Effect of the accommodation-vergence conflict on vergence eye movements.

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Effect of the accommodation-vergence conflict on vergence eye movements

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Technicolor R&D, 975 avenue des Champs Blancs, CS 17616, 35576 Cesson-Sévigné Cedex, France Laboratoire de Psychologie de la Perception, Université Paris Descartes/CNRS, 45 rue des Saints-Pères, 75006 Paris, France

a r t i c l e

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Cyril Vienne a,b,⇑, Laurent Sorin a, Laurent Blondé a, Quan Huynh-Thu a, Pascal Mamassian b

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Article history: Received 2 December 2013 Received in revised form 24 April 2014 Available online xxxx

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Keywords: Accommodation-vergence conflict Vergence dynamics Focus Disparity Stereopsis Visual fatigue Disparity step

a b s t r a c t With the broader use of stereoscopic displays, a flurry of research activity about the accommodationvergence conflict has emerged to highlight the implications for the human visual system. In stereoscopic displays, the introduction of binocular disparities requires the eyes to make vergence movements. In this study, we examined vergence dynamics with regard to the conflict between the stimulus-to-accommodation and the stimulus-to-vergence. In a first experiment, we evaluated the immediate effect of the conflict on vergence responses by presenting stimuli with conflicting disparity and focus on a stereoscopic display (i.e. increasing the stereoscopic demand) or by presenting stimuli with matched disparity and focus using an arrangement of displays and a beam splitter (i.e. focus and disparity specifying the same locations). We found that the dynamics of vergence responses were slower overall in the first case due to the conflict between accommodation and vergence. In a second experiment, we examined the effect of a prolonged exposure to the accommodation-vergence conflict on vergence responses, in which participants judged whether an oscillating depth pattern was in front or behind the fixation plane. An increase in peak velocity was observed, thereby suggesting that the vergence system has adapted to the stereoscopic demand. A slight increase in vergence latency was also observed, thus indicating a small decline of vergence performance. These findings offer a better understanding and document how the vergence system behaves in stereoscopic displays. We describe what stimuli in stereo-movies might produce these oculomotor effects, and discuss potential applications perspectives. Ó 2014 Published by Elsevier Ltd.

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1. Introduction

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The mismatch between accommodation and vergence is considered to be the main difference between stereoscopic and natural viewing conditions. It is also recognized as the predominant factor entailing visual fatigue and discomfort (Hoffman, Girshick, Akeley, & Banks, 2008; Howarth, 2011; Rushton & Riddell, 1999; Shibata, Kim, Hoffman, & Banks, 2011; Ukai & Howarth, 2008; Wann, Rushton &Mon-Williams, 1995). For example, a significant proportion of stereoscopic observers have reported symptoms of eye strain, blurred vision, headache or dizziness symptoms (Hoffman et al., 2008; Shibata et al., 2011). As such, understanding the reason for these oculomotor issues is of major concern for optimal and safe use of stereoscopic systems. Because

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⇑ Corresponding author at: Laboratoire de Psychologie de la Perception, Université Paris Descartes/CNRS, 45 rue des Saints-Pères, 75006 Paris, France. E-mail addresses: [email protected] (C. Vienne), [email protected] (L. Sorin), [email protected] (L. Blondé), quan.huynh-thu@technico lor.com (Q. Huynh-Thu), [email protected] (P. Mamassian).

depth perception is based on vergence, it is crucial to evaluate how the vergence system can be altered by stereoscopic viewing. This study thus examines both the effect and after-effect of the accommodation-vergence conflict on vergence response, using the main sequence analysis (Bahill, Clark & Stark, 1975). In natural vision, binocular disparity and focus cues provide comparable signals about object distance (Held, Cooper & Banks, 2012), leading to a normal correlation between accommodation distance and vergence distance (Hoffman et al., 2008). These two cues are involved in depth and distance perception (Cutting & Vishton, 1995) and are complementary cues to depth (Held, Cooper & Banks, 2012). In stereoscopic displays, focus cues are, however, inconsistent with the displayed pattern of disparity because they signal a flat object, whose distance tends to be perceived closer to the display as compared to what indicates binocular disparity (Hoffman et al., 2008). Additionally, there is a conflict beyond these stimulations, because accommodation and vergence systems are intrinsically coupled (Schor, 1992). The two oculomotor systems operate together to provide a clear and single view of the world, leading to synkinesis, as the eyes simultaneously

http://dx.doi.org/10.1016/j.visres.2014.04.017 0042-6989/Ó 2014 Published by Elsevier Ltd.

Q1 Please cite this article in press as: Vienne, C., et al. Effect of the accommodation-vergence conflict on vergence eye movements. Vision Research (2014), http://dx.doi.org/10.1016/j.visres.2014.04.017

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accommodate and converge to the distance of the target object. Both systems can thus be stimulated through crosslink components (i.e. convergence accommodation and accommodative convergence) and, therefore, stereoscopic displays can strongly influence this synkinesis (Eadie, Gray, Carlin, & Mon-Williams, 2000). Models of the vergence system imply two components in vergence response, an initial ‘transient‘ fast pre-programmed component and a slow ‘sustained‘ feedback component (Hung, Ciuffreda & Rosenfield, 1996; Schor, 1992). The first component yields the motor signal to rapid depth changes, and the second minimizes the vergence error within neurological tolerances. The vergence response also depends on the contribution of different motor controllers that respond to specific inputs, such as binocular disparity, retinal defocus and proximity (Howard & Rogers, 1995). Here, we consider the contribution of disparity vergence and accommodative vergence to the overall response, because proximal vergence should barely participate in the response to disparity below 4° (Schor, 1992). There are a few studies dealing with the possibility that vergence dynamics could vary when disparity and focus cues are available (Hung, Semmlow & Ciuffreda, 1983; Hung, Ciuffreda, Semmlow, & Horng, 1994; Maxwell, Tong & Schor, 2010). The dynamics of disparity vergence when accommodation is open-loop has been shown to be similar to the one when correct blur cues are presented (Maxwell, Tong & Schor, 2010). However, no quantitative study has been conducted to explore whether vergence dynamics could differ between a condition with correct blur cues and a condition with constant accommodation (Maxwell, Tong & Schor, 2010), although a number of points suggest that the accommodation-vergence conflict could affect vergence dynamics. Firstly, the conflict has been shown to increase time to fuse (Hoffman et al., 2008). Secondly, it has also been demonstrated that accommodative vergence and disparity vergence have different dynamics (i.e. different velocities Maxwell, Tong & Schor (2010)). Thirdly, the contribution of disparity vergence drives the transient response, while that of accommodative vergence only occurs at the end of the transient response (Hung, Semmlow & Ciuffreda, 1983; Semmlow & Wetzel, 1979). Lastly, because of synkinesis, accommodation would tend to inhibit vergence that conflicts with itself (Patel, Jiang, White, & Ogmen, 1999). The dynamics of vergence response could thus vary when both cues provide different information, because of the influence of each controller on the initial response. The contribution of accommodative vergence to the total vergence response can be explored using a cue-conflict paradigm, where focus and disparity cues are either conflicting or congruent. In these conditions, the conflict can alter the normal functions of the visual system (Hoffman et al., 2008; Howarth, 2011; Rushton & Riddell, 1999; Ukai & Howarth, 2008; Wann, Rushton & Mon-Williams, 1995). For instance, binocular fusion can be slower and stereoacuity thresholds can be worse (Hoffman et al., 2008). Furthermore, displaying discrepant stimuli can both provide an immediate effect and an after-effect on the vergence system (Emoto, Niida & Okano, 2005; Hoffman et al., 2008). Changes in the dynamic characteristics of such a system can be studied using main sequence analysis (Bahill, Clark & Stark, 1975; Munoz, Semmlow, Yuan, & Alvarez, 1999). It has been used extensively in the literature, for example, to assess the dynamic changes to repetitive step stimuli (Munoz et al., 1999). It also portrays how the dynamic responses of a system can change with increasing amplitude (e.g., Kasthurirangan, Vilupuru & Glasser, 2003). In a first experiment, we assessed the effect of the conflict on vergence response. In a second experiment, we examined the effect of prolonged exposure to the accommodation-vergence conflict on vergence response.

2. Experiment 1

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Vergence responses were examined in a conflict viewing condition and a match viewing condition. The conflicting stimuli presented incongruent disparity and blur information for the second fixation position, i.e., after a disparity step (in front or behind the screen plane). The matching stimuli provided corresponding disparity and blur information at the target depth. Based on previous work (Hoffman et al., 2008), the conflict condition was expected to reduce the velocity of the vergence system, as well as its response amplitude and its reaction time.

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2.1.1. Participants A total of 14 observers took part in the study. Two participants were discarded, both because they revealed very poor performances in judging relative disparities (under the chance level) and because of their difficulty in fusing the stimuli (presenting overly long reaction times). Two more were discarded because they had large difficulties performing the task (less than 50% of trials were valid). The ten remaining participants were tested according to a full counterbalanced order. They were on average 29.3 years old (ranging from 22 to 49 years old). All had normal or corrected vision and presented stereoacuity threshold at least inferior to 30 arc minutes as assessed by the Randot Stereo Test. They gave their informed consent before beginning the experiment.

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2.1.2. Apparatus We designed a specific apparatus depicted in Fig. 1(A). The participants’ head was placed in a chinrest located 1.3 meters from the 3D display (Hyundai S465D 4600 HDTV LCD Polarized monitor), on an optical table (Newport, 120 90 cm), which served as a firm mechanical connection for all elements of the system. The apparatus was composed of a vertical beam splitter (360 255 mm, Edmund Optics), located in front of the eyes of the participants, and tilted 45° to the sagittal plane. Perpendicular to the sagittal plane, an optical bench (2.8 m) was used to move a 2D display (Dell 1908FP 19’’ LCD monitor) at the desired distances thanks to a slider device mounted on the bench. The center of both displays was carefully aligned along the subject’s midline using visible light. We visually checked that alignment was correct by displaying a set of vertical and horizontal lines crossing at the center of each screen. Participants wore polarized glasses to fuse left/right views; the displaying method was to present left/right views interleaved line-by-line. To minimize display crosstalk visibility, we placed the participant’s cyclopean eye on the axis perpendicular to the center of the screen. Stimuli were displayed using Matlab and the Psychtoolbox extensions (Brainard, 1997). Vergence movements were recorded using a binocular eye-tracker (Eyelink 1000, SR-Research) with a sampling rate of 2000 Hz and a spatial resolution of 0.01°.

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2.1.3. Procedure and stimuli There were two conditions labeled (1) the conflict viewing condition, and (2) the match viewing condition. Participants had to fuse disparity step stimuli, which always started in the middle of a 3D screen plane. Convergent and divergent vergence responses were measured for the disparity amplitudes of 0.75°, 1.0°, 1.25° and 1.5° (see Fig. 2(A)). We used a fixation target (35 arc minutes radius) formed of a white fixation cross (18 arc minutes) surrounded by a frame composed of small squares of various shades of grey (5 by 5°) to help maintain stereoscopic fusion (see Fig. 1(B)). This visual pattern yielded the perception of relative depth between the fixation target and the surrounding frame. Each

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Fig. 1. (A) Schematic representation of the experimental apparatus used in the study. S3D refers to the stereoscopic 3D display used in the conflict viewing condition and for the initial fixation in the match condition. BS means beam splitter. The second display presents the fixation target for the match viewing condition. To compare the vergence response in both conditions, the second display distance was adjusted on the optical bench to match the 3D stereoscopic distance used in the S3D condition. The disparity amplitude is determined by the convergence angle at initial position (h) minus the angle at stimulus depth (d). In the present case, the disparity amplitude is negative and corresponds to a convergent disparity step. Drawing is not to scale. (B) Stimulus used to measure the vergence response to a disparity step. (a) is the fixation target and (b) is the frame composed of small squares to help maintain stereoscopic fusion. This stimulus introduces relative disparity between the fixation target and the frame.

Fig. 2. Schematic representation of the disparity step value used in Experiment 1 and portrayal of their relation to the accommodative demand. The panels plot absolute vergence demand as a function of stimulus presentation time (A) and accommodative demand as a function of vergence demand (B). The left figure (A) shows the possible (unsigned) disparity step magnitudes (0.75°, 1°, 1.25° and 1.5°) as a function of trial duration (in seconds). The left ordinate axis represents the corresponding vergence demand scale (MA = Meter Angles). The random step stimulus is represented with a black rectangle with the three possible latencies (0.5, 1 and 1.5 s). The right figure (B) plots the accommodative demand as a function of vergence demand for the possible step stimuli in the conflict condition (white squares) and the match condition (white circles). The main diagonal black line corresponds to the natural vision line. The dark grey zone is the Panum0s fusion area (15 arc min, Hoffman et al., 2008), the medium grey zone is the depth-of-focus (0.3 diopter) and the light grey zone corresponds to the Percival0s zone of comfort. 208 209 210 211

trial started with a zero-screen-disparity fixation target, whose presentation duration was randomly either 0.5, 1 or 1.5 s so as to limit any anticipation behavior and to weaken step predictability. In the conflict condition, the left and right views of the fixation

target were then shifted according to eight possible disparity amplitudes; there was no blur step because the accommodative distance stayed on the 3D display. This condition provides a conflict between disparity and focus cues that creates a


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stereoscopic demand (see Fig. 2(B)). In the match condition, the fixation target was now presented on the second 2D display, after the step, so as to provide the disparity step with the appropriate blur step. This condition displays step stimuli located on the natural vision line (main diagonal in Fig. 2(B)). The step stimulus duration was 2.5 s. The fixation target was then presented back on the first display so as to control vergence has returned back to its initial position. Both viewing conditions were performed on two different days. Each level of the conflict condition was repeated ten times and each level of the match condition nine times.

2.1.4. Data analysis To investigate how the viewing conditions affected the dynamic properties of vergence responses, we analyzed changes in the slopes of the linear regression between amplitude and peak velocity of each participant, vergence gain (response amplitude to stimulus amplitude) and latency. The slope of the regression between amplitude and peak velocity of the response, which describes the first-order dynamics of that movement (Alvarez, Semmlow, Yuan, & Munoz, 1999), provides a way to study changes in velocity as a function of stimulus amplitudes. Vergence gain allows the study of changes in response amplitude for different stimulus amplitudes. Latency indicates which delay a physiological system needs to produce a response and therefore informs about how easily inputs are processed by the system. Blinks (during any portion of the response), saccades (during the transient portion of the response) or missing samples were discarded from data analysis. Vergence movements were analyzed to detect the response onset (latency), offset, peak velocity and initial amplitude of each trial, using the procedure described in Maxwell, Tong and Schor (2010). The raw eye position records from the two eyes were first smoothed by a ten-point sliding average filter (see example in Fig. 3). Vergence was calculated as left-eye position minus right-eye position. Vergence velocity was calculated using a two-point central difference algorithm (Bahill, Kallman & Lieberman, 1982). The onset of the vergence response was defined as the point where the vergence velocity of five successive points

Fig. 3. Ocular vergence and vergence velocity of a response to a 1.5° convergent disparity-step. Ocular vergence is represented by the upper thick solid line and vergence velocity by the lower thin solid line. The dashed line represents the stimulus disparity amplitude in degree; in this case, the 2D fixation duration is 0.5 s and is followed by a (stereoscopic) disparity step of 1.5° during 2.5 s, followed the by a 1 s of a return 2D fixation. In the lower part of the figure, the reference lines (solid and dashed lines) represent the thresholds used to find the onset and offset of the vergence movement. Detected onset and offset points are represented with grey dots for the velocity distribution and with dark dots for the vergence position of the eyes.

first exceeded 2 deg. s1. Latencies larger than one second were not considered in the analysis. The peak of velocity was taken as the highest velocity within the first second of each trial. The offset of the vergence response was defined as the point where the vergence velocities of five successive points were less than 5% of the peak velocity. The amplitude was calculated as the amplitude at offset minus the amplitude at onset. Raw data represent the combination of four disparity values, two vergence directions and two stimulus conditions. The amplitude, velocity peak and latency of vergence responses were then averaged per disparity amplitude in order to plot main sequences and to perform data analysis. Averaging was only performed when at least four repetitions were available per disparity amplitude. To compute the slopes of the linear regression between peak amplitude and peak velocity, we used the raw data, not the averaged data. Each of the three dependent variables was analyzed separately using a repeated measures analysis of variance (ANOVA). Before running statistical analysis, the normality and sphericity assumptions were verified. Where applicable, we used the Greenhouse-Geisser correction for correcting against violations of sphericity.

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Fig. 4(A) presents the relationship between amplitude and peak velocity of vergence responses. A two-way ANOVA on the slopes of the regression lines revealed an effect of viewing condition (Fð1; 9Þ ¼ 31:78; p < 0:0001). Slopes were larger in the match condition (2.37 deg. s1 (sd ¼ 0:67) and 2.35 deg. s1 (sd ¼ 0:69) for divergence and convergence, respectively) than in the conflict condition (1.6 deg. s1 (sd ¼ 0:5) and 1.46 deg. s1 (sd ¼ 0:43) for divergence and convergence, respectively), thereby indicating a greater velocity in the match viewing condition (on average about 35% faster). There was no statistical difference between slopes for convergence and divergence (Fð1; 9Þ ¼ 0:12; p ¼ 0:74) and no interaction effect between these two variables was observed (p > 0:05). A three-way ANOVA was conducted on vergence gain and showed an effect of disparity amplitudes (Fð3; 27Þ ¼ 5:39; p < 0:03 using the Greenhouse-Geisser correction). Vergence gain decreased with larger disparity amplitudes. An effect of vergence direction was also observed (Fð1; 9Þ ¼ 4:5; p < 0:01). Vergence gain was thus significantly larger for convergence than for divergence. However, there was no effect of viewing condition (Fð1; 9Þ ¼ 3:37; p ¼ 0:1) and no interaction effect was observed between viewing condition and vergence direction (Fð1; 9Þ ¼ 3:57; p ¼ 0:092). The results are displayed in Fig. 4(B). A three-way ANOVA revealed a significant effect of viewing condition on latency (Fð1; 9Þ ¼ 27:42; p < 0:001), indicating it was larger for conflicting stimuli than for matched stimuli (194 ms (sd ¼ 9) vs. 149 ms (sd ¼ 7)). An effect of disparity amplitudes was observed (Fð3; 27Þ ¼ 9:03; p < 0:001). Vergence latency decreases with increasing disparity amplitude. Lastly, an interaction effect between vergence direction and disparity amplitudes was found (Fð3; 27Þ ¼ 6:85; p < 0:001). The effect of disparity amplitudes on latency was larger for divergence than for convergence. The results of a post hoc analysis (Duncan’s test) are represented in Fig. 4(B).

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In this experiment, the accommodation-vergence conflict reduced vergence dynamics: peak velocity was reduced both for convergence and divergence. Hung et al. (1994) found no difference in vergence dynamics of three subjects between a conflict condition and a no-conflict condition. Also, their participants were able to fuse a larger range of disparity amplitudes (i.e., from 0.5° to 10°). The difference between the stimuli used in their study and

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Fig. 4. (A) Plot of main sequence of amplitude vs. peak velocity data points, for symmetric vergence step responses for all participants combined (N = 10). The linear regression lines are represented for convergence on the left and divergence on the right in both the match condition (black lines) and the conflict viewing condition (gray lines). The Pearsons correlation coefficients range from 0.80 to 0.89. The black circles correspond to data averaged over participants for the match condition and the grey triangles represent the data for the conflict viewing condition. (B) Left: vergence gain as a function of viewing condition (match vs. conflict) and vergence direction (convergence vs. divergence). Right: mean latency of vergence movements as function of viewing condition and vergence direction. Error bars denote the standard error of means and, where applicable, brackets with accompanying stars indicate significant differences (p < 0:05 and p < 0:01).

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ours might explain the different observed effects on vergence response. They used similar fixation targets, except that ours were surrounded by a frame composed of small squares. Thus, it is likely that this frame, whose position was kept in the screen plane, has provided a substantial accommodative and possibly a proximal stimulus. As the disparity amplitudes exceeded Panums fusion area, the frame led to diplopia when observers fused the step stimulus. However, the frame was not optically blurred in the conflict condition, and this could potentially account for the effect on vergence response. Though vergence velocity was reduced by about 35% in the conflict condition, vergence gain was only marginally reduced. As a result, the accommodation-vergence conflict did cause a change in vergence dynamics, and not just a remapping of the amplitude of the preprogrammed movement to the perceived depth. In our study, the latency of vergence response was on average 150 ms when disparity and focus cues specified the same depth, while vergence movements typically show latency of about 100 to 200 ms, depending on the direction, initial position and predictability of the target (Alvarez, Semmlow & Pedrono, 2005; Heron, Charman & Schor, 2001; Hung, Zhu & Ciuffreda, 1997; Semmlow & Wetzel, 1979). Latency was significantly increased when the two stimuli were conflicting, with an increase up to 54 ms larger on average. The accommodation-vergence conflict has been shown as a factor increasing time to fuse (Hoffman et al., 2008). Here, we provide experimental results that confirmed this finding, indicating that time to fuse is increased, firstly because vergence latency is larger, and secondly because vergence velocity is reduced by the conflict condition.

Some distinctions in the timing components between divergent and convergent movements are not new (Alvarez, Semmlow & Pedrono, 2005). For example, vergence latency has been found to be smaller for convergence than for divergence response in some studies (Hung et al., 1994; Hung, Zhu & Ciuffreda, 1997; Zee, Fitzgibbon & Optican, 1992), whereas other studies found the inverse (Alvarez, Semmlow, Yuan, & Munoz, 2002; Krishnan, Farazian & Stark, 1973). Alvarez, Semmlow and Pedrono (2005) found that divergence latency is dependent on initial stimulus position. Here, vergence latency was slightly larger for convergence than for divergence in the match viewing condition (about 30–40 ms), as found in other studies (Alvarez et al., 2002; Krishnan, Farazian & Stark, 1973). Conversely, we observed that the conflict between accommodation and vergence not only increased latency, but also reduced the difference between convergence and divergence latency. However, no statistical differences were found between convergence and divergence kinematic properties, both in the conflict condition and the match viewing condition. This lack of effect could stem from the use of smaller disparity values in this study compared to those in past ones (e.g., up to 16° in Hung, Zhu & Ciuffreda (1997)).

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The conflict between accommodation and vergence can affect the responses of the visual system as well as the perceived depth from stereopsis (Eadie et al., 2000; Hoffman et al., 2008; Rushton & Riddell, 1999; Ukai & Howarth, 2008; Wann, Rushton & Mon-Williams,

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1995). In our work, a decrease in vergence dynamics was observed with a conflict condition. In this experiment, the focus is on the after-effect of such a conflict on vergence performance. The vergence system can adapt to continuous viewing through fixed prisms or to sustained stimuli in stereoscopic displays, causing phoria adaptation in subjects with normal binocular vision (e.g., Lee, GrangerDonetti, Chang, & Alvarez, 2009;North, Henson & Smith, 1993;Patel et al., 1999;Schor, 1979). Several studies have also shown the adaptation of dynamic parameters following sustained vergence (Lee et al., 2009; Patel et al., 1999), or specific conditioning step stimuli (e.g., Munoz et al., 1999). The vergence system can also adapt to consecutive changes in binocular parallax (Eadie et al., 2000; Emoto, Niida & Okano, 2005), and significant effects on vergence performance have been shown after a prolonged stereoscopic activity (Emoto, Niida & Okano, 2005). The dynamics of vergence eye movements are thus malleable, and somehow depend on the magnitude and direction of previous stimuli (Alvarez, Semmlow & Pedrono, 2005). This experiment thus proposes to investigate whether the recovery of performance occurs by adaptation. An experimental phase was used to strongly stimulate the visual system, and changes in vergence responses to disparity steps were assessed in pre- and post-tests. Participants were tested in two different sessions presenting different disparity amplitudes. The first session displayed a very slight stereo-demand (stimuli were easily fused) whereas the second session presented a range of disparity values around the theoretical zone of comfort. The later was expected to produce large vergence errors (i.e. fixation disparity) due to the use of larger stereo-demands. Given that the range of disparity amplitudes was relatively small, an increase in vergence dynamics was expected.

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The same ten observers participated in this second experiment. We also used the same experimental apparatus except the beam splitter and the second screen.

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3.1.1. Procedure and stimuli Participants were invited for two experimental sessions during which they were presented with a set of pre- and post-tests so as to estimate the outcomes of the experimental phase. This later

included a series of trials, each composed of a disparity step followed by an oscillating depth pattern (see Fig. 5(A)). The task was to perform eye movements on a fixation target identical to the one of Experiment 1 (see Fig. 1 (B)). Additionally, participants had to indicate, as quickly as possible, if an oscillating depth pattern (annulus) was in front or behind the fixation plane so as to ascertain they fused the stimulus. They had to discriminate the sign of the disparity at the end of the oscillatory motion. The use of such a stimulus was not expected to affect vergence, neither to set vergence as active as the range of amplitude was small (i.e. in the Panums area). More specifically, we used a motion-in-depth stimulus in order to make the task harder and to keep a relatively high attentional level. The experimental phase was designed to mimic the possible ocular displacements experienced by a viewer of a stereoscopic video including several shots. A typical trial began with the fixation target located at a random planar position (in a virtual frame of a 7.5 by 11° centered in the middle of the screen), thereby involving version eye movements. The fixation target then jumped in depth for a disparity amplitude selected in a given range, defining two different sessions. We defined Session 1 where fixation targets were displayed with disparity of 0, 10, 20, 30 or 40 arc minutes and Session 2 with disparity of 80, 90, 100, 110 or 120 arc minutes. Step stimuli were presented on the stereoscopic display and, therefore, the accommodation demand was constant and fixed on the screen plane (see Fig. 5 (B)). Once the fixation target jumped in depth, an annulus (47 arc minutes radius) oscillated back and forth from +6 arc min to 6 arc min relative to the fixation target with a mean velocity of 18 arc minutes per second (oscillation of 1.33 Hz). Participants had to wait until the annulus stopped oscillating to judge whether it was in front or behind the fixation target. Visual feedback was provided (green for correct response or red for incorrect response). In each session, ten possible values of disparity were used and repeated until the session duration reached 30 min. Trial duration was between 4 and 15 s, ensuring multiple sustained phases for the vergence system. To investigate changes in the response dynamics of vergence, participants performed vergence movements in pre- and post-tests. Thus, we presented crossed/uncrossed disparity-step stimuli of 0.75°, 1.0°, 1.25° or 1.5° simulated using the stereoscopic display and the same procedure as for the conflict condition in Experiment 1.

Fig. 5. Schematic representation of the disparity step values used in the experimental phase of Experiment 2 and their relation to the accommodative demand. (A) shows an example of disparity amplitudes observed during the experimental phase. The disparity steps of Session 1 (small disparity values) are represented in black and those of Session 2 are represented in grey (large disparity values). Negative values correspond to convergence steps and positive values to divergence steps. The stereo-demand implied by the moving-in-depth annulus is also represented with the oscillating pattern both in Session 1 and Session 2. (B) plots the accommodative demand as a function of vergence demand for the possible step stimulus in Session 1 (using small disparity values, white diamonds on the figure) and Session 2 (large disparity values, white squares on the figure). The main diagonal black line corresponds to the natural vision line. The dark grey zone is the Panum0s fusion area, the medium grey zone is the depth-of-focus and the light grey zone corresponds to the Percival0s zone of comfort.


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Each level of this condition was repeated nine times in a random order.

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3.1.2. Data analysis The procedure described in Experiment 1 was used to analyze the effect of the experimental phase in pre- and post-tests (i.e., changes in slopes between amplitude and peak velocity, vergence gain and latency). Raw data represent the combination of four disparity values, two vergence directions, two tests (pre- and posttests) and two experimental sessions (Session 1 vs. Session 2). Each dependent variable was analyzed separately using repeated measures ANOVA and the assumptions of normality and sphericity were verified. When the normality assumption was violated, we report the results of the non-parametric tests and we use the Bonferroni correction for post hoc testing. Concerning the experimental phase, data were arranged so that we obtained two factors with two levels: session (Session 1 vs. Session 2) and vergence direction (convergence vs. divergence). The analysis of success rate assessed whether participants fused and tracked the fixation target.

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3.2.1. Results of the experimental phase By performing a non-parametric ANOVA (Wilcoxon Matched Pairs Test, non-normal samples), we compared the results (success/failure) obtained in the two sessions of the experimental

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phase (Session 1 vs. Session 2). The analysis revealed an effect of Session, the success rate was larger in Session 1 (87% 19 SD) than in Session 2 (67% 33 SD) (T ¼ 26; Z ¼ 2:95; p < 0:004). There was no statistically significant effect of vergence direction between the success rate for divergent targets (67% 35 SD) and the one for convergent steps (85% 17 SD) (T ¼ 53; Z ¼ 1:94; p ¼ 0:052). In order to test for a possible interaction effect between the two factors, we ran a Friedman’s test on the four subgroups (two factors of two levels). The analysis revealed a significant difference between the subgroups (v2ð3Þ ¼ 9; p ¼ 0:029). A post hoc analysis was then conducted and only revealed a significant difference between the success rate for convergence response in Session 1 and the one for the divergence response in Session 2 (p < 0:01).

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3.2.2. Results of the disparity steps An exemplar of the relationship between amplitude and peak velocity of the vergence responses can be observed in Fig. 6, for one participant. In this case, the main sequence analysis suggests that peak velocity is increased in convergence responses of session 2. In order to provide a quantitative analysis, a two-way ANOVA was conducted on the slopes of the regression lines for each participant. This analysis revealed a first interaction effect between vergence direction and experimental phase (Fð1; 9Þ ¼ 5:63; p < 0:05). A post hoc analysis (Duncan’s test) was performed and showed that slopes for divergence and convergence could not be differentiated in pre-test but slopes for divergence were significantly larger by about 0.31 deg. s1 (22% slower) than for convergence in

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Fig. 6. Main sequences for one observer, for symmetric vergence responses based on raw data. The upper part of the figure represents the data for Session 1 (small disparity amplitudes) and the lower part represents the data for Session 2 (large disparity amplitudes). The black triangles are data in pre-test and the grey squares represent data in post-test. The linear regression lines are represented for convergence on the left and divergence on the right for both the pre-test (black lines) and the post-test (grey lines). The Pearson’s correlation coefficients range from 0.65 to 0.86.


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post-test (p < 0:05, see Fig. 7 (A)). A second interaction effect was observed between the factors session (range of disparities) and experimental phase (Fð1; 9Þ ¼ 7:44; p < 0:025). A post hoc analysis revealed that, although slopes were about 0.24 deg. s1 higher (17% faster) for Session 1 than Session 2 in pre-test (p < 0:05), slopes of Session 2 increased by about 0.18 deg. s1 (12% faster) after the experimental phase (p < 0:05) whereas no difference was found for slopes of Session 1 between pre-test and post-test (p > 0:05, see Fig. 7 (B)). Vergence dynamics were thus increased following the experimental phase with large disparities near the comfort limits but not with small disparities. A four-way ANOVA performed on vergence gain revealed a significant effect of disparity amplitudes (Fð3; 27Þ ¼ 4:84; p < 0:01), vergence amplitude decreased with larger stimulus disparity amplitude. There was no statistically significant effect of vergence direction (Fð1; 9Þ ¼ 4:99; p ¼ 0:051). The results are represented in Fig. 8. A four-way ANOVA performed on vergence latency revealed an effect of disparity amplitudes (Fð3; 27Þ ¼ 7:83; p ¼ 0:001), the latency of vergence movements decreased with larger disparity amplitude. An interaction effect between the factors session and experimental phase was also observed (Fð1; 9Þ ¼ 7:16; p ¼ 0:025). A post hoc analysis (Duncan’s test) revealed no difference between the effects of session in pre-tests (p > 0:05) but showed that the latency was about 11 ms larger in Session 2 than in Session 1 in post-test (p < 0:02). This effect suggests that latency was increased following the experimental phase in Session 2 but not in Session 1 (see Fig. 8). No other effect was observed.

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3.3. Discussion

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During each experimental session, participants judged whether a moving-in-depth pattern was in front or behind the fixation plane. It was harder to perform the task in the session with disparity amplitudes near the comfort zone than in the session with small disparity amplitudes. This finding suggests that the session with large disparity amplitudes led to larger vergence errors than the session with small disparity. As the disparity amplitudes did not exceed the comfort zone, the experimental phase could have served as a training period for the visual system (Emoto, Niida & Okano, 2005). The results should thus indicate an adaptive behavior of the vergence system when the disparity step amplitudes are close to the comfort zone. When a range of disparities around the comfort zone was displayed, the experimental phase caused an increase both in peak velocity and vergence latency. This improvement in vergence dynamics could be caused by a process known as vergence

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adaptation. Vergence adaptation typically refers to a component that prevents visual fatigue, and attempts to minimize the vergence error during binocular fixation. Fixation disparity can be seen as a result of visual stress, or as a purposeful steady-state error for the vergence system (Schor, 1979). Fixation disparity is tightly linked to vergence adaptation because the adaptive control requires an error signal and typically uses the sustained feedback component to adjust the oculomotor parameters to decrease the error amplitude (Schor, 1979). Previous studies have observed adaptation to sustained vergence or to repetitive step stimuli, which led to changes in the tonic and/or phasic elements (e.g., Kim, Vicci, Han, & Alvarez, 2011;Lee et al., 2009;Munoz et al., 1999;Patel et al., 1999). Even though adaptation of the tonic component is well predicted by most vergence models (e.g., Hung, 1992; Schor, 1979), only one accounts for adaptation of the transient component (Patel, Ogmen, White, & Jiang, 1997). Accordingly, changes in vergence dynamics are associated with changes in tonic vergence (i.e. phoria), and the asymmetry between convergence and divergence would predict fixation disparity. These predictions have received support from recent experimental evidence (e.g., Kim et al., 2011;Patel, Jiang & Ogmen, 2001). Therefore, the increase in vergence dynamics observed in this report might have been accompanied by changes in tonic elements (i.e., phoria or fixation disparity), although we should keep in mind the potential for crosslink adaptation given the nature of our stimuli. In studies concerned with vergence adaptation to sustained vergence or to dynamic stimuli, the magnitude and direction of vergence step were systematically the same, such that vergence changes were more easily predicted. In contrast, in our study and in a previous one (Emoto, Niida & Okano, 2005), the changes in binocular parallax involve various amplitudes and include convergent as well as divergent steps. Emoto and colleagues (2005) speculated about the factors causing visual stress in stereoscopic displays. They argued that not only accommodation-vergence conflict could alter visual performance, but also the incapacity to adapt to rapid and continuous vergence changes. Using rotary prisms, they varied the vergence demand according to various convergent or divergent steps, whose amplitudes could exceed the zone of comfort, and found an overall decrease of vergence performance attributed to viewers’ fatigue. As the time to complete adaptation is reduced with smaller stereo-demand (i.e., possibly less than 1 s, Larson & Faubert (1994)), vergence adaptation could have occurred in our experiment, thereby increasing vergence dynamics. Though the effect of stereoscopic viewing on vergence has been conceived as adaptation of the tonic elements and adaptation of vergenceaccommodation crosslinks (Eadie et al., 2000; Rushton & Riddell, 1999; Wann, Rushton & Mon-Williams, 1995), the objective of this

Fig. 7. (A) represents main sequence slopes as a function of pre- and post-tests and of vergence direction (convergence vs. divergence). (B) represents main sequence slopes as a function of pre- or post-tests and of sessions of the experimental phase (Session 1 = range of small disparities vs. Session 2 = range of large disparities). Error bars denote the standard error of means and, where applicable, brackets with accompanying stars indicate significant differences (p < 0:05 and p < 0:01).


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Fig. 8. Left: mean latency of vergence movements as a function of pre- or post-tests and of sessions of the experimental phase. Right: vergence gain as a function of pre- or post-tests and of sessions of the experimental phase (Session 1 = range of small disparities vs. Session 2 = range of large disparities). Error bars denote the standard error of means and, where applicable, brackets with accompanying stars indicate significant differences (p < 0:05 and p < 0:01).

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study was mainly to document the overall response in this condition. Accordingly, the present findings reveal how the vergence system behaves and what consequences can be expected from a prolonged stereoscopic activity. An interesting point to note is the slight increase of vergence latency observed after the experimental phase with the range of disparity amplitudes near the comfort zone. This finding was unexpected, as the adaptation effect on vergence velocity was not thought to be accompanied by an indicator of visual stress (Lambooij, Fortuin, Heynderickx, & IJsselsteijn, 2009) or conflicting Q4 stimulations (Hoffman et al., 2008). Neveu et al. (2013) observed a decrease of vergence amplitude in the time course of a prolonged sinusoidal disparity change (Neveu, Philippe, Priot, Fuchs, & Roumes, 2012). Although the stimuli and task used in their study notably differ from those of our study, they used disparity amplitudes that were much larger than those of our study and this could have account for these different results. Neveu and colleagues (2013) were in favor of a fatigue explanation, considering the decreased gain of vergence as a decline in visual functions, although the participants did not report significant signs of subjective fatigue. In our study, the increase of vergence latency could thus indicate a slight decline of visual performance while an adaptation effect was observed. These oculomotor effects are interesting from an applied perspective because they can be added to the long list of objective indicators of visual fatigue/adaptation in stereoscopic viewing. These oculomotor-effects led us to consider how likely they might be produced in conventional stereo-displays. Disparity-step stimuli in the experimental phase of our study included both transient and sustained vergence activities. Conventional stereo displays would provide the motor effects observed in this study as they mainly present such stimuli. Indeed, the magnitude of binocular parallax fluctuates over time according to the scene cutchanges, depth changes of the target object and shifts in visual attention. Although rapid scene cut-changes can be observed in professional films, they are mainly expected to be fatigue-generating in movie trailers, where the vergence angle will abruptly change over time. When the vergence angle changes due to shifts in visual attention within a shot, vergence movements are under the control of the viewer, and are driven by the understanding of the narrative and other cues in the visual scene (e.g. blur can influence where we look, Huynh-Thu, Vienne & Blondé, 2013). When a cut occurs, in contrast, the new fixation point will generally occur toward the position that is nearest the screen position of the previous fixation. This might be problematic as the subsequent change in binocular parallax can relate to a substantial vergence demand, thereby increasing visual fatigue.

4. Conclusion

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The conflict between accommodation and vergence portrays a potent influence on the dynamic characteristics of vergence responses to step stimuli. We found a marked and immediate effect on the dynamics of vergence responses (i.e. the peak of velocity); the latter being slower than with natural stimuli. This finding has theoretical implications because this means that not only vergence cues but also focus cues can influence the velocity of the initial component of vergence, the later being not triggered by feedback. This result has strong implications for the design of stereoscopic programs, where the scene cut-changes are often too fast to fuse the stimulus due to the latency and time to fuse of the vergence system in stereoscopic viewing. Additionally, the conflict between accommodation and vergence proved to have an after-effect on vergence dynamics, when the disparity amplitudes do not excessively exceed the theoretical comfort zone. Thus, two types of after-effects were found: an increase of peak velocity that could ensue from adaptation allowing the oculomotor system to counteract visual fatigue (e.g., Schor, 2009) and, an increase of vergence latency that seems to be a slight decline of the visual system responding to conflicting stimuli. From an applied perspective, changes in vergence dynamics can thus be considered as objective indicators of visual fatigue/ adaptation in stereoscopic viewing.

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Vergence eye movements in patients with schizophrenia.

The relationship between fusional vergence eye movements and fixation disparity.

Dynamic properties of medial rectus motoneurons during vergence eye movements.

Vergence eye movements are not essential for stereoscopic depth.

Vergence movements in comitant strabismus.

Fast vergence eye movements are disrupted in Parkinson's disease: A video-oculography study.

The movement after-effect and eye movements.

Controlling Posture and Vergence Eye Movements in Quiet Stance: Effects of Thin Plantar Inserts.

Dynamics and efficacy of saccade-facilitated vergence eye movements in monkeys.

Smooth pursuit eye movements in children with strabismus and in children with vergence deficits.

Effect of repetition proportion on language-driven anticipatory eye movements.

Lithium effect on smooth pursuit eye movements of healthy volunteers.

Eye movements during motion after-effect.

Effects of lithium treatment on eye movements.

The effect of foveal and parafoveal masks on the eye movements of older and younger readers.

The effect of electromagnetic stimulation of the posterior parietal cortex on eye movements.

The effect of horizontal eye movements on free recall: a preregistered adversarial collaboration.

A physiological perspective on fixational eye movements.

A note on eye movements and relaxation.

Saccadic eye movements on Béla Julesz' figure.

Following eye movements on the absence of central vision.

The Influence of Monocular Spatial Cues on Vergence Eye Movements in Monocular and Binocular Viewing of 3-D and 2-D Stimuli.

The rate of change of vergence-accommodation conflict affects visual discomfort.

Hypnosis and eye movements.