Microstrabismus (also called microtropia) is a strabismus of small magnitude that affects between 1 and 3% of the general population.1,2 This condition may be complex to diagnose because of the difficulty in identifying all the clinical aspects that characterize it, a difficulty that may be particularly important in children.3 Several definitions and characteristics of microstrabismus have been proposed,4–6 with that of Lang7 being most widely accepted in the literature and favored in the present study. Accordingly, the angle of deviation of microstrabismus is defined as being between 0 and 5 degrees of visual angle (0 to 10 prism diopters).5,6,8 This deviation is subtle or completely invisible and can easily go unnoticed during a visual examination.9 The deviation is convergent in most cases, although microexotropia or vertical microtropia is also possible.8,10,11 Microtropia can arise either as an initial condition, as a primary microtropia, or as a consequence of a surgery for a larger strabismus, consecutive/secondary microtropia.1 Amblyopia is present in the deviated eye, which can be of strabismic-only origin, but it can be exacerbated by anisometropia and/or eccentric fixation.1,8,12 Visual examination also shows a decrease in stereoscopic vision and a harmonious abnormal retinal correspondence, which allows fusion despite the ocular deviation.6,8 In such a condition, the angle of anomaly is considered to be equal to the angle of ocular deviation. In subjects presenting an eccentric fixation, the angle of anomaly is equal not only to the angle of ocular deviation but also to the angle of eccentric fixation. In such cases, the deviation is undetectable; this condition is known as microstrabismus with identity.13 Visual examination of microstrabismus typically reveals a central suppression scotoma, which corresponds approximately to the angle of deviation.
Stereoscopic vision results from encoding of retinal disparity, and it is the gold-standard indicator of binocular vision integrity. Two types of tests are clinically used to determine stereothresholds, differentiated by whether they involve visible contours or not. Clinical tests without defined contours are typically composed of a multitude of randomly arranged dots called random-dot stereograms (RDSs).14,15 When RDSs are viewed monocularly, structured visual information is not perceived. However, under dichoptic viewing, a three-dimensional form emerges and becomes perceptible. On the other hand, for stereograms with defined contours, the shape of the target object perse is visible, with depth computation depending on both binocular and monocular cues. The stereoscopic processing associated with the presence or absence of visible contours is classically referred to as local and global stereopsis.14
In the presence of strabismus, regardless of the magnitude of the deviation, a loss of binocular vision is usually noticed. However, anomalous retinal correspondence (ARC) and central suppression of one eye prevent diplopia. Anomalous retinal correspondence also permits binocular fusion, and even central suppression preserves peripheral fusion.16 In fact, most subjects with microstrabismus seem able to discriminate some levels of local depth stereopsis, although their stereovision is much more impaired when measured using RDS. Using the Randot Stereotest, a standard clinical test to measure local and global stereoscopic capacities with polarized glasses, Pageau et al.17 have reported that almost 70% of subjects with microstrabismus can perceive local stereopsis, although their stereothresholds were about five times higher than normal, whereas only 3.8% of subjects were able to perceive global stereopsis. In accordance with this observation, previous studies18,19 suggest that global stereopsis may be particularly affected by the central scotoma typically seen in the deviated eye in microstrabismic subjects. Indeed adequate point-by-point correlation between the two eyes is mandatory to optimally perceive stereopsis in RDS.20 Microstrabismic central suppression may therefore interfere with interocular correspondence and, consequently, stereoperception. Interestingly, it has been estimated that in a microstrabismic adult, an interocular correlation of at least 80% is necessary to discriminate global stereo depth.18
An intriguing question is why microstrabismic subjects have some form of local stereopsis when assessed with standard tests. For example, the local task of the Randot Stereotest involves discriminating the depth of a circle relative to two others located on the frontal plane, which is used as a reference. This suggests that the brain succeeds, despite the area of central suppression, in establishing sufficient binocular correspondence to perform local depth discrimination. Such a residual stereoscopic capacity may be attributed, at least in part, to use of stimulus contours available in the tests. Indeed, stereoscopic matching, which consists of linking elements of the left image to the corresponding elements of the right one, is a fundamental and mandatory step for depth perception. This process is dependent on a set of “constraints” (e.g., gradient, contrast/brightness, segment geometry, uniqueness, etc.) that limit ambiguity and false matches.21 The continuity of contours, which is maximal in local stereoscopy, is one of the most important principles for stereo computation.22 For instance, depth perception of RDS is faster when the disparate regions are delimited by the addition of a continuous line.23
To date, few studies have systematically compared local and global depth discrimination in the same microstrabismic subjects, and the available data come almost exclusively from adult participants.18,24–28 As such, one cannot assume that the pattern of stereoscopic performance in microstrabismic adults necessarily translates to children. There is strong evidence that there are two distinct stereoscopic mechanisms in relation to the disparity magnitude coding, which have been referred to as “fine” (small disparities) and “coarse” (large disparities) stereopsis.29 Whereas the coarse stereoscopic system develops during the first years of life, recent evidence suggests that fine stereopsis is not completely mature at the onset of adolescence.30 Because microstrabismic adults are typically compared with control subjects who have optimal stereovision, including fine stereopsis, which fails to develop in amblyopia, their primary deficits might have been overestimated. Therefore, it is of interest to study microstrabismic stereoacuity in children at a developmental stage where fine stereopsis is not completely mature.
The goal of the present psychophysical study was twofold. First, similarly to the adult, we aimed to test the hypothesis that local depth discrimination is present in school-aged microstrabismic children, whereas global stereopsis is severely impaired. Second, given the potential adverse role of the central zone of suppression in stereoperception, we aimed to assess the performance of microstrabismic children when the size of the stimuli largely exceeded the area of suppression. Because such a manipulation would inherently increase the interocular correlation, an increase of global stereopsis performance was predicted.
Measurements of distance visual acuity were obtained from the linear crowded Lea symbols, the Snellen scale, or the ETDRS (Early Treatment Diabetic Retinopathy Study) scale, and stereopsis measurements were obtained from the Randot Stereotest. Nine children (mean [±SD] age, 10.6 [±2.7] years) with primary microesotropia and nine children (mean [±SD] age, 11.9 [±1.9] years) with normal visual acuity (20/20 in each eye) and stereopsis (20 arcsec at the Randot Stereotest) participated in the study. The details of the visual examination of subjects with microstrabismus are described in Table 1. None of the microstrabismic subjects complained of double vision. Microstrabismus diagnosis was based on the presence of the following signs: an ocular deviation; 0 to 10 prism diopters, as revealed by the cover test and the simultaneous prism cover test31; an eccentric fixation if null ocular deviation; a low to moderate (visual acuity of at least 20/80) unilateral amblyopia; a positive test result from the Irvine prism test (4Δ base-out test)32; a spontaneous sensory fusion using the Worth 4-dot; and an altered stereopsis, as measured by the Randot Stereotest. A thorough examination of refraction with cycloplegia (Cyclogyl 1%) was also performed to assess whether amblyopia was uniquely strabismic in nature or presented in combination with anisometropia (anisohyperopia ≥1 diopter [D] and/or anisoastigmatism ≥1.5 D). Amblyopia was considered to be any repeatable difference in visual acuity between the two eyes.33 Three subjects (1, 6, and 8) had microstrabismus with identity, as confirmed by a nasal eccentric fixation at the visuoscopy and no recovery movement of the affected eye with the cover test (cover test at near = 0, suggesting that the angle of anomaly is equal to the angle of eccentric fixation). The presence of ARC in subjects with microstrabismus without identity was confirmed by the simultaneous presence of both an ocular deviation and a spontaneous sensorial fusion with the Worth 4-dot test. In microstrabismic subjects with identity, the presence of ARC was assessed by spontaneous fusion with the Worth 4-dot test, an eccentric fixation of the affected eye, and absence of a shift movement with the cover test. The study was approved by the Research Ethics Committee of the Centre Hospitalier Universitaire Sainte-Justine (approval #2921), and all subjects were treated in accordance with the tenets of the Declaration of Helsinki.
For each experiment described below, two stimulus sizes were used: 4 and 12 degrees in diameter. Because the attention capacity of children is relatively limited, only two sizes were tested in this study to keep the duration of each experiment manageable. These sizes were chosen to indirectly manipulate interocular correlation. For example, if a stimulus with a diameter of 4 degrees is presented centrally to an observer with a right eye microesotropia of 2 degrees of visual deviation or more, we can presume that half of the stimulus information (left side from the fixation point) will not be available to the deviated eye because of central suppression or scotoma. In fact, some studies showed that the size of the scotoma might be larger considering the possibility that, in some cases, suppression can cross the midline to the ipsilateral visual field.8,34 Given that microstrabismic scotoma is only present when both eyes are open, it is very difficult to estimate its size.35 Nevertheless, suppression, and therefore interocular correlation, was estimated in subjects with noticeable strabismus (six out of nine subjects) using their ocular misalignment as measured with the simultaneous prism cover test. Interocular correlation for stimuli smaller than 4 degrees in diameter was minimal (i.e., 50% or less) for all subjects except for subject 2, who presented a very small deviation. Because his performance on the Randot Stereotest did not differ from the others (Z score, p > 0.05), this subject was kept in the sample. On the other hand, with a stimulus size of 12 degrees in diameter, the impact of suppression on total binocular correspondence becomes negligible in the microstrabismics, ranging from 83 to 99% in our sample, that is, well above the critical threshold of 80% reported by Garzia and Richman.18
Stimuli were presented dichoptically through a head-mounted virtual-reality display (Model Z800 3DVisor; eMagin Corp, Bellevue, WA) driven by a MAC G4 Desktop with an NVIDIA graphics card (GeForce 9400M, Santa Clara, CA). The device can be fitted over existing spectacles and the center-to-center distance between the left and right eye screens can be adjusted to match the interpupillary distance of each subject. The resolution of each monocular OLED (organic light emitting diode) screen was 800 by 600 pixels with a simultaneous refresh rate of 60 Hz and a corresponding visual field of 32 by 23 degrees for each eye. The size of a pixel subtends an angle of 144 arcsec (0.04 degrees). The granularity of the random-dot texture was 2 pixels, that is, 156 elements/degree.2 The RDS were generated with a two-dimensional uniform noise between −1 and 1 intensity, so that the RDS stimuli involved 100% of the dots.
For each experiment, trials consisted of 1-second stimulus presentations and the participants did not have limited time to respond. Stereothresholds for all conditions were obtained using a 1-up/2-down staircase procedure in which the disparity of the target increased by 25% with each incorrect response and decreased by 12.5% after two consecutive correct responses (threshold criterion = 79.7%). The staircase was interrupted after eight inversions and thresholds were estimated from the last six inversions. When required, subjects wore their refractive correction during the evaluation. Stereoacuity scores (in seconds) were transformed to log units for statistical analysis using a mixed analysis of variance with group and stimulus size as factors.
EXPERIMENT 1: CONTOUR STEREOPSIS
Stimuli and Methods
Circles with a diameter of 4 and 12 degrees with defined white contour (line width, 0.1 degrees) over a uniform grayscale noisy background (size of single dots, 2 pixels; contrast, 50%; mean luminance, 70 cd/m2) were presented stereoscopically to the subjects. A small horizontal reference line in the center (0.15 degrees by 0.4 degrees) was used to determine the relative depth of the circles. Subjects were instructed to indicate whether the circle was above (crossed disparity) or behind (uncrossed disparity) the frontal plane using two response keys. Local stereopsis thresholds were measured for each target size.
Analyses of variance revealed a main effect of group (F1,16 = 37.52, p < 0.00001) with no main effect of stimulus size (F1,16 = 0.28, p = 0.61) and no significant stimulus size × group interaction (F1,16 = 0.58, p = 0.46). As such, stereothresholds of the microstrabismic subjects were significantly higher (3.14 and 3.16 log arcsec for small and large stimuli, respectively) than those of the control group (2.10 and 2.21 log arcsec for small and large stimuli, respectively), regardless of the size of the stimuli (Fig. 1A). Interestingly, no correlation was found between microstrabismic subjects’ performances at this task and those measured with the Randot Stereotest (r = 0.11, p = 0.81).
This experiment involved a task in which stimulus presentation was relatively brief with a subtle reference cue relative to the frontal plane. This procedure was designed to minimize ceiling effects. As a result, stereothresholds measured in both groups were higher than expected. Normal stereopsis discrimination threshold is usually found at 40 arcsec or better5,36; this is about five times better than thresholds measured in the present experiment. Although this performance was expected in our child sample, a recent study showed that contour stereopsis for this age group is still immature for small (fine) disparities.30 Moreover, the lack of normal control values in this experiment, which could be attributed to the stimuli (e.g., interferences caused by the uniform background noise) and/or a nonoptimal stimulus presentation by the head-mounted display, raises the possibility that our task was too difficult and may therefore have introduced floor effects that prevented differences between the stimulus size conditions. Interestingly, even with such a difficult task, local stereopsis in most of our microstrabismic children was quite similar to those obtained in adults by Harwerth et al.’s study,26 in which dichoptic viewing was achieved using a 60-Hz liquid-crystal shutter system. However, with respect to stereoacuity measured by the clinical Randot Stereotest, the local stereo deficit of the microstrabismic children here was drastically more pronounced than with the control subjects.17 It is important to note that for the Randot Stereotest, there is no time limit to complete the task, and the target stimulus is surrounded by two adjacent frontal-plane circles of the same size that may facilitate determining the relative depth of the target.6 Such test characteristics may favor strategies or bias ability, such as use of monocular cues that overestimate stereopsis performance in observers who have inefficient binocular disparity coding.36–39 The lack of correlation between the Randot Stereotest and the present experiment supports this possibility, although our experiment was not exempt from monocular cues.
Unexpectedly, the increase of the stimulus size did not improve performance in the microstrabismic group. One factor that may explain this result is that visual information (i.e., contour) available from the unsuppressed part of the stimulus was sufficient to perform the task in both size conditions. Alternatively, the increase of interocular correlation and/or the Panum fusional area using large stimuli may have improved stereopsis, but this effect was masked by the greater difficulty of performing the task owing to the larger distance between the frontal-plane reference (fixation point) and the stimuli contour. This hypothesis is supported by the fact that thresholds for large stimuli tended also to be higher in the control group.
EXPERIMENT 2: RDS DEPTH STEREOPSIS
Stimuli and Methods
This experiment aimed at assessing depth discrimination by controlling for local/monocular cues using RDS instead of visible contours. To this end, dichoptic disks with diameters of 4 and 12 degrees embedded in RDS (size of single dots, 2 pixels; contrast, 50%; mean luminance, 70 cd/m2) were presented to the subjects. As in experiment 1, subjects were instructed to indicate whether the disk was above (crossed disparity) or behind (uncrossed disparity) the frontal plane using two response keys. Stereopsis thresholds were obtained for each target size.
Three out of nine microstrabismic subjects showed a complete absence of global stereopsis perception under 4 degrees stimulus size condition. Because these missing observations were not missing at random, a single missing data imputation technique was used by assigning the extreme upper limit of the distribution of the actual data (4 SD of the mean).40 This statistical approach allows the inclusion of the stereoblind subjects in the analysis without overestimating the differences between the two groups (i.e., minimizing type 1 error) or losing power. Analyses of variance revealed no main effect of stimulus size (F1,16 = 1.88, p = 0.19) but a significant main effect of group (F1,16 = 81.65, p < 0.000001). No significant stimulus size × group interaction was found (F1,16 = 2.79, p = 0.1), although it is worthwhile to point out a statistical trend (p ≤ 0.1) so that microstrabismic subjects tended to improve with large targets (3.17 vs. 2.99 log arcsec), whereas performance of the control subjects was identical in both conditions (1.82 vs. 1.84 log arcsec) (Fig. 1B). Interestingly, stereoacuity was measurable for all microstrabismic subjects under large stimuli condition, which was not the case with small stimuli.
This experiment aimed at assessing global depth discrimination in two stimulus size conditions. Regardless of the size of the target, the performance of the control observers was comparable and similar to previous studies.36,41,42 In contrast, the very low ability of microstrabismic observers to discriminate global depth for small stimuli tended to improve by presenting stereoscopic targets outside of their area of central suppression (threshold linear decrease of 27%). However, their performance was still abnormal compared with the control subjects.
The small sample size may account for the absence of significant improvement in the microstrabismic group as a function of stimulus size. On the other hand, because we chose to limit the missing data imputation technique to the distribution of the measurable stereoacuity, it is possible that the performance for the three “stereoblind” subjects was actually worse. If so, this would have yielded a more important, perhaps significant, gain induced by the large targets. Interestingly, two microstrabismic observers reported at the end of the task experiencing partial ring-like or donut-like percepts during the experiment with the large stimuli. In other words, their shape perception of the disks may have been incomplete because of the central suppression, but sufficient to have some depth discrimination ability. Indeed, although the area of central suppression prevented the perception of the whole shape, this was not mandatory to perform the task, and some depth discrimination was possible. A third experiment was then conducted to assess stereopsis under a condition where extraction of both depth and shape from the RDS is required.
EXPERIMENT 3: RDS SHAPE STEREOPSIS
Stimuli and Methods
This experiment aimed at assessing three-dimensional (3D) shape discrimination to assess stereovision beyond the computation of depth perception, as tested in experiment 2. Stimuli of 4 and 12 degrees in diameter were embedded in RDS (size of single dots, 2 pixels; contrast, 50%; mean luminance, 70 cd/m2). Subjects were instructed to perform a global form discrimination task by indicating with two response keys whether the stimulus in crossed-disparity depth was a disk or a square. Global stereopsis thresholds were measured for each stimulus size.
Stereothresholds of microstrabismic observers for this task were 0.2 to 0.5 log units higher in comparison to experiment 2 (Fig. 1C). In addition, it was impossible to measure any threshold for four out of the nine tested observers with small targets, and for one observer, stereopsis was impossible for large targets. As a consequence, the same missing data imputation technique used in experiment 2 was used here for statistical analysis. Analyses of variance showed a significant main effect of stimulus size (F1,16 = 34.12, p < 0.00002) and a significant main effect of group (F1,16 = 69.99, p < 0.00003). A significant stimulus size × group interaction was found (F1,16 = 33.73, p < 0.00003). No difference between stimulus sizes was found in the control group (1.88 log arcsec in both conditions). This performance in control subjects based on shape discrimination was almost identical to the one observed in experiment 2. By contrast, a significant improvement was noticed in the microstrabismic group for large stimuli with a linear gain of more than 62% of stereoscopic sensitivity (6712 vs. 2545 seconds). A strong correlation (r 2 = 0.94, p < 0.0001) was found between results with two stimulus sizes (Fig. 2).
This experiment aimed at assessing global shape depth discrimination. For both groups, threshold was slightly higher for this task compared with simple depth discrimination (experiment 2). Stereoscopic sensitivity of control observers was identical for both stimulus size conditions. However, performance of microstrabismic observers was greatly improved when most of the stereoscopic target exceeded the central scotoma, although their stereoperception remained abnormal. This drastic effect of large stimulus size, in addition to minimizing central suppression and maximizing interocular correlation, may also be related to higher-level processing. Indeed, the thresholds measured in this experiment resulted from a combination of depth (disparity) and shape discrimination. Thus, the effect of stimulus size on performance, which was much stronger than that found in experiment 2, which was based on depth discrimination only (i.e., on spatial retinal correspondence), is likely related to better cortical processing related to 3D shape perception.
This study aimed at better determining the factors underlying deficits of binocular depth discrimination in children with microstrabismus using different stereoscopic stimuli. In all experiments, stereopsis of the control group was about the same regardless of the type of target (local and global) or the size of the target. However, stereothresholds of microstrabismic subjects differed depending on the condition. In experiment 1 (contour stereopsis), the size of the target was not a critical factor in improving performance, whereas in experiments 2 and 3 (RDS stereopsis), the thresholds were better with large stimuli, particularly for global shape discrimination. In all experiments, for both local (contour) and global43 stereopsis, only coarse disparities could be measured in children with microstrabismus.
The improvement of stereopsis when the targets were presented in the periphery might appear paradoxical because stereoacuity thresholds usually increase as a function of eccentricity.44,45 However, the Panum fusional area, that is, the maximum disparity that can be fused binocularly, increases from fovea to periphery.46 Thus, stimuli that extend further into the periphery may be fused to produce coarse stereopsis, even in the presence of a small deviation between the optic axes. Consistently, it has been reported that the retinal correspondence of individuals with microstrabismus is normal with peripheral binocular stimuli but anomalous with central fusion stimuli.26 In the present study, this could explain why the large stimuli were able to improve RDS depth perception compared with the smaller ones, where suppression is a more important factor in reducing depth perception. Although this hypothesis is plausible for microstrabismus subjects, it does not apply to the control subjects, given the presence of a normal retinal correspondence across the retina and the common decrease of stereo performance as a function of eccentricity, although we did not observe such a decrease in the present study.
Regardless of the stimulus size, RDS thresholds in the microstrabismic group were lower when the task involved a discrimination of depth (in front of vs. behind) rather than shape (disk vs. square). This difference, which could be explained by the fact that shape discrimination truly requires global processing, was interestingly absent in control participants, suggesting a deficit in the microstrabismic to extract 3D shapes. This result is in agreement with the common clinical observation that microstrabismic patients are often able to correctly identify the stereograms of the Randot Stereotest containing no disparity (two out of eight stereograms), whereas they are usually unable to identify the shape in the other crossed-disparity stereograms, although they are often reported to “feel” some depth (see Pageau et al.17).
For each experiment, the stimuli were presented for 1 second. One could argue that longer stimulus duration would have improved performances of the microstrabismic subjects. In fact, this duration is largely sufficient to obtain normal stereovision (considered to be 20 arcsec in standard tests) in observers with normal binocular vision.47,48 Furthermore, Harwerth et al.49 have shown that local and global stereopsis thresholds saturate soon before 1000 milliseconds in both normal and amblyopic subjects. Therefore, the stimulus duration used in the present study appears sufficient to assess valid stereopsis thresholds in both groups, although we cannot exclude the possibility that longer stimulus duration in microstrabismic subjects would have yielded different results.
Our results suggest that the impairment of stereopsis in microstrabismus may be caused, at least in part, by the area of central suppression that affects the correlation between the two retinal images, thus reducing the efficiency of disparity coding and/or binocular correspondence processed by the brain. Although this interpretation is reasonable,50 one needs to be cautious about it. First, the fact that the performance of the microstrabismic subjects was still abnormal under large stimulus condition, which favored interocular correlation (>90% in average), suggests that other disrupting processes were involved. Second, the presence of suppression does not necessarily preclude stereopsis. As reviewed by Wolfe,51 there are strong evidences that rivalry-based suppression and stereopsis involve independent and parallel pathways through the early stages of visual processing, which allows both processes to coexist under some circumstances. Third, impairment of stereoacuity in comparison to control subjects has been reported in some microesotropic observers in the absence of identifiable central scotoma when assessed with the stereoperimetric technique.52 Therefore, the reduced stereopsis observed in microstrabismic subjects may also result, and perhaps primarily, from the abnormal and amblyogenic experience during early development.26 Further research is necessary to better distinguish the respective contribution of suppression and abnormal experience. For instance, one could assess the impact of suppression on stereopsis when controlling for abnormal visual experience by using a simulated central scotoma in healthy observers, induced by degrading the stimulus or uncorrelating the images presented to both eyes.
Anomalous retinal correspondence, that is, the shift of space coordinate of the deviated eye in which an extrafoveal element adopts the visual direction of the fovea of the fellow eye, was present in all of our microstrabismic subjects. Although ARC allows retrieving binocular single vision, including stereopsis, such a sensory adaptation is clearly not as efficient as in normal observers.13 An important question is whether the central and peripheral ARC have the same magnitude.4,5,53,54 Although early studies suggested that ARC involved a uniform shift throughout the visual field,55 the current consensus points to a nonuniformity. In fact, it has been reported that strabismic subjects have normal retinal correspondence with peripheral binocular stimuli and anomalous retinal correspondence with central fusion stimuli,26 although other data suggest that the retinal correspondence tends to be closer to normal in the central field and more anomalous in the periphery.56 Thus, the performance improvement observed in the present study, although abnormal, is likely attributed to the stimulus exceeding the size of either the suppression scotoma or the central area of ARC.
This study showed that the suppression zone might contribute to the poor stereoscopic performance of microstrabismic children. By increasing the angular size of the RDS stereoscopic stimuli, the adverse impact of the suppression area in binocular matching was decreased, and, as a result, the stereopsis of subjects with microstrabismus was increased. Despite this improvement, the microstrabismic performance did not reach control levels, supporting the view that the abnormal visual experience prevents the fine stereopsis mechanism from developing. Further research is necessary to better distinguish the respective contribution of suppression and abnormal experience.
Département de psychologie
Université du Québec à Montréal
We thank the FRSQ Visual Heath Research Network for their financial support awarded to Dave Saint-Amour and the Fonds de Recherche en Ophtalmologie de l’Université de Montréal for a scholarship awarded to Mariline Pageau.
The authors would like to express their gratitude to Mathieu Simard, who provided invaluable technical assistance. We also thank Dr. Armando Bertone for his valuable comments on earlier versions of the paper.
Received February 21, 2014; accepted October 31, 2014.
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