Objective: The performance of a new video-based infrared eye tracker (IR) was compared to the magnetic search coil technique (SC). Since the IR offers interesting possibilities as a diagnostic tool in neuro-ophthalmology, it was investigated whether the new device has overcome shortcomings that were reported from former IR systems. Methods: Horizontal saccades were recorded using the IR and the SC. The IR allowed eye movement recordings at different sampling rates ranging from 250Hz to 1000Hz while the SC recorded at 1000Hz. Results/Conclusions: The results show that the IR and the SC were in good agreement and produced similar results. In contrast to other studies, the influence of the sampling rate of the IR was small. The saccade main-sequences did not show significant differences. The latency times observed for both systems were mainly in the short-latency rang

Lanz, Christian

Muser, Markus

Schmitt, Kai-Uwe

Schwarz, Urs

Walz, Felix

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Technical NoteCOMPARING EYE MOVEMENTS RECORDED BYSEARCH COIL AND INFRARED EYE TRACKINGKai-Uwe Schmitt, PhD1, Markus H. Muser, PhD2,Christian Lanz, MD2, Felix Walz, MD2 andUrs Schwarz, MD3From the 1Institute for Biomedical Engineering, ETH Zurich andUniversity of Zurich, Gloriastrasse 35, 8092 Zurich, Switzerland;2AGU Zu¨rich, Winkelriedstrasse 27, 8006 Zurich, Switzerland;3Department of Neurology, University Hospital Zurich, Frauen-klinikstrasse 26, 8091 Zurich, SwitzerlandReceived 31 August 2006. Accepted for publication 17 October2006Address correspondence to Kai-Uwe Schmitt, Institute forBiomedical Engineering, ETH Zurich and University of Zurich,Gloriastrasse 35, 8092 Zurich, Switzerland.E-mail: schmitt@ethz.chSchmitt K-U, Muser MH, Lanz C, Walz F, Schwarz U. Comparing eyemovements recorded by search coil and infrared eye tracking.J Clin Monit Comput 2007ABSTRACT. Objective. The performance of a new video-basedinfrared eye tracker (IR) was compared to the magnetic search coiltechnique (SC). Since the IR offers interesting possibilities as adiagnostic tool in neuro-ophthalmology, it was investigatedwhether the new device has overcome shortcomings that werereported from former IR systems. Methods. Horizontalsaccades were recorded using the IR and the SC. The IRallowed eye movement recordings at different sampling ratesranging from 250 Hz to 1000 Hz while the SC recorded at1000 Hz. Results/Conclusions. The results show that the IRand the SC were in good agreement and produced similar results.In contrast to other studies, the influence of the sampling rate ofthe IR was small. The saccade main-sequences did not showsignificant differences. The latency times observed for bothsystems were mainly in the short-latency range.KEY WORDS. eye movements; human; magnetic search coil;saccade; video; infrared; eye tracking.INTRODUCTIONPrecise recording and detailed analysis of eye movementshas become an important diagnostic tool in neurology[1, 2]. Due to its very high spatial and temporal resolution,the magnetic search-coil technique (SC) is regarded as thegold standard for recording these movements [3, 4].However, since this method is invasive, it has significantdisadvantages. In contrast, video-based systems forrecording eye movements are non-invasive and, there-fore, are a desirable alternative to the SC.In the past, several studies have compared the twomethods [3–6]. Van der Geest and Frens [6], for instance,compared a video system (sampling rate 250 Hz) and theSC (500 Hz) using saccadic eye movements and con-cluded that the main disadvantage of the video method isits low sampling rate, which was thought to be responsiblefor noisier estimates of all parameters. Nonetheless, theyreport that the main sequence was similar for both systemsand that its peak velocity saturation level was only slightlyoverestimated by the video system. The SC, however,was suspected to inhibit viscoelastic coupling between theannulus and the cornea, i.e., the SC motion produces afiltered version of the actual eye movement with under-estimation of the true peak velocity. A similar explanationwas given by Tra¨isk et al. [7] who analysed saccades usingthe SC and video-based device both sampling at 500 Hzand found significant differences between the twomethods. A slower peak velocity was recorded for the SCJournal of Clinical Monitoring and Computing (2007) 21:49–53DOI: 10.1007/s10877-006-9057-5  Springer 2006and explained by a slippage of the SC on the surface of theeye, hence inducing a low-pass filter. Furthermore, it wassuggested that the presence of the SC alters the ocu-lomotoric command signal. The finding that the video-based results show a higher variability was explained byreduced alertness of the volunteers, i.e., a subject is morealert when wearing the uncomfortable device.This study further investigates the differences betweenthe SC and a video-based infrared eye tracker (IR). Usinga more advanced IR system with adjustable sampling ratesup to 1000 Hz, it was possible to elaborate on the fre-quency dependence in more detail. Additionally, the IRwas used to record the SC in the eye of the volunteerswhile performing saccades. As a result, it was possible toanalyse the behaviour of the SC during eye movements.METHODS (INCLUDING ETHICS ASPECT)SubjectsA total of 5 healthy subjects (3 women, 2 men, age 29–50 years, including the male author US) participated inthis study. All of them gave informed consent and hadnormal or corrected-to-normal vision. Three of the fivesubjects were familiar with wearing SCs.Eye movement recordingsEye movements were recorded with two different sys-tems:1. A typical SC set-up (SKALAR Medical, Delft, TheNetherlands) with a sampling rate of 1000 Hz of thehorizontal and vertical positions of the centre-alignedright eye and a nominal spatial resolution of 30 sec arc.The stimulus was back-projected onto a translucenttangent screen (viewing distance 1 m, subtending ±42horizontally and ±26 vertically, pixel size 0.2) usingan active-matrix LCD video projector (SHARP). Theframe rate of the display system was 70 Hz.2. A table-mounted IR prototype (OCULOMETRICS,Zurich, Switzerland) with an adjustable sampling ratebetween 250 and 1000 Hz. The stimulus was pre-sented on a 18-inch LCD monitor (Belinea 101920,1280 1024 Pixel, 60 Hz) positioned 490 mm fromthe right eye and aligned the same way as the SCsystem. Eye movement traces were evaluated bytracking both the pupil and the corneal-scleral junc-tion. Tracking was performed by first determining thecentre of the pupil using a modified circular Houghtransform, and subsequently detecting the left andright pupil edges as well as the left and right border ofthe iris using an adaptive edge detection algorithm.Because of the spherical shape of the eye, theattainable angular resolution depends on the rotationof the eye, i.e. a rotational displacement smaller than10 can be tracked with a resolution of 0.1approximately, while a larger displacement will yielda resolution of 0.2.Raw CS and IR signals were stored for off-line analysis.ParadigmA standard paradigm to elicit visually guided saccades wasused in all experiments. The stimulus (0.8 0.8, targetluminance 25 cd/m2, background luminance 0 cd/m2)was presented at 0, 5, 10, 15, and 20 eccentricityright- and leftwards with the same randomised sequencein all experiments and for all subjects. All subjects werefamiliarised with the experimental procedure before thefirst run.Each subject performed the following set of consecu-tive experiments:1. IR, sampling rate: 250 Hz2. IR, 500 Hz3. IR, 1000 Hz4. SC, 1000 Hz5. IR, coil in the eye, 500 Hz6. IR, coil removed, eye still anaesthetised, 500 HzData analysisData analysis was performed off-line using commercialsoftware (MATLAB R2006a, The MathWorks Inc.,Natick, MA) with a purpose-built toolbox (available uponrequest). Blinks and grossly abnormal eye traces wereexcluded. The saccade onset of the adaptive smoothingspline filtered raw eye position traces [8, 9] was deter-mined by a velocity and acceleration threshold. Subse-quently, latency, amplitude, duration, and peak velocityof all valid saccades as well as their main sequence weredetermined [5].RESULTSThe main sequence was computed for each subject andfor every experiment (Figure 1). Table 1 summarises theparameters.The post-saccadic positions recorded for all measure-ments (Figure 2) showed a very small standard deviation.50 Journal of Clinical Monitoring and ComputingAt 1000 Hz sampling rate, the saccade parameters ofthe main sequences obtained from the SC and the IRsystem were in agreement. In general, the saccade laten-cies recorded by the SC system were slightly longer thanthose recorded by the IR system, but the difference wasstatistically not significant (p > 0.05).Comparing the results from the IR system at samplingrates of 250 Hz, 500 Hz and 1000 Hz showed no sys-tematic difference for the saccade main-sequence param-eters and latencies (p > 0.05).The outcome of the IR recordings before and after theSC experiments (both with the SC still in place andwithout the SC) did not reveal significant differences.Furthermore, the video sequences taken with the SCshowed no movement at all of the SC relative to the eye.In line with the literature, it was generally observed thatthe subjects performed slightly faster abducting saccadesthan adducting saccades.DISCUSSIONDespite the limited number of tested subjects, the datapresented in this study is of excellent quality (see Fig-ures 1, 2). The derived parameters and their variance arewell within the known range for horizontal saccadic eyemovements (see Table 1).The comparison of the data and parameters from the IRand the SC system at 1000 Hz sampling rate showed verygood agreement without statistically significant differ-ences. This finding indicates that both methods arecapable of reliably recording horizontal saccades. Hence,the IR system exhibits a quality that allows recordings at aspatial and temporal resolution that is comparable to theSC.Furthermore, these findings are comparable with thosepublished by van der Geest and Frens [6] who comparedthe two methods at lower sampling rates. They concludedthat the main disadvantage in their study was the lowsampling rate of their video system.However, there are several disagreements with theresults of Tra¨isk et al. [7]: In our set-up, IR measured peakvelocities of 20 saccades varied on average by 10%compared to the SC measurements. Tra¨isk et al. reportedthat their video system resulted in a 31% higher maximumvelocity at 20. Moreover, we did not observe a slippageof the coil as they did. Also, we could not confirm ahigher variability of the IR parameters derived in ourstudy; and the order of the experiments (IR first, then SC)was not important either since we did not find significantdifferences between the results of the measurements takenbefore and after the SC experiment. There is no satis-factory explanation of these differences. However, someof them might be due to simple technical differences ofthe systems (e.g., head vs. table mounted video systems)rather than the recording methodology itself.Because it was hypothesised in several studies that thesampling rate of the video system might be a critical factorwith great impact on the results, we compared IR eyemovement recordings at sampling rates ranging from 250to 1000 Hz. Our results revealed only minor differences,which did not show a systematic pattern. This excludes amethodological or technical error in these recordings andmust therefore be attributed to biological variance. In fact,independence of the sampling frequency is to beexpected, since a sampling rate of 250 Hz is sufficient forrecording saccades with a velocity range as employedhere, according to the Whittaker–Shannon samplingtheorem. We therefore conclude that the range of fre-quencies used in our study is sufficient to analyse hori-zontal saccades, and a lower spatial resolution (comparedto the SC) is not a limiting factor for the video systems.Unfortunately, the methods of saccade identificationand parameter computation were not always sufficientlypresented in some other studies, although they play animportant role (e.g., filtering, threshold values, fittingtechnique). Different SC systems as well as different videosystems might have used different methods to analyse thedata, which would explain some discrepancies in otherreports. In addition, the experimental set-up could be aFig. 1. Post-saccadic positions for all subjects and all conditions: IR250 Hz (1), IR 500 Hz (2), IR 1000 Hz (3), SC 1000 Hz (C), IR500 Hz and coil in place (5), IR 500 Hz eye still anaesthetised (6). Theshaded area represents the normal position gain between 0.75 and 1.00 foreach excursion.Schmitt et al.: Recording Eye Movements 51cause for differences, in particular higher variability, inresults obtained by some other assessments [video results,6, CS results, 4]. Some factors that possibly account forthese inconsistencies are the positioning of the head, headmovements, the alignment of the optical and the geo-metrical axes as well as differences in the eye trackingalgorithms.The interpretation of the saccade latencies, which weretypically in the short-latency range, is somewhat difficultsince they are not explicitly presented in similar studies [3–6].In our view, the fact that two subjects (including one of theauthors) were used to wearing a scleral SC restricts conclu-sions byother groups with respect to its influence on alertness.We did not find systematic differences in the databefore and after wearing the SC and with the SC in place.In particular, the comparison of the measurements beforeand after wearing the SC, but with the eye still anaes-thesised, did not suggest that the behaviour was influencedby anaesthesia, which is in line with similar findings byFrens and van der Geest [10].CONCLUSIONSModern IR systems can produce results of a qualitycomparable to SC systems. Furthermore, the influence ofthe sampling frequency is smaller than suggested in otherstudies, and its importance might be overestimated. Amovement of the coil relative to the eye during saccadeswas not observed.Table 1. Summary of the saccade main-sequences and latencies from all subjects for all conditionsSubject Condition tc Conf Lim Conf r2 Latency Std1 1 video, 250 Hz 8.85 0.71 390.48 15.46 1.00 154.67 27.422 video, 500 Hz 7.54 0.56 398.56 13.22 1.00 156.29 25.593 video, 1000 Hz 5.79 0.70 379.11 16.85 0.99 160.22 25.60C MSC, 1000 Hz 7.10 0.21 396.17 4.56 1.00 182.31 29.885 video, 500 Hz 7.01 0.62 364.66 13.85 0.99 159.42 29.576 video, 500 Hz 5.13 0.42 356.05 9.44 0.99 151.65 17.802 1 video, 250 Hz 13.89 1.00 538.86 23.55 1.00 153.61 35.432 video, 500 Hz 10.79 0.74 492.70 18.09 1.00 157.67 25.673 video, 1000 Hz 9.67 1.20 479.04 29.47 1.00 168.40 23.55C MSC, 1000 Hz 11.47 1.01 517.42 24.78 0.99 186.99 25.315 video, 500 Hz 3.26 4.34 333.57 107.22 0.44 157.84 25.696 video, 500 Hz 11.37 0.80 501.70 19.52 1.00 160.47 21.753 1 video, 250 Hz 9.80 1.27 424.39 28.89 0.99 111.55 15.432 video, 500 Hz 6.77 1.12 398.83 28.19 0.98 120.20 17.273 video, 1000 Hz 5.99 1.11 427.41 30.56 0.99 118.08 10.54C MSC, 1000 Hz 7.04 0.36 384.34 8.49 1.00 147.16 18.875 video, 500 Hz 7.68 0.78 383.47 17.93 0.99 119.47 21.346 video, 500 Hz 9.80 1.88 456.53 45.48 0.98 111.94 10.984 1 video, 250 Hz 10.69 1.50 570.76 41.89 0.99 218.56 63.782 video, 500 Hz 11.00 1.18 599.64 33.23 0.99 200.14 50.183 video, 1000 Hz 10.42 1.70 604.43 49.94 0.99 176.52 46.17C MSC, 1000 Hz 9.22 0.40 517.48 10.34 1.00 222.48 56.635 video, 500 Hz 9.52 1.69 540.09 47.17 0.98 202.15 68.446 video, 500 Hz 10.22 1.10 560.51 30.23 0.99 182.14 57.325 1 video, 250 Hz 6.65 0.88 417.71 21.42 0.99 128.75 36.452 video, 500 Hz 7.07 0.42 449.89 10.70 1.00 124.94 20.593 video, 1000 Hz 7.09 1.04 459.29 25.86 0.99 137.75 20.23C MSC, 1000 Hz 6.34 0.34 383.60 7.16 0.99 165.93 24.465 video, 500 Hz 6.70 0.64 394.06 14.44 0.99 132.00 17.676 video, 500 Hz 7.14 0.39 427.30 9.53 1.00 125.64 22.29Data for right- and leftward eye movements are merged (conf represents the 95% confidence interval, r2 gives an estimate on the quality ofthe exponential fit, and std denotes for the standard deviation).52 Journal of Clinical Monitoring and ComputingThis work was financed by the Swiss Council for Traffic Safety, theUniversity Hospital Zurich and the Institute for BiomedicalEngineering.REFERENCES1. Schwarz U. Neuroophthalmology – a brief vademecum. Eur JRadiology 2004; 49: 31–63.2. Leigh RJ, Kennard C. Using saccades as a research tool in theclinical neurosciences. Brain 2004; 127: 460–477.3. Robinson DA. A method of measuring eye movements using asearch coil in a magnetic field. IEEE Trans Biomed Eng 1963;10: 137–145.4. Collewijn H, van der Mark F, Jansen TC. Precise recording ofhuman eye movements. Vision Res 1975; 15: 447–450.5. DiScenna AO, Das V, Zivotofsky AZ, Seidman SH, Leigh RJ.Evaluation of a video tracking device for measurement ofhorizontal and vertical eye rotations during locomotion.J Neurosci Methods 1995; 58(1–2): 89–94.6. van der Geest JN, Frens MA. Recording eye movements withvideo-oculography and scleral search coils: A direct comparisonof two methods. J Neurosci Methods 2002; 114(2): 185–195.7. Traisk F, Bolzani R, Ygge J. A comparison between themagnetic scleral search coil and infrared reflection methods forsaccadic eye movement analysis. Graefes Arch Clin ExpOphthalmol 2005; 243: 791–797.8. Schwarz U, Miles FA. Ocular responses to translation and theirdependence on viewing distance. I. Motion of the observer.J Neurophysiol 1991; 66: 851–864.9. Eubank RL. Spline smoothing and nonparametric regression.Statistics: Textbooks and monographs, Vol. 90. New York:Dekker, 1988.10. Frens MA, van der Geest JN. Scleral search coils influencesaccade dynamics. J Neurophysiol 2002; 88(2): 692–698.Fig. 2. Raw data, which was used to compute the main-sequences, of subject 1. All data points (D eye position vs. peak saccade velocity) for each conditionare plotted. Note the similarity (+ excursion/velocity = movement to the right, ) excursion/velocity = movement to the left).Schmitt et al.: Recording Eye Movements 53

Comparing eye movements recorded by search coil and infrared eye tracking

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Comparing eye movements recorded by search coil and infrared eye tracking

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