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A compact equipment package for vestibular experiments during spaceflight
Acta Asrronautiuz Vol. 23, pp. 307-309, Printed in Great Britam
1991
0094-5765!91 $3.00 + 0.00 Pergamon Press plc
A COMPACT EQUIPMENT PACKAGE FOR VJZSTIBULAREXPERIMENTS DURING SPACEFLIGHT A H Clarke, W Teiwes, H Scherer Department
of Otorhinolaryngology, Steglitz Medical Center, Freie Universitat Berlin, FRG
The basic system is assembled around a multifunctional signal processing computer; a number of hard- and software modules provide options for signal conditioning, online display and storage of multichannel time series data, and control of experimental profiles.
ABSTRACT
A compact measurement and stimulus equipment package for vestibular testing is described. The package is designed on a modular concept so that a customised version can be assembled for each primarily Although experimental situation. conceived for space-related research, the equipment has also been introduced successfully into the clinical diagnostic procedure. An essential function of the equipment is the recording and evaluation of eye movements. This is performed by a video-based measurement system which permits evaluation of horizontal, vertical and torsional components of eye movement. Objective testing of the vestibulo-ocular reflex in all three orthogonal planes is therefore possible. Furthermore evaluation of the otolithic function in weightlessness is made feasible by the possibility of measuring dynamic ocular counterrolling. Some applications of the equipment are described.
Video I PCY System
Figure 1. Overview of signals recorded synchronously onto a video/PCM tape format.
This concept permits an arbitrary set of stimulus and measurement modules to be integrated into a robust package which remains compact enough to be mounted on a backpack structure for use in a predefined experimental situation, or installed as an autonomous middeck experiment during an orbital mission.
INTRODUCTION The requirements of vestibular experimentation very often demand custom-designed stimulus and measurement equipment. In particular, the requirements of comprehensive testing during spaceflight has led to extremely complex equipment, as exemplified by the vestibular helmet and its support system, which was designed for the European Spacelab experiments of 1983 and 1985. Although this equipment generally fulfilled its purpose, experimental objectives were jeopardised to some extent by its inflexibility. To avoid such problems in future applications and to exploit the advantages of more recent technology a modular system design has been employed for the development of a stimulus and measurement equipment package.
An essential component of vestibular experimental equipment remains the accurate recording of reflex eye movements. In the present configuration the standard electrooculographic method (EOG) is replaced by a video measurement system that permits higher resolution and more reliable recording of eye movements. Depending on available rack facilities and manpower, these recordings can be either stored on high resolution videotape for subsequent, postmission laboratory analysis or evaluated online and downlinked for PI supervision. This approach yields a more reliable procedure than those previous solutions which relied on realtime analysis of the video images. 307
308
A.H. CLARKEet al.
RECORDING EYE MOVEMENTS
Of essential importance in the study of the vestibular system is the observation and measurement of eye movements. Over the course of this century a variety of technical devices have been employed by vestibular researchers and clinicians for these purposes. At present, objective measurement of eye movements is dominated - on the one hand by the conventional EOG technique, - and on the the other, by the more precise search coil technique 4. Nevertheless, there remains considerable advantage in observing eye movements directly, as in the clinical situation with Frenzel lens, or by means of a video image of the eye. One approach which yields the essential advantages of both Frenzel lens examination and quantitative measurement techniques is video-ocutography (VOG); this involves the use of solid state CCD devices for acquiring a visual image of the eye, and the application of digital image processing for measuring eye position on a frame by frame basis. Thus, eye movements can be observed directly on a video monitor during examination, recorded on videotape for documentation or demonstration purposes, and furthermore, be analysed by digital image processing techniques to obtain quantitative measurement data equivalent to, or better than state-of-the-art techniques. A major advantage of this approach is that it provides the preconditions for registering all three dimensions of eye movement, i.e. not only the horizontal and vertical components but also the torsional component of eye movement. Evaluation of the torsional component promises to yield information on the canal-otolith contributions to ocular counterrolling. Static measurement of this phenomenon has already proven useful in both clinic and research as an indicator of otolith function 2.
With the present equipment, videooculographic recordings are obtained by means of a miniaturised CCD video sensor mounted in a lightweight mask. This mask can be assembled to seal out all visible light from the environment, in order to perform vestibular testing in the absence of visual fixation; an infrared source mounted concentrically with the video lens provides the necessary lighting. Alternatively, the video sensor can be mounted to permit binocular vision by employing dichroic mirrors. The eye image can be recorded onto videotape for later evaluation or fed directly to the image processing system. The first stage of image processing involves the determination of centre of pupil, which is assumed to coincide with the visual axis. One efficient solution involves thresholding of the luminance signal in order to separate the dark pupil ifrom the remainder of the image. This binarisation then permits calculation of the geometric centre of pupil by address averaging. The present sampling rate of 50 Hz renders adequate temporal resolution for pursuit eye movements (100°/s), but is, of course restricted in its usage for saccade measurement. Spatial resolution is of the order of 0.25 - 0.5 degrees. This algorithm also yields a frame-by-frame measure of pupil size and can be used alone for pupillometry. Parallel to the realtime determination of the pupil centroid, each current video frame is read into a digital frame store, so that as soon as the centroid coordinates are known, an annular profile in the iris structure can be defined and the luminance data extracted and filtered (Fig. 2). In this way, realtime operations are a priori restricted to pupil centroid measurement and extraction of iris p r o n e data. [: .......................
iv
/
×
Video image
The importance of considering the threedimensional aspects of spatial orientation and visual-vestibuiar interaction should hardly need be mentioned, but only with the recent application of search-coil techniques have the associated eye movemens become accessible to comprehensive objective measurement. This development has already enabled substantial new inroads into the understanding of the three-dimensional nature of the vestibular and ocutomotor systems 1. Nevertheless, the semi-invasive nature of the search coil technique make it unlikely that it will find widespread use in human experimentation in either the clinical situation or in the space research environment. A video-based measurement system is certainly more convenient for these applications.
.... t
]
Object-Tracker .........
|
1
l ..........
I~ddress
L .o. or-i ~1~
~
.
.
.
.
.
F, _~,~=o!,
.
Profile co.elation
a
Figure 2. Principle of operation of three-dimensional eye tracker unit. Pupil centroid (x,y vs t) and iris profile are extracted in realtime. Subsequentprocessingyieldsa measure of eye torsion (13vs t ) on a frame by framebasis. Thus, by reducing realtime operations to determination of pupil centroid and extraction of the corresponding annular profile, the digital image data stream (380kB/frame) is effectively reduced to a managable amount (3kB/frame). This relevant
8th 1AA Man in Space Symposium
subset of image data can be stored conveniently on standard mass storage devices. Subsequent offline processing permits frame-by-frame measurement of the torsional angle of the eye. This is performed by crosscorrelating each current iris profile with a zero reference profile determined during calibration. This permits a reconstruction of the dynamic torsional component of eye movement. This strategy permits interactive evaluation of the acquired data. Whilst pupil tracking can be performed accurately in realtime, a valid measurement of ocular torsion requires some degree of user intervention during image analysis ; in this manner the influence of such factors as pupil diameter, lid closure, partial loss of iris image can be taken into consideration. This provides for more valid and reliable results than would be the case with those implementations 3'5 which favour a complete, one-off, realtime solution.
309
SYSTEM INTEGRATION A variety of system implementations can be assembled from the available sensor devices, recording media and processing units. A minimal solution would include only the VOG assembly together with a small, high quality VTR unit. This is currently in use in our clinic as a portable system for the collection of transient eye movement disorders and for diagnosing bedridden patients. A flight version could be flown as middeck equipment, for use in a large variety of shortform vestibular experiments. A number of configurations have been tested during experimentation during parabolic flight and in other test facilities, the modular concept offering customisation to each scientific objective. A more comprehensive solution for employment in the space environment would include a portable PC workstation, thus providing "local intelligence" and enabling online monitoring or telescience applications via downlink communication.
AUXILIARY SIGNALS In addition to the registration of the elicited eye movements, it is usually necessary to record dynamic experimental parameters, such as head movement, accelerations etc. A PCM data encoder permits acquisition of up to eight such auxiliary signals, and their synchronous recording onto an additional track of the videotape recorder. Audiovisual recording of the experimental environment (commentary, general view of experiment) can be included on the audio channels and an additional lower quality video chatmel of the same recording system. This comprehensive data and audiovisual recording of the experiment permits near total reconstruction of the experimental situation for the subsequent analysis and evaluation in the laboratory. This is of particular importance for space experiments where experimental time is at a premium and the investigator's effort is required to be concentrated on optimal exploitation of the timeline.
S
This concept of employing a compact, autonomous system can be regarded as complementary to larger integrations such as the ESA Biorack or Anthrorack. In the future, the inclusion of advanced computer architectures should further increase the flexibility required for both standalone solutions and interfacing to communications and data transmission standards. REFERENCES
1. Cohen B, HenriV Representation of three-dimensionalspace in the vestibular, oculomotor,and visualsystems.Ann N Y Acad Sci Vo1545.(1988) 2. Diamond S G, Markham C It. Ocular Counterrolllng as an Indicator of Vestibular function. Neurology; 33; 1460-1469. (1983) 3. Hatamian M, Anderson D J. Design Considerations for a Real-Time Ocular CounterroU Instrument. IEEE Transactions on BiomedicalEngineering;Vol. BME-30;278-288.(1983) 4. Robinson D A. A Method of Measuring Eye Movements using a Scleral Search Coil in a Magnetic Field. IEEE Transaction on BiomedicalElectronics; 137-145.(1963). 5. Vi~villeT, Masse D. Ocular Counter-rolling during Active Head Tilting in Humans. Acta Otolaryngol (Stockh); 103; 280-290;(1987)
Figure 3. High quality recordingof video and data during the li~ted experimental time in space provide a comprehensive basis for postmission reexamination of the experiment and subsequent image and signal processing. In contrast to direct acquisition of final data, requiring complex realtime analysis, the above approach improves both experiment time usage and final data quality.
ACKNOWLEDGEMENT This project was supported by research grant No. 01 QV 8801 from the Bundesministerium fiir Forschung und Technologic, FRG.
1991
0094-5765!91 $3.00 + 0.00 Pergamon Press plc
A COMPACT EQUIPMENT PACKAGE FOR VJZSTIBULAREXPERIMENTS DURING SPACEFLIGHT A H Clarke, W Teiwes, H Scherer Department
of Otorhinolaryngology, Steglitz Medical Center, Freie Universitat Berlin, FRG
The basic system is assembled around a multifunctional signal processing computer; a number of hard- and software modules provide options for signal conditioning, online display and storage of multichannel time series data, and control of experimental profiles.
ABSTRACT
A compact measurement and stimulus equipment package for vestibular testing is described. The package is designed on a modular concept so that a customised version can be assembled for each primarily Although experimental situation. conceived for space-related research, the equipment has also been introduced successfully into the clinical diagnostic procedure. An essential function of the equipment is the recording and evaluation of eye movements. This is performed by a video-based measurement system which permits evaluation of horizontal, vertical and torsional components of eye movement. Objective testing of the vestibulo-ocular reflex in all three orthogonal planes is therefore possible. Furthermore evaluation of the otolithic function in weightlessness is made feasible by the possibility of measuring dynamic ocular counterrolling. Some applications of the equipment are described.
Video I PCY System
Figure 1. Overview of signals recorded synchronously onto a video/PCM tape format.
This concept permits an arbitrary set of stimulus and measurement modules to be integrated into a robust package which remains compact enough to be mounted on a backpack structure for use in a predefined experimental situation, or installed as an autonomous middeck experiment during an orbital mission.
INTRODUCTION The requirements of vestibular experimentation very often demand custom-designed stimulus and measurement equipment. In particular, the requirements of comprehensive testing during spaceflight has led to extremely complex equipment, as exemplified by the vestibular helmet and its support system, which was designed for the European Spacelab experiments of 1983 and 1985. Although this equipment generally fulfilled its purpose, experimental objectives were jeopardised to some extent by its inflexibility. To avoid such problems in future applications and to exploit the advantages of more recent technology a modular system design has been employed for the development of a stimulus and measurement equipment package.
An essential component of vestibular experimental equipment remains the accurate recording of reflex eye movements. In the present configuration the standard electrooculographic method (EOG) is replaced by a video measurement system that permits higher resolution and more reliable recording of eye movements. Depending on available rack facilities and manpower, these recordings can be either stored on high resolution videotape for subsequent, postmission laboratory analysis or evaluated online and downlinked for PI supervision. This approach yields a more reliable procedure than those previous solutions which relied on realtime analysis of the video images. 307
308
A.H. CLARKEet al.
RECORDING EYE MOVEMENTS
Of essential importance in the study of the vestibular system is the observation and measurement of eye movements. Over the course of this century a variety of technical devices have been employed by vestibular researchers and clinicians for these purposes. At present, objective measurement of eye movements is dominated - on the one hand by the conventional EOG technique, - and on the the other, by the more precise search coil technique 4. Nevertheless, there remains considerable advantage in observing eye movements directly, as in the clinical situation with Frenzel lens, or by means of a video image of the eye. One approach which yields the essential advantages of both Frenzel lens examination and quantitative measurement techniques is video-ocutography (VOG); this involves the use of solid state CCD devices for acquiring a visual image of the eye, and the application of digital image processing for measuring eye position on a frame by frame basis. Thus, eye movements can be observed directly on a video monitor during examination, recorded on videotape for documentation or demonstration purposes, and furthermore, be analysed by digital image processing techniques to obtain quantitative measurement data equivalent to, or better than state-of-the-art techniques. A major advantage of this approach is that it provides the preconditions for registering all three dimensions of eye movement, i.e. not only the horizontal and vertical components but also the torsional component of eye movement. Evaluation of the torsional component promises to yield information on the canal-otolith contributions to ocular counterrolling. Static measurement of this phenomenon has already proven useful in both clinic and research as an indicator of otolith function 2.
With the present equipment, videooculographic recordings are obtained by means of a miniaturised CCD video sensor mounted in a lightweight mask. This mask can be assembled to seal out all visible light from the environment, in order to perform vestibular testing in the absence of visual fixation; an infrared source mounted concentrically with the video lens provides the necessary lighting. Alternatively, the video sensor can be mounted to permit binocular vision by employing dichroic mirrors. The eye image can be recorded onto videotape for later evaluation or fed directly to the image processing system. The first stage of image processing involves the determination of centre of pupil, which is assumed to coincide with the visual axis. One efficient solution involves thresholding of the luminance signal in order to separate the dark pupil ifrom the remainder of the image. This binarisation then permits calculation of the geometric centre of pupil by address averaging. The present sampling rate of 50 Hz renders adequate temporal resolution for pursuit eye movements (100°/s), but is, of course restricted in its usage for saccade measurement. Spatial resolution is of the order of 0.25 - 0.5 degrees. This algorithm also yields a frame-by-frame measure of pupil size and can be used alone for pupillometry. Parallel to the realtime determination of the pupil centroid, each current video frame is read into a digital frame store, so that as soon as the centroid coordinates are known, an annular profile in the iris structure can be defined and the luminance data extracted and filtered (Fig. 2). In this way, realtime operations are a priori restricted to pupil centroid measurement and extraction of iris p r o n e data. [: .......................
iv
/
×
Video image
The importance of considering the threedimensional aspects of spatial orientation and visual-vestibuiar interaction should hardly need be mentioned, but only with the recent application of search-coil techniques have the associated eye movemens become accessible to comprehensive objective measurement. This development has already enabled substantial new inroads into the understanding of the three-dimensional nature of the vestibular and ocutomotor systems 1. Nevertheless, the semi-invasive nature of the search coil technique make it unlikely that it will find widespread use in human experimentation in either the clinical situation or in the space research environment. A video-based measurement system is certainly more convenient for these applications.
.... t
]
Object-Tracker .........
|
1
l ..........
I~ddress
L .o. or-i ~1~
~
.
.
.
.
.
F, _~,~=o!,
.
Profile co.elation
a
Figure 2. Principle of operation of three-dimensional eye tracker unit. Pupil centroid (x,y vs t) and iris profile are extracted in realtime. Subsequentprocessingyieldsa measure of eye torsion (13vs t ) on a frame by framebasis. Thus, by reducing realtime operations to determination of pupil centroid and extraction of the corresponding annular profile, the digital image data stream (380kB/frame) is effectively reduced to a managable amount (3kB/frame). This relevant
8th 1AA Man in Space Symposium
subset of image data can be stored conveniently on standard mass storage devices. Subsequent offline processing permits frame-by-frame measurement of the torsional angle of the eye. This is performed by crosscorrelating each current iris profile with a zero reference profile determined during calibration. This permits a reconstruction of the dynamic torsional component of eye movement. This strategy permits interactive evaluation of the acquired data. Whilst pupil tracking can be performed accurately in realtime, a valid measurement of ocular torsion requires some degree of user intervention during image analysis ; in this manner the influence of such factors as pupil diameter, lid closure, partial loss of iris image can be taken into consideration. This provides for more valid and reliable results than would be the case with those implementations 3'5 which favour a complete, one-off, realtime solution.
309
SYSTEM INTEGRATION A variety of system implementations can be assembled from the available sensor devices, recording media and processing units. A minimal solution would include only the VOG assembly together with a small, high quality VTR unit. This is currently in use in our clinic as a portable system for the collection of transient eye movement disorders and for diagnosing bedridden patients. A flight version could be flown as middeck equipment, for use in a large variety of shortform vestibular experiments. A number of configurations have been tested during experimentation during parabolic flight and in other test facilities, the modular concept offering customisation to each scientific objective. A more comprehensive solution for employment in the space environment would include a portable PC workstation, thus providing "local intelligence" and enabling online monitoring or telescience applications via downlink communication.
AUXILIARY SIGNALS In addition to the registration of the elicited eye movements, it is usually necessary to record dynamic experimental parameters, such as head movement, accelerations etc. A PCM data encoder permits acquisition of up to eight such auxiliary signals, and their synchronous recording onto an additional track of the videotape recorder. Audiovisual recording of the experimental environment (commentary, general view of experiment) can be included on the audio channels and an additional lower quality video chatmel of the same recording system. This comprehensive data and audiovisual recording of the experiment permits near total reconstruction of the experimental situation for the subsequent analysis and evaluation in the laboratory. This is of particular importance for space experiments where experimental time is at a premium and the investigator's effort is required to be concentrated on optimal exploitation of the timeline.
S
This concept of employing a compact, autonomous system can be regarded as complementary to larger integrations such as the ESA Biorack or Anthrorack. In the future, the inclusion of advanced computer architectures should further increase the flexibility required for both standalone solutions and interfacing to communications and data transmission standards. REFERENCES
1. Cohen B, HenriV Representation of three-dimensionalspace in the vestibular, oculomotor,and visualsystems.Ann N Y Acad Sci Vo1545.(1988) 2. Diamond S G, Markham C It. Ocular Counterrolllng as an Indicator of Vestibular function. Neurology; 33; 1460-1469. (1983) 3. Hatamian M, Anderson D J. Design Considerations for a Real-Time Ocular CounterroU Instrument. IEEE Transactions on BiomedicalEngineering;Vol. BME-30;278-288.(1983) 4. Robinson D A. A Method of Measuring Eye Movements using a Scleral Search Coil in a Magnetic Field. IEEE Transaction on BiomedicalElectronics; 137-145.(1963). 5. Vi~villeT, Masse D. Ocular Counter-rolling during Active Head Tilting in Humans. Acta Otolaryngol (Stockh); 103; 280-290;(1987)
Figure 3. High quality recordingof video and data during the li~ted experimental time in space provide a comprehensive basis for postmission reexamination of the experiment and subsequent image and signal processing. In contrast to direct acquisition of final data, requiring complex realtime analysis, the above approach improves both experiment time usage and final data quality.
ACKNOWLEDGEMENT This project was supported by research grant No. 01 QV 8801 from the Bundesministerium fiir Forschung und Technologic, FRG.