined as the period when theabsolute value of the mean of all four velocity signals exceeded thesteady-state fixation baseline by 3 SD (usually about 75°/s). Todetermine saccade amplitude and direction, position was compared atthe beginning and end of each eye movement. Saccades under 3° wereexcluded because of the limited resolution of the video eye trackers.The amplitude of saccades made by each eye were compared asfollows (Walton et al. 2014):
When the left eye and right eye saccades are equal, the amplituderatio is equal to 1. Unequal saccades by the left eye and the right eyeyield amplitude ratio values that are asymmetrical around 1. There-fore, amplitude ratios were normalized for statistical comparisons byusing their common logarithm as the final measurement unit.
The difference between the polar angle of each eye’s saccade wascalculated as follows:
For any given tectal site, amplitude ratio, log amplitude ratio, anddirection difference were calculated for each stimulation trial and thenaveraged to derive means � SD for each parameter. For the logamplitude ratio and direction difference, confidence intervals werecalculated on the basis of the critical value of the t-distribution foreach sample size, with an � � 0.05.
RESULTS
In all three exotropic monkeys, stimulation of the superiorcolliculus resulted in a series of consecutive saccades (Fig. 2).The cardinal finding was that for any given stimulus site, thesaccades in each eye evoked by current application wereapproximately equal in size and direction. In other words,tectal stimulation elicited saccades that were essentially con-jugate. The eyes moved in parallel, by the same amount, withtheir relative positions determined by the animal’s underlying
strabismic deviation. In the 3 monkeys, 48 tectal sites werestimulated in total. Stimulation of the right superior colliculusalways produced leftward movement of the eyes, and viceversa. Ipsiversive saccades were never evoked.
In each animal, we stimulated repeatedly the same site in thesuperior colliculus. Figure 3 shows data from monkey 1, ananimal with an alternating exotropia measuring 35–40°. Hisocular deviation remained relatively constant with changes ingaze angle. The relative positions of monkey 1’s eyes over a40° range of vertical and horizontal static fixations have beenreported previously (see Fig. 9 in Economides et al. 2007). Theanimal was rewarded for fixation with either eye on a targetlocated at the center of the tangent screen (Fig. 3, A and B). Atthis location he preferred to use his right eye. After fixationwas acquired, current was delivered to drive a staircase of sac-cades. The amplitude ratio (saccade amplitudeleft eye/saccade am-plituderight eye) was 1.00 � 0.09 for trials initiated with the righteye (n � 55) and 0.90 � 0.07 for trials initiated with the left eye(n � 28). The direction difference (polar directionleft eye � polardirectionright eye) was 2.2° � 4.8° for right eye trials and 2.1° �3.8° for left eye trials. Regardless of which eye fixated on thetarget at the origin, the saccades in each eye evoked by stimulationdrove movements of the eyes that differed little in amplitude ordirection.
When the location of the initial fixation point was movedalong the horizontal meridian, the monkey’s fixation behaviorchanged. He preferred to acquire targets on the left with the lefteye, and on the right with the right eye (Fig. 3C). Combining
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Fig. 2. Saccades elicited by stimulation of the superior colliculus are essen-tially conjugate. Representative eye position traces from 3 exotropic monkeysshow staircases of saccades after current application in the right superiorcolliculus. The ocular deviation remains constant in each animal as the eyes aredriven to the left. Positive values � upgaze or rightgaze.
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Fig. 3. Repeated stimulation at a single site in the superior colliculus yieldssaccades of similar size and direction in each eye of monkey 1. A: saccadevectors for the right eye (red arrows) fixating at a target (green dot) at thescreen center. Vectors were computed from initial saccades of staircases drivenby electrical stimulation. B: saccade vectors for the left eye fixating at theorigin. C: saccade vectors for intermediate target positions along the horizontalmeridian. D: mean saccadic vectors (bold arrows) for all trials (A–C) aresimilar for the 2 eyes. AR, amplitude ratio; DD, direction difference.
the data for all fixation starting positions tested in Fig. 3(n � 149), the right eye saccades had a mean amplitude of21.3° � 3.9° [95% CI: 20.7°, 21.9°] and a mean polar angleof 124.7° � 7.4° [123.5°, 125.9°]. The left eye saccades hadan amplitude of 20.4° � 3.6° [19.8°, 21.0°] and a polarangle of 127.9° � 8.2° [126.5°, 129.2°]. The confidenceintervals for saccade amplitude overlapped, indicating thatthere was no significant difference in the magnitude ofsaccades driven in each eye by electrical stimulation at thissite in the superior colliculus. For saccade direction, how-ever, there was a small yet significant difference (Fig. 3D).
In monkey 1, five sites were stimulated, all in the rightsuperior colliculus (Fig. 1B). For these sites, the mean ampli-tude ratio was 1.07 � 0.16. The mean log amplitude ratio was0.017 � 0.067 [�0.066, 0.101]. The 95% confidence intervalincluded 0, indicating that there was no significant differencein the size of saccades for the left eye and right eye. The meandirection difference for saccades was 3.9° � 7.7° [�5.7, 13.5].The direction difference also was not significant.
In monkey 1, the superior colliculus was stimulated while theanimal was being rewarded for fixating a target. In monkey 2,we tested the effect of applying stimulation while the monkeywas free-viewing either random dot noise patterns or naturalscenes (Fig. 4A). The images were needed to maintain alert-ness, because no reward was being provided. Given that tectalstimulation was not linked to behavior, the eye of fixation wasindeterminate and saccade starting points were widely scat-tered. Nonetheless, the saccades made by each eye weresimilar, with an amplitude ratio of 1.13 � 0.13 and directiondifference of 1.2° � 9.2° (Fig. 4B).
In monkey 2, electrical stimulation was applied at 18 col-licular sites, 16 on the right and 2 on the left (Fig. 1B), whilethe monkey was either free-viewing or fixating a target. Themean amplitude ratio was 1.14 � 0.17. The mean log ampli-tude ratio was 0.038 � 0.066 [0.005, 0.071], indicating thatsaccades for the left eye were significantly larger than those forthe right eye. The mean direction difference was �3.6° � 8.2°[�7.6°, 0.49°]. This difference was not significant.
The positions of the two eyes for monkey 2 during staticfixation by the left eye on a nine-point grid is shown in Fig. 5A.In addition to an alternating exotropia of 40°–45°, the visualaxis of the animal’s left eye was nearly 20° above that of his
right eye. This left hypertropia accounts for the fact that lefteye vectors were always higher than corresponding right eyevectors (Fig. 4A). This vertical offset had a negligible impacton the conjugacy of saccades. However, the arrays of staticfixations revealed another feature of this animal’s strabismusthat did affect the conjugacy of saccades. The horizontalseparation of left eye positions and corresponding right eyepositions increased with left gaze. For example, left eye posi-tions on the vertical meridian (0° horizontal) matched right eyepositions at a mean of 40.5°, whereas left eye positions at �40°matched right eye positions at a mean of 5.5°. Moving the lefteye from primary gaze to �40° increased the horizontal devi-ation of the eyes from 40.5° to 45.5°. This effect of gaze angleon the horizontal separation between the eyes was also evidentduring smooth pursuit (Fig. 5B). Tracking a target with the lefteye that moved from the midline leftward by 40° resulted in anincrease in the exotropia. To summarize, shifts in gaze angle bythe monkey toward the left side caused an increase in exotropiadue to a 12% greater movement of the left eye compared withthe right eye.
Monkey 3 had the largest alternating exotropia, measuring60–65° (Fig. 6A). He preferred to fixate with the right eye. He
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Fig. 5. Horizontal incomitance in monkey 2. A: eye positions during staticfixations by monkey 2 with the left eye (blue points) on a grid of 9 targetsspaced 20° horizontally and vertically. The grid for the right eye (red points)is displaced downward because the animal has a left hypertropia. The cloudsof points are also elongated, because monkey 2 has a small vertical pendularnystagmus. Note that comparing matching positions for the 2 eyes (horizontalbrackets) shows that the exotropia increases when the animal moves the lefteye from the midline to the left. B: smooth pursuit by the left eye shows anincrease in the horizontal exotropia (black trace) from 40° to 45° when the eyemoves leftward from primary gaze to �40°. Note the low-gain, saccadicsmooth pursuit, which is typical of strabismus.
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Fig. 4. Stimulation of the superior colliculus during free-viewing producesnearly conjugate saccades in monkey 2. A: saccadic vectors for the left eye(blue arrows) and the right eye (red arrows) resulting from electrical stimula-tion applied every 5 s while the animal looked at patterns on a tangent screen.B: the amplitudes and directions of saccades made by each eye are similar,despite initiation from widely scattered positions in visual space.
had an unusual pattern deviation: his exotropia decreased onupgaze. In addition, his left eye became hypertropic on leftgaze but hypotropic on right gaze. As a result, a nine-point gridof targets fixated by the right eye corresponded to a rotated gridof matching left eye positions (Fig. 6A). The left eye grid wasrotated clockwise about 17° compared with the right eye grid.
Note that the plot does not show that the left globe wascyclorotated, but rather that the left eye array of static fixationpositions was rotated.
When engaged in smooth pursuit (Fig. 6B), monkey 3 usu-ally employed his right eye, but he switched to the left eyewhen the target moved far enough to the left side (Fig. 6C).The angle between the trajectory of right eye positions and lefteye positions equaled about 16° during smooth pursuit of atarget moving back and forth along the horizontal meridian(Fig. 6B). This rotation recapitulated the relative angle betweenright eye and left eye positions during static fixation.
Stimulation was delivered to 17 sites in the right superiorcolliculus and 8 sites in the left superior colliculus while themonkey was either free-viewing or fixating a target (Fig. 1B).For all 25 sites, the mean amplitude ratio was 1.00 � 0.20. Themean log amplitude ratio was �0.015 � 0.084 [�0.050,0.020]; there was no significant difference in the magnitude ofsaccades by the left eye and the right eye. The mean directiondifference was �15.0° � 9.9° [�19.1°, �10.9°]. The polardirection of saccades was significantly different, with left eyesaccades rotated clockwise relative to right eye saccades(Fig. 7). The mean direction difference was close to thedifference observed in the angular alignment of the staticfixation grids (Fig. 6A) and in the relative trajectory of eyemovements during smooth pursuit (Fig. 6, B and C).
The data for all stimulation sites in each monkey areshown in Fig. 8. As mentioned previously, the recordingchamber was mounted on the right side of the head in eachanimal, causing us to stimulate the right superior colliculusmore often than the left superior colliculus. Consequently,the majority of saccades were elicited to the left. There wasvariability in the log amplitude ratio from site to site in each
Fig. 6. Pattern deviation in monkey 3. A: static fixation by monkey 3 with theright eye on a grid of points separated horizontally and vertically by 20°.The corresponding positions for the nonfixating left eye produce a gridrotated by �17°. The left inset depicts schematically an incomitant devi-ation known as an A-pattern. It results in a clockwise direction differencefor the vertical component of gaze shifts (compare orientation of linesconnecting central 3 grid points). In addition to an A-pattern, the left eyeof monkey 3 moves down relative to the right eye as the animal looks to theright (the right inset shows this effect schematically). B: smooth pursuit bymonkey 3 of a target oscillating sinusoidally �30° (green line). He tracksit with his right eye (red), except beyond 20° to the left. Note the slopingtrajectory (blue line) of the nonfixating left eye positions, which forms an�16° angle with the trajectory of the right eye’s positions. C: monkey 3switches to fixate with the left eye when the target in B moves more than15° to the left of the midline. Note the corresponding rotation of thedeviated right eye’s positions.
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Fig. 7. Saccades generated by tectal stimulation differ in polar angle becauseof pattern deviation. Families of saccades and their mean vectors obtained inmonkey 3 are shown for 4 representative stimulation sites. At all sites, left eyesaccades were rotated clockwise in polar angle with respect to right eyesaccades, due to the animal’s combination of pattern deviations (Fig. 6).Number in the polar plots represents scale for saccade amplitude.
animal, but only monkey 2 showed a systematic bias (Fig.8A). The saccades made by his left eye were greater inamplitude than those made by his right eye, reflected bypositive values for log amplitude ratio. For direction differ-ence, there was also variability among stimulation sites (Fig.
8B). The mean direction difference was close to 0, except inmonkey 3, the animal with a clockwise rotation of left eyesaccades relative to right eye saccades.
Conjugacy of saccades in normal subjects. It is possiblethat the rather small disconjugacies observed in these stra-bismic monkeys were due to measurement error by the eyetrackers. It would be ideal to perform parallel experimentsin normal monkeys, but we had none available in ourlaboratory. As an alternative, we tested five human subjectswith normal visual function and orthotropic eye alignment,following a protocol approved by the UCSF Committee onHuman Research. The eye movement recordings were madewith the same apparatus used for the strabismic monkeys.
Subjects fixated a central target on a tangent screen at adistance of 57 cm. A peripheral target appeared briefly at agiven eccentricity and polar angle. The subjects made asaccade to the peripheral target. Trials were repeated fora single peripheral target locus before the next target locuswas tested. A total of 30 trials were analyzed for each targetlocus. Figure 9 shows examples of saccades to a target at20° eccentricity made by two subjects. In the first subject,the amplitude ratios ranged between 0.92 and 1.08; thedirection differences ranged between �1.3° and 5.4°. In thesecond subject, the amplitude ratios ranged between 0.93and 1.11; the direction differences ranged between �1.5°and 4.4°. Many of the mean saccade vectors for each eyediffered significantly in amplitude or direction.
One can assume the human subjects made saccades thatwere nearly perfectly conjugate (the change in vergence anglerequired to maintain eye alignment on a flat screen inducesonly minor disconjugacy). The discrepancy in the size anddirection of saccades that was measured in our subjects is duemostly to inaccuracy in eye position measurements generatedby each eye tracker. Although they are calibrated carefully,they are susceptible to errors from many sources, such asfluctuations in eyelid height, pupil thresholding, pupil size,ocular surface reflectivity, image quality, and subject position.
DISCUSSION
Our main finding was that in strabismus, although the eyes’fixation points are offset in position, they move nearly conju-gately when the superior colliculus is stimulated electrically.
Stimulation did not cause a major change in the magnitudeof the strabismic deviation or any tendency for the eyes toconverge on a location in visual space. It has been reported that
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Fig. 8. Data from tectal stimulation sites in 3 monkeys. A: log amplitude ratioplotted against polar angle; most points are on the left side because the rightsuperior colliculus was stimulated in 38/48 experiments. The points arescattered around 0 (dotted line), which equals an amplitude ratio of 1. Onlymonkey 2 had a mean log amplitude ratio that differed significantly from 0. B:histograms of direction differences (10° bins) for stimulation sites in eachmonkey. Monkey 3 had a mean direction difference of �15.0°.
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*
*AR=0.99DD=4.4°
AR=1.11DD=-1.5°
AR=0.97DD=2.0°
AR=0.93DD=3.5°
AR=1.07DD=-1.0°
AR=1.06DD=-0.6°
AR=0.96DD=1.9°
AR=1.02DD=0.91°
**
* *
*
Fig. 9. Noise in video eye tracker measure-ments of saccade conjugacy. Examples arefrom 2 normal human subjects making 30consecutive saccades to targets at an eccen-tricity of 20° (circle) spaced every 45°. Boldblue (left eye) and red (right eye) vectorsrepresent mean saccades. Both subjects shownonsystematic discrepancies in saccade am-plitude (AR � left eye saccade/right eye sac-cade) and direction (DD � left eye polarangle � right eye polar angle). Asterisk de-notes significant difference at 95% confidencelevel.
disconjugate movements related to shifts in vergence angle canbe evoked by stimulation at the rostral pole (Chaturvedi andVan Gisbergen 2000; Van Horn et al. 2013). We did notstimulate this region, and hence, only conjugate eye move-ments were observed in our experiments.
At individual stimulation sites, there were often minor dif-ferences in the amplitude or direction of saccades evoked ineach eye (Fig. 8). Some differences attained statistical signif-icance, but they were not significant in biological terms. Forexample, the stimulation site illustrated in Fig. 3 yieldedsaccades with a 3.2° direction difference. However, this direc-tion difference fell within the range of direction differencesencountered in normal subjects engaged in conjugate eyemovements. The trackers monitoring each eye, even whencalibrated fastidiously, yield small errors in position signalsthat can make conjugate saccades appear slightly disconjugate(Fig. 9). The problem is compounded in strabismic monkeysbecause of their large ocular deviations. For example, withextreme globe rotations, the tracking of position can be de-graded by movement of the illuminator light reflex from thecornea to the sclera.
The discrepancies we measured in saccade size or directionfrom one stimulation site to another were usually small andvariable in sign (Fig. 8). Most importantly, except as notedbelow, there was no systematic trend or pattern to the saccadeasymmetries. For this reason, it seemed valid to average thedata obtained from tectal stimulation sites in each animal.
In monkey 2, the mean amplitude ratio compiled from all 18stimulation sites was 1.14. This was a significant deviationfrom an equal amplitude ratio and well outside the range oftracker error. Comparison of relative left eye and right eyepositions during static fixation (Fig. 5A) and smooth pursuit(Fig. 5B) showed that with shifts in gaze angle to the left, theanimal’s exotropia increased by 12%. The most likely expla-nation is that the surgery on the medial rectus muscles per-formed during infancy resulted in asymmetrical weakness. Ifthe right medial rectus were weaker than the left medial rectus,then the animal’s exotropia would increase on left gaze, just asshown in Fig. 5. In this animal we happened to stimulate theright superior colliculus on 16/18 trials, driving the eyes to theleft. Accordingly, tectal stimulation evoked 14% larger sac-cades in the left eye compared with the right eye.
In strabismus, a shift in the relative positions of the two eyesthat occurs with changes in gaze angle is known as “incomi-tance.” The increase in exotropia on left gaze in monkey 2represents a horizontal gaze incomitance. In monkey 3, therewas a different type of incomitance. Left eye saccades showeda mean clockwise shift of 15° relative to right eye saccades(Fig. 8). A clockwise shift was observed for saccades of allamplitudes, directions, and starting points. The most likelycause was an unusual property that this animal displayed,namely, a 16°–17° rotation in the relative positions of the eyes’fixation points that occurred with either vertical or horizontalchanges in gaze angle (Fig. 6A). With upgaze, the animal’sexotropia was reduced, a form of incomitance known as anA-pattern. An A-pattern produces a clockwise direction differ-ence (i.e., left eye clockwise relative to right eye), but only forthe vertical component of saccades (Fig. 6A, left inset). Con-sequently, the direction difference is maximal for pure verticalsaccades and absent for pure horizontal saccades. In addition to anA-pattern exotropia, monkey 3 had a left hypertropia on left gaze
and a left hypotropia on right gaze. This change in relative verticalposition of the eyes as a function of horizontal gaze induces aclockwise direction difference, but only for the horizontal com-ponent of saccades (Fig. 6A, right inset). The combination of arelative shift in the horizontal separation of the two eyes withvertical gaze (A-pattern), and a relative shift in the verticalseparation with horizontal gaze, account for the 15° clockwiserotation of the left eye’s saccades.
It is debated whether incomitant strabismus patterns are dueto orbital mechanical forces, such as oblique muscle dysfunc-tion, altered rectus muscle action, or displaced pulleys (Ghasiaand Shaikh 2013; Ghasia et al. 2015; Hao et al. 2016; Kushner2010; Narasimhan et al. 2007; Oh et al. 2002). The surgeryperformed to induce strabismus in monkeys 2 and 3 causedchanges in eye muscle mechanics that contributed to theincomitance of their ocular deviation. Monkey 3 also had a verylarge deviation, which would be likely to alter extraocularmuscle action in different gaze positions. However, such pat-tern deviations have been observed in monkeys with a muchsmaller deviation, whose strabismus was induced without eyemuscle surgery. For example, Das and Mustari (see their“monkey S1” in Fig. 1; 2007) reported a pattern deviation in amonkey rendered strabismic by early bilateral visual depriva-tion that was nearly identical to the pattern in monkey 3.Incomitance, especially A- and V- pattern deviations, is alsoencountered frequently in humans with strabismus who havenot undergone eye muscle surgery (Deng et al. 2013; Dick-mann et al. 2012). This suggests that the incomitance inmonkeys 2 and 3 was not simply an odd feature of the surgicalmodel of strabismus.
Recently, Fleuriet et al. (2016) reported observations fromelectrical stimulation of the superior colliculus in strabismicmonkeys. They also recorded small differences in the size anddirection of saccades made by each eye. Our results are in closeagreement with their findings. They suggested that in strabis-mus, activation of a single location in the superior colliculus isnot interpreted by the saccade generator as the same desireddisplacement for each eye. This notion implies two separate,shifted motor maps in the superior colliculus, one for each eye.If offset monocular motor maps were present, it would beinteresting to consider the outcome of microstimulation appliedto a single site. The result can be predicted from experimentsstimulating at two different sites in the superior colliculus ofnormal animals. The ensuing saccade resembles the vector sumof the saccades driven at either site alone (Katnani et al. 2012;Noto and Gnadt 2009; Robinson 1972; Vokoun et al. 2014).The saccade is conjugate, because tectal output is conveyed toa saccade generator that drives yoked eye movements. For thesame reason, one would expect microstimulation to driveconjugate saccades in strabismus, even if there were monocularmaps comprising separate populations of neurons encodingdifferent vectors for each eye. It seems doubtful that thetechnique of microstimulation could reveal separate monocularmaps in strabismus, even if present. This point holds true,regardless of which eye is engaged in fixation at the moment ofmicrostimulation.
Fleuriet et al. (2016) found that the eyes’ peak saccadicvelocities remain equal, leading them to conclude that discon-jugate saccades could not be fully explained by changes in theoculomotor plant. We previously compared adducting andabducting saccades after tenotomy of the medial rectus and
also found equal saccadic velocities (Economides et al. 2007).Saccadic velocity, however, is only one facet of muscle func-tion. Other changes in muscle action could contribute tosaccadic disconjugacy.
In monkeys 2 and 3, evoked saccades were disconjugate, butsmooth pursuit and static fixations also showed disconjugacy(Figs. 5B, 6B, and 6C). This fact provides the strongest evi-dence that disconjugate saccades did not arise from plasticityof the map in the superior colliculus, but rather were a generalproperty of all gaze angle shifts in these animals. Evidence hasemerged that pattern deviations are due to cross-axis eyemovements generated by the firing behavior of ocular mo-toneurons (Das 2011; Das and Mustari 2007; Walton et al.2013, 2015). Even in normal animals, saccade metrics canadapt without any change in the tectal saccade map (Quessy etal. 2010). Incomitance in strabismus is likely due to perturba-tions downstream from the colliculus at sites that controlyoking of eye position in different gaze angles for saccades,smooth pursuit, and steady fixation.
In this context, “monkey XT1” in the report by Fleuriet et al.(2016) had a pattern deviation that was extremely similar tothat recorded in our exotropic monkey 3 (compare Fig. 6A withWalton et al. 2014, Fig. 1, C and D). Their monkey XT1 had a�20° direction difference for saccades, similar to the value of�15° in monkey 3. Reasons suggested for this direction dif-ference included density of eye muscle innervation, site ofmedial rectus reattachment, altered vergence tone, and abnor-malities in the saccade generator (Walton et al. 2014).
When strabismic subjects plan a saccade to a target, theymust decide which eye to use and then calculate the appropriatesaccade vector for that eye. Sometimes the target is detected bythe same eye that acquires it, but strabismic subjects are alsocapable of perceiving the location of a target with one eye andthen making a saccade to it with the other eye, a phenomenonknown as a “crossover” saccade (Economides et al. 2014). Itremains unclear how this process is controlled in the visualsystem. Any given visual target evokes a sensory response attwo different locations in the superior colliculus, but motoractivity to drive a saccade occurs at only one site. Single-unitrecordings, currently underway, should provide further insightinto how sensory responses are gated to yield subsequent motoractivation at a single site in the superior colliculus.
ACKNOWLEDGMENTS
Jessica Wong assisted with computer programming and Brittany C. Raponehelped to perform the experiments.
GRANTS
This work was supported by National Eye Institute Grants EY10217 (toJ. C. Horton) and EY02162 (to Beckman Vision Center) and by a PhysicianScientist Award from Research to Prevent Blindness.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
J.R.E., D.L.A., and J.C.H. conception and design of research; J.R.E.,D.L.A., and J.C.H. performed experiments; J.R.E., D.L.A., and J.C.H. ana-lyzed data; J.R.E., D.L.A., and J.C.H. interpreted results of experiments;J.R.E., D.L.A., and J.C.H. prepared figures; J.R.E., D.L.A., and J.C.H. drafted
manuscript; J.R.E., D.L.A., and J.C.H. edited and revised manuscript; J.R.E.,D.L.A., and J.C.H. approved final version of manuscript.
REFERENCES
Adams DL, Economides JR, Jocson CM, Horton JC. A biocompatibletitanium headpost for stabilizing behaving monkeys. J Neurophysiol 98:993–1001, 2007.
Agaoglu MN, LeSage SK, Joshi AC, Das VE. Spatial patterns of fixation-switch behavior in strabismic monkeys. Invest Ophthalmol Vis Sci 55:1259–1268, 2014.
Breznen B, Lu SM, Gnadt JW. Analysis of the step response of the saccadicfeedback: system behavior. Exp Brain Res 111: 337–344, 1996.
Bucci MP, Bremond-Gignac D, Kapoula Z. Speed and accuracy of saccades,vergence and combined eye movements in subjects with strabismus beforeand after eye surgery. Vision Res 49: 460–469, 2009.
Chaturvedi V, Van Gisbergen JA. Stimulation in the rostral pole of monkeysuperior colliculus: effects on vergence eye movements. Exp Brain Res 132:72–78, 2000.
Cooper J, Feldman J. Panoramic viewing, visual acuity of the deviating eye,and anomalous retinal correspondence in the intermittent exotrope of thedivergence excess type. Am J Optom Physiol Opt 56: 422–429, 1979.
Cooper J, Record CD. Suppression and retinal correspondence in intermittentexotropia. Br J Ophthalmol 70: 673–676, 1986.
Crawford ML, von Noorden GK. Optically induced concomitant strabismusin monkeys. Invest Ophthalmol Vis Sci 19: 1105–1109, 1980.
Das VE. Alternating fixation and saccade behavior in nonhuman primates withalternating occlusion-induced exotropia. Invest Ophthalmol Vis Sci 50:3703–3710, 2009.
Das VE. Cells in the supraoculomotor area in monkeys with strabismus showactivity related to the strabismus angle. Ann NY Acad Sci 1233: 85–90, 2011.
Das VE, Fu LN, Mustari MJ, Tusa RJ. Incomitance in monkeys withstrabismus. Strabismus 13: 33–41, 2005.
Das VE, Mustari MJ. Correlation of cross-axis eye movements and motoneu-ron activity in non-human primates with “A” pattern strabismus. InvestOphthalmol Vis Sci 48: 665–674, 2007.
Das VE, Ono S, Tusa RJ, Mustari MJ. Conjugate adaptation of saccadic gainin non-human primates with strabismus. J Neurophysiol 91: 1078–1084,2004.
Deng H, Irsch K, Gutmark R, Phamonvaechavan P, Foo FY, Anwar DS,Guyton DL. Fusion can mask the relationships between fundus torsion,oblique muscle overaction/underaction, and A- and V-pattern strabismus. JAAPOS 17: 177–183, 2013.
Dickmann A, Parrilla R, Aliberti S, Perrotta V, Salerni A, Savino G,Petroni S. Prevalence of neurological involvement and malformative/sys-temic syndromes in A- and V-pattern strabismus. Ophthalmic Epidemiol 19:302–305, 2012.
Economides JR, Adams DL, Horton JC. Eye choice for acquisition of targetsin alternating strabismus. J Neurosci 34: 14578–14588, 2014.
Economides JR, Adams DL, Horton JC. Perception via the deviated eye instrabismus. J Neurosci 32: 10286–10295, 2012.
Economides JR, Adams DL, Jocson CM, Horton JC. Ocular motor behaviorin macaques with surgical exotropia. J Neurophysiol 98: 3411–3422, 2007.
Fleuriet J, Walton MM, Ono S, Mustari MJ. Electrical microstimulation ofthe superior colliculus in strabismic monkeys. Invest Ophthalmol Vis Sci 57:3168–3180, 2016.
Gandhi NJ, Katnani HA. Motor functions of the superior colliculus. AnnuRev Neurosci 34: 205–231, 2011.
Ghasia FF, Shaikh AG. Pattern strabismus: where does the brain’s role endand the muscle’s begin? J Ophthalmol 2013: 301256, 2013.
Hao R, Suh SY, Le A, Demer JL. Rectus extraocular muscle size and pulleylocation in concomitant and pattern exotropia. Ophthalmology 123: 2004–2012, 2016.
Herzau V. How useful is anomalous correspondence? Eye (Lond) 10: 266–269, 1996.
Herzau V. Untersuchungen über das binokulare Gesichtsfeld Schielender. DocOphthalmol 49: 221–284, 1980.
Horwitz GD, Newsome WT. Separate signals for target selection and move-ment specification in the superior colliculus. Science 284: 1158–1161, 1999.
Hubel DH, LeVay S, Wiesel TN. Mode of termination of retinotectal fibers inmacaque monkey: an autoradiographic study. Brain Res 96: 25–40, 1975.
Joosse MV, Simonsz HJ, van Minderhout HM, de Jong PT. Quantitativemeasurements of suppression in divergent strabismus. Invest OphthalmolVis Sci 38: 532, 1997.
Katnani HA, Gandhi NJ. The relative impact of microstimulation parameterson movement generation. J Neurophysiol 108: 528–538, 2012.
Katnani HA, Van Opstal AJ, Gandhi NJ. A test of spatial temporal decodingmechanisms in the superior colliculus. J Neurophysiol 107: 2442–2452,2012.
Kiorpes L. Effect of strabismus on the development of vernier acuity andgrating acuity in monkeys. Vis Neurosci 9: 253–259, 1992.
Kushner BJ. Effect of ocular torsion on A and V patterns and apparent obliquemuscle overaction. Arch Ophthalmol 128: 712–718, 2010.
Narasimhan A, Tychsen L, Poukens V, Demer JL. Horizontal rectus muscleanatomy in naturally and artificially strabismic monkeys. Invest OphthalmolVis Sci 48: 2576–2588, 2007.
Niechwiej-Szwedo E, Goltz HC, Chandrakumar M, Wong AM. Effects ofstrabismic amblyopia and strabismus without amblyopia on visuomotorbehavior: III. Temporal eye-hand coordination during reaching. InvestOphthalmol Vis Sci 55: 7831–7838, 2014.
Noto CT, Gnadt JW. Saccade trajectories evoked by sequential and collidingstimulation of the monkey superior colliculus. Brain Res 1295: 99–118,2009.
Oh SY, Clark RA, Velez F, Rosenbaum AL, Demer JL. Incomitantstrabismus associated with instability of rectus pulleys. Invest OphthalmolVis Sci 43: 2169–2178, 2002.
Quessy S, Quinet J, Freedman EG. The locus of motor activity in thesuperior colliculus of the rhesus monkey is unaltered during saccadicadaptation. J Neurosci 30: 14235–14244, 2010.
Robinson DA. Eye movements evoked by collicular stimulation in the alertmonkey. Vision Res 12: 1795–1808, 1972.
Schall JD. Neural basis of deciding, choosing and acting. Nat Rev Neurosci 2:33–42, 2001.
Schiller PH. The superior colliculus and visual function. In: The Handbook ofPhysiology. The Nervous System. Sensory Processes. Bethesda, MD: AmPhysiol Soc, 1984, sect. 1, vol. III, part 1, chapt. 11, p. 457–505.
Song JH, McPeek RM. Neural correlates of target selection for reachingmovements in superior colliculus. J Neurophysiol 113: 1414–1422, 2015.
Stryker MP, Schiller PH. Eye and head movements evoked by electricalstimulation of monkey superior colliculus. Exp Brain Res 23: 103–112,1975.
Tychsen L, Burkhalter A. Nasotemporal asymmetries in V1: ocular domi-nance columns of infant, adult, and strabismic macaque monkeys. J CompNeurol 388: 32–46, 1997.
Van Horn MR, Waitzman DM, Cullen KE. Vergence neurons identified inthe rostral superior colliculus code smooth eye movements in 3D space. JNeurosci 33: 7274–7284, 2013.
Vokoun CR, Huang X, Jackson MB, Basso MA. Response normalization inthe superficial layers of the superior colliculus as a possible mechanism forsaccadic averaging. J Neurosci 34: 7976–7987, 2014.
Walton MM, Mustari MJ, Willoughby CL, McLoon LK. Abnormal activityof neurons in abducens nucleus of strabismic monkeys. Invest OphthalmolVis Sci 56: 10–19, 2015.
Walton MM, Ono S, Mustari M. Vertical and oblique saccade disconjugacyin strabismus. Invest Ophthalmol Vis Sci 55: 275–290, 2014.
Walton MM, Ono S, Mustari MJ. Stimulation of pontine reticular formationin monkeys with strabismus. Invest Ophthalmol Vis Sci 54: 7125–7136,2013.
Wiesel TN. Postnatal development of the visual cortex and the influence ofenvironment. Nature 299: 583–591, 1982.
Wurtz RH, Albano JE. Visual-motor function of the primate superior col-liculus. Annu Rev Neurosci 3: 189–226, 1980.
