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1Department of Neurophysics, Philipps-University Marburg, Marburg, Germany; and 2Center for Molecular and Behavioral Neuroscience, Rutgers University, Newark, New Jersey
Submitted 7 January 2008; accepted in final form 20 February 2008
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ABSTRACT |
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INTRODUCTION |
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; Rotman et al. 2004; van Beers et al. 2001). During voluntary saccades localization errors follow a characteristic spatiotemporal pattern, which heavily depends on experimental conditions (Ross et al. 2001; Schlag and Schlag-Rey 2002). In complete darkness, transient stimuli are mislocalized in the direction of the eye movement from about 100 ms before saccade onset (shift) (Cai et al. 1997; Honda 1989). The maximum shift is observed around saccade onset. Mislocalization is then inverted and stimuli are perceived as being shifted opposite to the saccade direction for 
100 ms. In contrast, when visual references are available, the mislocalization strongly depends on the position of the target relative to the saccade goal and all stimuli are shifted toward the landing point of the eye, resulting in a perceptual compression of visual space (Kaiser and Lappe 2004; Lappe et al. 2000; Ross et al. 1997).
Mislocalization is commonly interpreted as a temporary mismatch between the actual eye position and eye-position signals in the brain (Dassonville et al. 1992; Honda 1991). Given this context it is of interest to determine how different eye movements, with very similar kinematics, affect localization. Recently, two studies investigated mislocalization of transient visual stimuli during optokinetic nystagmus (OKN) (Kaminiarz et al. 2007; Tozzi et al. 2007). OKN is a reflexive eye movement evoked by large-field moving patterns. OKN consists of two alternating phases: a slow phase in the direction of the stimulus motion and a fast phase opposite to the stimulus motion. Stimuli presented during OKN slow phase were found to be mislocalized in the direction of the eye movement. However, contrary to smooth pursuit, the size of the error did not depend on the position of the target relative to the fovea. The error pattern observed during the OKN fast phase resembled the one described for voluntary saccades in darkness (perisaccadic shift). The biphasic mislocalization pattern during OKN, however, occurred earlier with respect to fast-phase onset.
In this study we continue our investigation of mislocalization during reflexive eye movements. Most important, we sought to address the issue that during OKN a moving textured background is permanently visible. This background in itself might contribute to the observed localization errors. Hence, to show that perceptual errors occur in the complete absence of visual stimulation, we tested localization during optokinetic afternystagmus (OKAN), which is an alternation of slow and fast phases observed in total darkness in subjects who previously performed prolonged OKN.
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METHODS |
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Nine subjects participated in the experiments. Six were naive as to the purpose of the experiment. All subjects had normal or corrected-to-normal visual acuity and gave informed written consent. All procedures used in this study conformed to the Declaration of Helsinki.
Stimulus presentation and eye-movement recordings
Experiments were carried out in a completely dark experimental room to avoid visual references that otherwise could 1) prevent OKAN and/or 2) influence visual localization. Computer-generated stimuli were projected onto a large tangent screen using a CRT projector (Marquee 8000, Electrohome) running at a spatial resolution of 1,152 x 864 pixels and a frame rate of 100 Hz. The screen was viewed binocularly at a distance of 114 cm and subtended 70 x 55° of visual angle. During the experiments each subject's head was supported by a chin rest. Eye position was sampled at 500 Hz using an infrared eye tracker (Eye Link 2, SR Research). The system was calibrated prior to each session using a 9 (3 x 3) point calibration grid. During sessions drift correction was performed before each trial. Recording sessions lasted between 5 and 10 min, depending on experiment and subject. Eye movement and behavioral data were stored on hard disk for off-line analysis.
Visual stimuli
To induce OKN/OKAN we presented a random-dot pattern (RDP) consisting of black dots (size: 2.0°, luminance <0.1 cd/m2, number of visible dots: 250) moving left- or rightward on the screen. All dots moved coherently and a new RDP was generated for each trial. The visual localization target [white circle, 0.5° (OKAN) or 1.0° (OKN) in diameter, luminance 22.5 cd/m2] was flashed for 10 ms at one of three positions (x = –8°, 0°, +8°) on the horizontal meridian. In all experiments different target positions were displayed with equal probability in pseudorandom order. To determine the perceived position of the target a horizontal ruler was displayed on a gray background (luminance 12.5 cd/m2) at the end of each trial (see also Kaminiarz et al. 2007). The ruler's tick-mark positions were equally spaced, but random numbers were assigned to the tick marks for each trial to prevent subjects from developing stereotypical response strategies due to the limited number of targets. Subjects reported the perceived position of the target by entering the number of the tick mark closest to the target flash.
Baseline trials
In OKAN baseline trials, subjects freely viewed a white (luminance 22.5 cd/m2) screen for 3,000 ms. Thereafter the screen turned black for another 3,000 ms. The target was flashed 2,500 ms after the luminance change (Fig. 1B). This background luminance change mimicked the change that occurred in the actual OKAN trials (see following text). Here and in all other cases, the ruler was displayed 490 ms after target presentation and the trial ended once the subject entered the perceived position on the keyboard. In OKN baseline trials, subjects freely viewed a homogeneous gray (luminance 12.5 cd/m2) screen for 4,000 ms. The target was presented after 3,500 ms (Fig. 1D). Each baseline session consisted of 30 trials.
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In the OKN condition, the RDP moved across a gray background (luminance 12.5 cd/m2) for 4,000 ms. The target was flashed after 3,500 ms (Fig. 1C). The RDP velocity in the OKN condition was set individually for each subject such that the amplitudes of the fast phases during OKN and OKAN matched as closely as possible. Thirty trials were recorded per session.
OKAN trials
In the OKAN condition the RDP moved on a white background (luminance 22.1 cd/m2) at a speed of 80°/s. After 15 s of stimulus motion the screen turned completely dark. After 2,500 to 4,500 ms (depending on subject and session) in darkness, the target was flashed. The ruler was displayed 490 ms later (Fig. 1A). A single session consisted of 15 trials.
Data analysis
Data were analyzed using Matlab 7.3.0 (The MathWorks) and SigmaStat 3.10 (Systat Software). Eye-position data for all OKN/OKAN trials were inspected off-line. Trials were excluded from further analysis if 1) subjects had not performed any systematic OKN/OKAN, 2) the fast phase closest to the target flash did not match the previously defined velocity/acceleration criterion, or if 3) the fast phase closest to the flash was in the same direction as the slow eye movement and was initiated <100 ms after the target flash. Due to these criteria 38% of all OKAN and 21% of all OKN trials were excluded from further analysis.
For the remaining valid trials, we determined as a first step the error in the free-viewing condition (baseline error). Then we computed the errors during OKN/OKAN slow phases. For this analysis only trials in which no fast phase was initiated in a 200-ms time window centered on the onset of the flash were considered. Net errors were estimated by subtracting baseline errors from OKN/OKAN slow-phase errors.
To determine the dynamics of the localization error around the fast phases of the OKN/OKAN we first identified the fast phase closest (in time) to the flash. Then we determined the (baseline-corrected) localization error as a function of the time of the flash relative to the onset of the fast phase and computed a moving average for this data set. The moving average was smoothed with a Gaussian-shaped weighing function (
= 7 ms). Data were recorded until data from 150 valid trials in the relevant time window were available for each subject.
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RESULTS |
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During OKN trials (leftward pattern motion only) fast-phase frequency averaged across subjects was 2.41 (SD 0.34) Hz with fast phases having a mean horizontal amplitude of 5.3 (SD 1.2) deg. The slow-phase gain (gain = [eye velocity/stimulus velocity]) was 0.89 (SD 0.04) at an average stimulus velocity of 14.54 (SD 1.50) deg/s. Mean preflash slow-phase velocity (determined in the last 50 ms before flash onset) was 12.85 (SD 1.15) deg/s, whereas the average eye position at flash onset was 5.33 (SD 1.73) deg. The analysis was based on 8,545 fast phases in 930 trials.
During leftward/rightward OKAN trials mean fast-phase frequency during optokinetic stimulation (80°/s) was 3.07/3.17 (SD 0.5/0.49) Hz with an average horizontal fast-phase amplitude of 13.7/14.4 (SD 2.8/2.9) deg. Mean slow-phase gain was 0.66/0.72 (SD 0.06/0.04). During OKAN the fast-phase frequency and the horizontal fast-phase amplitude dropped to 1.5/0.97 (SD 0.23/0.33) Hz and 3.32/4.8 (SD 0.97/1.96) deg, respectively. Average preflash slow-phase velocity was –4.07/4.58 (SD 0.82/2.16) deg/s and the average horizontal eye position at flash onset was –2.71/4.46 (SD 2.41/3.36) deg. The analysis was based on 101,987 and 5,977 fast phases during OKN and OKAN, respectively, performed during 1,388 valid trials.
To summarize, we achieved our goal to match the fast-phase amplitudes during OKN and OKAN; they were within 1SD from each other. We could not, however, simultaneously match the slow-phase velocities; they were slower during OKAN than during OKN.
Localization during OKAN slow phase
Figure 2 (left column) shows the results of the first experiment in head-centered coordinates. During free viewing in darkness (Fig. 2, top left) perception was not veridical. Instead, we observed a heterogeneous pattern of misperceptions. Three subjects (1, 2, and 8) showed an outward bias (centrifugal shift), whereas two subjects (5 and 9) showed an inward bias (centripetal shift). The remaining subjects showed a tendency for an overall shift either to the left (subjects 4, 6, and 7) or to the right (subject 3). Across subjects (mean) we found no consistent bias in any direction.
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). These results show that there is a large degree of intersubject variability. To confirm this finding, we repeated the experiment with rightward OKAN for seven of nine subjects (Fig. 3). Again, we found no bias in any particular direction when data were averaged across all subjects. Visual comparison of the baseline-corrected findings for leftward and rightward OKAN, however, showed that subjects were consistent in their mislocalization. For instance, subject 7 mislocalized against the direction of the slow-phase eye movement for both leftward and rightward OKAN. Similarly, subject 3 showed a clear centrifugal effect irrespective of whether the OKAN slow phase was leftward or rightward.
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Localization during OKN slow phase
The kinematics of the eye movements during OKAN are quite similar to those during OKN; thus it is instructive to directly compare mislocalization during OKAN and OKN. Our previous OKN study used small-field (monitor size: circular aperture with 25° diameter) rather than the large-field visual stimulation (screen size: 70 x 55°) of the current study. To exclude this factor as a possible cause for any differences, we repeated some of the OKN experiments in the large-field setup.
The results for localization during OKN are shown in Fig. 2, right column. In the control condition (top right) we observed for all but one subject (8) a centripetal shift of perceived target locations. This confirms our previous findings and matches errors of mislocalization found in the absence of eye movements (Müsseler et al. 1999) but is clearly different from our findings for the OKAN baseline in total darkness where we observed a much more heterogeneous error pattern (top left). Mislocalization during OKN slow phase, however, closely matched results from our previous study; position perception during the OKN slow phase was biased in the direction of the eye movement (right middle and bottom). This error was independent of flash position (P = 0.71, ANOVA on ranks).
Error as a function of retinal eccentricity
We determined the effect of retinal eccentricity on localization error for OKAN and large-field OKN as well as during free viewing in darkness and in light. To do so we calculated localization errors as a function of retinal stimulus eccentricity independently for each subject and performed linear regressions for all three stimulus positions (see Fig. 4 for data from a single subject). In Fig. 5 regression lines for all single subjects (thin lines) as well the population mean (thick lines) are shown. Extending our earlier findings for both free viewing in light (top right) and OKN (middle right), respectively, mislocalization did not depend on retinal eccentricity as indicated by the flat regression curves. However, the localization errors during both free viewing in darkness (top left) and slow-phase OKAN (middle left) depended strongly on the retinal eccentricity of the flashed target. To further analyze this behavioral difference we performed for each individual subject an eccentricity-dependent baseline correction by subtracting the baseline fits from the corresponding OKAN fits (bottom left). The data clearly show that on average when a target is flashed on the lagging side of the retina (i.e., on the right when the eye moves to the left and vice versa), it is mislocalized in a direction opposite that of the eye movement. When a target is flashed on the leading side of the retina, however, it is mislocalized in the direction of the eye movement. The OKAN data were obtained during leftward slow phases, but the same effect was found for rightward slow phases (not shown). Hence, we can rephrase this finding as showing a horizontal expansion of visual space in retinal coordinates during horizontal OKAN. The linear regressions of the single-subject data provide us with a quantification of this expansion. First, we can determine the focus of the expansion by calculating the eccentricity for which the mislocalization is zero. When averaged across all subjects and all stimulus positions, the focus was found to lie at x = 0.2°. Across subjects, this focus of expansion ranged from x = –1.6 ° to x = 0.76°. Second, the slope of the regressions quantifies the foveofugal mislocalization error per degree of retinal eccentricity. Averaged across subjects the mislocalization increased by 0.12°/degree eccentricity. This measure was somewhat more variable across subjects (range: [0.03, 0.26]). Importantly, the correlation between eccentricity and mislocalization was positive and significant (P < 0.01; Spearman rank order) for all but one subject. These data confirm that horizontal OKAN slow phases consistently lead to a horizontal expansion of visual space in retinal coordinates.
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This perceptual expansion of visual space was found only during OKAN. The baseline-corrected OKN data (Fig. 2, bottom right) clearly show no such effect of retinal stimulus eccentricity on visual localization.
Localization during OKAN/OKN fast phase
To analyze the dynamics of the perceptual error in the temporal vicinity of the fast phases we computed the perceived stimulus position as a function of time between flash onset and the initiation of the temporally closest fast phase. To increase our data yield per time window, we merged data from all subjects and flash positions and calculated a moving average across these data points. The solid lines in Fig. 6 show the results for OKAN (top) and OKN (bottom), respectively. Dashed lines depict the underlying average eye-position traces (trials in which the time interval between flash onset and fast-phase onset was >100 ms were not considered for calculating the average eye-position trace). Although during OKN targets were on average mislocalized 4° in the direction of the slow-phase eye movement (thin straight horizontal line), no such shift was observed during OKAN. This mirrors the findings shown in Figs. 2 and 5. For both eye movements the observed bias depended on the time interval between stimulus presentation and the onset of the fast phase.
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During OKN we observed a mislocalization in the direction of the fast phase before its onset and mislocalization in the opposite direction about 50 ms after fast-phase onset. This fast-phase effect was superimposed on the general shift in the direction of the slow-phase eye movement. Contrary to the OKAN, however, the error in direction of the fast phase peaked 37 ms before fast-phase onset.
Error patterns during both OKAN and OKN fast phases were independent of target position; i.e., we did not observe any evidence for a compression of space around the fast phase (data not shown). As can be inferred from the eye-position traces the average amplitude of the saccade closest (in time) to the flash was nearly identical for both types of eye movements [OKAN: 3.7° (SD 2.7); OKN: 3.5 (SD 2.2)]. Statistical analysis comparing mean fast-phase amplitudes across subjects revealed no significant difference (P = 0.69, Mann–Whitney rank-sum test). Surprisingly, however, the amplitude of the biphasic modulation in perceived position induced by the fast phase was considerably larger during OKAN (3.8°) than during OKN (2.15°).
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DISCUSSION |
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In this discussion we first compare our OKAN findings to those previously reported on other fast and slow eye movements. Second, we discuss the role of visual references in localization. Third, we discuss the claim that a combination of erroneous eye position signals and veridical retinal signals could underlie these phenomena.
Mislocalization around slow eye movements
OKN, OKAN, and smooth pursuit share similar phases of slow eye movements. Mislocalization during these eye movements, however, is quite different. During smooth pursuit and OKN slow phase, but not OKAN slow phase, stimuli are mislocalized in the direction of the eye movement. During pursuit and OKAN the error pattern depends on the retinal position of the flash (Mateeff et al. 1982; Mitrani and Dimitrov 1982; Rotman et al. 2004; van Beers et al. 2001), whereas the error is independent of retinal position during OKN (Kaminiarz et al. 2007). This clearly shows that the kinematics of the eye movements alone are not sufficient to account for the observed mislocalizations.
There are two clear differences that may in principle account for the disparate localization errors. First, the visual scene is quite different during OKN, smooth pursuit, and OKAN and these visual factors may contribute to mislocalization in various ways (see following text for further discussion). Second, the eye-movement control networks involved in OKN, OKAN, and pursuit are distinct and thus they may interact in different ways with the visual system. To be specific, smooth pursuit and OKN are accompanied by neural activity within identical cortical areas or networks (Bremmer et al. 2002; Dieterich and Brandt 2000; Konen et al. 2005; Schlack et al. 2003), but with stronger activation during smooth pursuit (Konen et al. 2005). OKAN, on the other hand, is driven by the so-called velocity storage mechanism whose neuronal substrate is located in the vestibular nucleus (Leigh and Zee 2006; Waespe and Henn 1977). Although we are not aware of any functional MRI study investigating human brain activity during OKAN, studies in the macaque suggest that OKAN is not accompanied by specific cortical activity at all (Ilg 1997). Furthermore, the subcortical nucleus of the optic tract/dorsal terminal nucleus is active during OKN but not during OKAN (Ilg and Hoffmann 1996). This raises the possibility that the involvement or absence of cortical as well as specific subcortical processing may contribute to the observed perceptual differences during slow eye movements.
Mislocalization around fast eye movements
A temporally biphasic mislocalization has been found during voluntary saccades (Dassonville et al. 1992; Honda 1991), fast-phase OKN (Kaminiarz et al. 2007; Tozzi et al. 2007), and now also fast-phase OKAN. In the spatial domain, these biphasic mislocalizations are quite similar, although a direct comparison across all three fast eye movements is difficult given that the sizes of the fast movements in the different studies were not matched. In our study, however, the OKN fast phases matched the OKAN fast phases and we nevertheless found a much larger spatial modulation during OKAN. This is in line with reports showing that localization errors during voluntary saccades increase when fewer visual references are available (Dassonville et al. 1995; Honda 1999). In other words, the relatively large effects found during OKAN, compared with OKN, may be due to the complete absence of visual references. Interestingly, the errors during OKAN are not only larger than those during OKN but are also larger than those found during voluntary saccades in darkness. During saccades errors are in the range of 50% of the saccadic amplitude. During OKAN the error is about 100% of the fast-phase amplitude. Analogous to the preceding line of arguments this could be due to the total absence of visual references. In saccade experiments two visual references are available for the subjects: the initial fixation point and the saccade target. Even if both are not present at the time of the flash they allow subjects to build up an internal representation of the environment. During OKAN, on the other hand, subjects performed eye movements without visual goal or feedback for 
2,500 ms, which should severely constrain the buildup of a representation of the environment. Summarizing, this line of arguments suggests that perceptual bias increases when the internal visual representation of the environment is poor.
In the temporal domain, the biphasic mislocalization differs considerably across eye movements. For visually guided saccades, the peak error generally occurs at saccade onset (Honda 1991), whereas during OKN it occurs about 40 ms before fast-phase onset and even earlier during OKAN.
Localization and visual references in the absence of OKAN/OKN
As noted earlier, differences in visual input could be an important factor affecting mislocalization. This has been suggested before (Lappe et al. 2000) and our current data provide further evidence in favor of this view. Not only do we find very different patterns of mislocalization during OKAN (no visual references) and OKN (with some visual references), our free-viewing baseline trials support a similar view. We tested the same subjects during free gaze in light and in darkness and found a systematic centripetal (inward) bias in light but a centrifugal (outward) bias in complete darkness. This may also explain why previous studies reported disparate results concerning localization of targets during fixation. Some reported an overall centripetal bias (Kaminiarz et al. 2007; Mateeff and Gourevich 1983; Müsseler et al. 1999), whereas others reported a centrifugal bias (Honda 1989; Königs et al. 2007). Our data suggest that the details of the visual references are critical in those experiments and may explain some of the observed discrepancies.
Neural basis of visual mislocalization
It has been argued that localization errors during smooth eye movements could be due to sluggish or delayed eye-position signals that combine with veridical retinal signals to determine the (world) position of the flashed stimulus (Schlag and Schlag-Rey 2002). However, as discussed earlier, different patterns of mislocalization are observed during fast and slow eye movements with very similar kinematics. To explain all mislocalizations with the same mismatch between eye-position signals and veridical retinal signals, one would have to assume that the eye-position signals generated by these three eye movements are very different. For smooth pursuit mislocalization, the eye-position signal should be sluggish, but the sluggishness/delay should vary with retinal eccentricity; for slow-phase OKN, the eye-position signal should be sluggish throughout the visual field; and for slow-phase OKAN, the eye-position signal should be veridical foveally, whereas it should lead on the leading side of the retina and lag on the lagging side of the retina. For fast phases, these eye-position signals should then be modulated appropriately to account for the spatiotemporal mislocalization around fast eye movements.
We cannot exclude that such complex eye-position signals exist, but note that there is no evidence beyond that gathered in mislocalization experiments that supports their existence. Accordingly, based on our current results, we believe that mislocalization is not caused by the algebraic summation of a veridical retinal signal with an erroneous eye-position signal. Instead, these findings support the view that the underlying retinal signals are distorted. When distorted retinal position information is combined with (veridical or otherwise) eye-position signals, perceptual mislocalization in head-centered coordinates occurs. Explicit support for errors in eye-centered neural position signals comes from recordings in the middle temporal (MT) and medial superior temporal (MST) areas of the macaque. Neurons in MT and MST encode position information, although this information is distorted around saccades in a manner that mimics the perisaccadic compression of space (Krekelberg et al. 2003). The fact that a neural correlate of space compression is already found in an area encoding eye-centered position information suggests that the effect is not caused by a combination of retinal and eye-position information. Our current finding—that mislocalization during slow-phase OKAN is best understood in retinal rather than in head-centered coordinates—also suggests that it arises from distortions of the representation in early visual areas, with a retinocentric encoding of space. These changes of the early visual representation may be related to mechanisms whose aim is to hide retinal motion that is caused by the eye movements themselves (Kleiser et al. 2004). Such an early, purely visual basis for mislocalization would also be consistent with the fact that the details of the visual scene have such a great influence on mislocalization.
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GRANTS |
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FOOTNOTES |
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Address for reprint requests and other correspondence: A. Kaminiarz, Department of Neurophysics, Philipps-University Marburg, Renthof 7, D-35032 Marburg, Germany (E-mail: andre.kaminiarz{at}physik.uni-marburg.de)
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ABSTRACT
INTRODUCTION
