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The effect of saccade spike on saccadic peak velocity
Clinical Neurophysiology 122 (2011) 1476–1478
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Letters to the Editor The effect of saccade spike on saccadic peak velocity
In the October issue of Clinical Neurophysiology, Lappe-Osthege et al. (2010) present the inaccuracy of saccade velocity measurement from the EOG signal. The reason for this is the spike artefact, which is also readily visible in their illustration (Fig. 1). This artefact has been known for decades. Papers on this topic have also been published in this journal, the latest being by Shan et al. (1995). We refer to it and other references for further references and history. The origin of this spike potential has been suggested to be facial musculature or extraocular muscles. However, it is still present in facial palsy and disappears in sixth nerve palsy, so the most likely source is extraocular muscles. It could be due to similar synchronization of motor units as in the case of M-wave in motor nerve conduction velocities. The fact that the spike has usually at least two phases also fits this model. The saccade spike is useful in the analysis of EEG and sleep recordings as it often helps to identify small saccadic eye movements such as corrective saccades, low amplitude nystagmus, or saccadic eye movements of REM-sleep (Jäntti et al., 1983). However, when velocity of saccade is measured, the spike can make this impossible. The maximum velocity, as illustrated by Lappe-Osthege et al., reflects the saccade spike, not the saccadic eye movement. By filtering, the spike can be removed and error can be decreased, but even with efficient low-pass filtering the spike is included in the signal and accurate velocity measurement of small saccades is impossible (Jäntti, 1982). With larger saccades, 20° and over, the problem is smaller, as the duration of spike is much shorter than the saccade. Furthermore, with drowsiness or in disease, when the saccades slow down, the spike decreases in amplitude. It is, however, important to notice, that any measure of eye position and velocity is only a relatively rough estimate due to the mechanic and other properties of the eye and its surroundings. This is particularly true for EOG, as, for instance, blinks sometimes synchronize with horizontal saccades so that the volume conductor for the EOG signals changes its shape. This is most obvious for vertical saccades, where the change in lid position and shape often causes curious waveforms of the recorded signal during saccade, and no meaningful saccade velocity therefore can be recorded with EOG. The lid is effectively an electrode on cornea, which conducts the current from the positive surface of the cornea to the surrounding tissues of face and orbit. As there is no way of specifically accurately removing the saccade spike from the EOG signal, the best we can do is to find electrode locations – closer or farther from outer canthus – so that the signal to noise ratio of saccade is optimal. Yet, we can never reach the accuracy of, for instance, search coil technique in eye movement recording, and for accurate measurement of vertical or oblique saccades EOG is useless. Search coil and video recordings, on the other hand, can record with high accuracy saccades in any direction, even rotational. For EOG recordings of horizontal saccade
parameters we must always remember the biophysical limitations of changing volume conductor shape, which can individually vary considerably. Taking all this into account it is actually surprising that we in many cases can measure the velocity of horizontal saccades with EOG relatively accurately. In conclusion, the saccade spike is an artefact which sometimes makes velocity measurement of small saccades virtually impossible, but on the other hand is of help in detecting small amplitude saccadic eye movements in EEG recordings. References Jäntti V. Spike artefact associated with fast eye movements in electronystagmography and its importance in the automatic analysis of saccades. ORL J Otorhinolaryngol Relat Spec 1982;44:216–25. Jäntti V, Aantaa E, Lang H, Schalén L, Pyykkö I. The Saccade spike. Adv Otorhinolaryngol 1983;30:71–5. Lappe-Osthege M, Talamo S, Helmchen C, Sprenger A. Overestimation of saccadic peak velocity recorded by electro-oculography compared to video-oculography and scleral search coil. Clin Neurophysiol 2010;121:1786–7. Shan Y, Moster ML, Roemer RA. The effects of time point alignment on the amplitude of averaged orbital presaccadic spike potential (SP). Electroencephalogr Clin Neurophysiol 1995;95:475–7.
Ville Jäntti Hannu Heikkilä Antti Kulkas Department of Clinical Neurophysiology, Seinäjoki Central Hospital, Hanneksenrinne 7, 60220 Seinäjoki, Finland Tel.: +358 40 75 85 401; fax: +358 6 415 4037. E-mail address: Ville.Jantti@epshp.fi (V. Jäntti) Available online 28 January 2011 1 Febraury 1388-2457/$36.00 Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2010.10.051
Spike artefact of extraocular eye muscles does not account for increased saccadic peak velocity in EOG recordings
We recently showed that saccade velocity is consistently higher in eye movement recordings obtained by electro-oculography (EOG) when compared with scleral search coil (SSC) or video-oculography (VOG). Jäntti et al. in this issue (Jäntti et al., 2011) commented that spike artefacts of extraocular eye muscles might be the source of increased peak velocity (Shan et al., 1995). The authors suggested that peak saccade velocity will decrease once spike artefacts are removed. Furthermore, they argued that lid movements may affect the EOG signal and hence saccade velocity. We thank Jäntti et al. for their comments. However, we will provide some lines of evidence why we do not think that spike artefacts of extraocular eye muscles
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Letters to the Editor The effect of saccade spike on saccadic peak velocity
In the October issue of Clinical Neurophysiology, Lappe-Osthege et al. (2010) present the inaccuracy of saccade velocity measurement from the EOG signal. The reason for this is the spike artefact, which is also readily visible in their illustration (Fig. 1). This artefact has been known for decades. Papers on this topic have also been published in this journal, the latest being by Shan et al. (1995). We refer to it and other references for further references and history. The origin of this spike potential has been suggested to be facial musculature or extraocular muscles. However, it is still present in facial palsy and disappears in sixth nerve palsy, so the most likely source is extraocular muscles. It could be due to similar synchronization of motor units as in the case of M-wave in motor nerve conduction velocities. The fact that the spike has usually at least two phases also fits this model. The saccade spike is useful in the analysis of EEG and sleep recordings as it often helps to identify small saccadic eye movements such as corrective saccades, low amplitude nystagmus, or saccadic eye movements of REM-sleep (Jäntti et al., 1983). However, when velocity of saccade is measured, the spike can make this impossible. The maximum velocity, as illustrated by Lappe-Osthege et al., reflects the saccade spike, not the saccadic eye movement. By filtering, the spike can be removed and error can be decreased, but even with efficient low-pass filtering the spike is included in the signal and accurate velocity measurement of small saccades is impossible (Jäntti, 1982). With larger saccades, 20° and over, the problem is smaller, as the duration of spike is much shorter than the saccade. Furthermore, with drowsiness or in disease, when the saccades slow down, the spike decreases in amplitude. It is, however, important to notice, that any measure of eye position and velocity is only a relatively rough estimate due to the mechanic and other properties of the eye and its surroundings. This is particularly true for EOG, as, for instance, blinks sometimes synchronize with horizontal saccades so that the volume conductor for the EOG signals changes its shape. This is most obvious for vertical saccades, where the change in lid position and shape often causes curious waveforms of the recorded signal during saccade, and no meaningful saccade velocity therefore can be recorded with EOG. The lid is effectively an electrode on cornea, which conducts the current from the positive surface of the cornea to the surrounding tissues of face and orbit. As there is no way of specifically accurately removing the saccade spike from the EOG signal, the best we can do is to find electrode locations – closer or farther from outer canthus – so that the signal to noise ratio of saccade is optimal. Yet, we can never reach the accuracy of, for instance, search coil technique in eye movement recording, and for accurate measurement of vertical or oblique saccades EOG is useless. Search coil and video recordings, on the other hand, can record with high accuracy saccades in any direction, even rotational. For EOG recordings of horizontal saccade
parameters we must always remember the biophysical limitations of changing volume conductor shape, which can individually vary considerably. Taking all this into account it is actually surprising that we in many cases can measure the velocity of horizontal saccades with EOG relatively accurately. In conclusion, the saccade spike is an artefact which sometimes makes velocity measurement of small saccades virtually impossible, but on the other hand is of help in detecting small amplitude saccadic eye movements in EEG recordings. References Jäntti V. Spike artefact associated with fast eye movements in electronystagmography and its importance in the automatic analysis of saccades. ORL J Otorhinolaryngol Relat Spec 1982;44:216–25. Jäntti V, Aantaa E, Lang H, Schalén L, Pyykkö I. The Saccade spike. Adv Otorhinolaryngol 1983;30:71–5. Lappe-Osthege M, Talamo S, Helmchen C, Sprenger A. Overestimation of saccadic peak velocity recorded by electro-oculography compared to video-oculography and scleral search coil. Clin Neurophysiol 2010;121:1786–7. Shan Y, Moster ML, Roemer RA. The effects of time point alignment on the amplitude of averaged orbital presaccadic spike potential (SP). Electroencephalogr Clin Neurophysiol 1995;95:475–7.
Ville Jäntti Hannu Heikkilä Antti Kulkas Department of Clinical Neurophysiology, Seinäjoki Central Hospital, Hanneksenrinne 7, 60220 Seinäjoki, Finland Tel.: +358 40 75 85 401; fax: +358 6 415 4037. E-mail address: Ville.Jantti@epshp.fi (V. Jäntti) Available online 28 January 2011 1 Febraury 1388-2457/$36.00 Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2010.10.051
Spike artefact of extraocular eye muscles does not account for increased saccadic peak velocity in EOG recordings
We recently showed that saccade velocity is consistently higher in eye movement recordings obtained by electro-oculography (EOG) when compared with scleral search coil (SSC) or video-oculography (VOG). Jäntti et al. in this issue (Jäntti et al., 2011) commented that spike artefacts of extraocular eye muscles might be the source of increased peak velocity (Shan et al., 1995). The authors suggested that peak saccade velocity will decrease once spike artefacts are removed. Furthermore, they argued that lid movements may affect the EOG signal and hence saccade velocity. We thank Jäntti et al. for their comments. However, we will provide some lines of evidence why we do not think that spike artefacts of extraocular eye muscles