Al. APPENDIX: INVENTORY OF INSTRUMENTAL PROBLEMS
D. Stability of the glottograms
The amplitude of the EGG is strongly dependent on the positions of the electrodes.
However, by adjusting the length of the path the high-frequency current has to travel through the tissues by changing the positions of the electrodes, it is easy to adjust the amplitude of the EGG until an acceptable level is obtained and saturation of the amplifiers does not occur.
Once the electrodes have been positioned correctly, the EGG waveforms are quite stable.
Only when the electrodes are not fixed properly, distortions during the recordings may arise.
It is our experience that once the electrodes have been brought in place, repositioning during the recordings is seldomly required. Nevertheless, to ascertain reliable waveforms during the entire experiment, a continuous visual check on an oscilloscope remains necessary.
The PGG on the other hand is probably the most instable signal in our experiment.
First, the way in which the perspex rod is pressed against the throat of the subject determines the luminicance in the trachea. Second, the amount of light which is picked up by the photocell depends on the direction it is "looking" in. Movements of the sensor are likely to yield fluctuations in amplitude (both of ac and dc components) which may give rise to very
odd waveform distortions. Third, there is the problem that the view of the photocell may be blocked by the epiglottis. This often requires a repositioning of the photocell when an other type of vowel is articulated. Positioning the photocell (and keeping the position right) is done by the subject himself by manipulating the tube with the photo sensor. This requires a skill of the subject which can only be acquired after considerable training.
A technique which relies much less on the ability of the subject to keep the light beam and the photocell aligned is to attach a flexible light guide to the subject's throat. An even more attractive method uses a flexible endoscope inserted via the nose. The light which is used to illuminate the image area that can be observed at the proximal end of the endoscope, can also be picked up by a photo sensor attached to the outside of the troat of the subject below the glottis. By connecting the endoscope to a video-camera and displaying the image on a video-monitor, visual feedback about the measurement situation can be obtained. Also, if this information would be recorded on a videorecorder and somehow synchronized with the recorded waveforms, valuable mformation could be preserved for interpretation of expected and/or unexpected phenonena in the recorded PGG waveforms.
Unfortunately, we were not in the position to use such luxureous equipment; thus, we tried to optimize our PGG measurements by experimenting with the way the photocell was mounted in the tube that serves to guide the electrical wires and that provides the necessary mechanical stiffness needed for manipulating the position of the photo sensor. In the first trials we used a photocell which was mounted in a transparent tube about 15 cm from the end which was swallowed (Boves, 1984). This has the advantage that the position of the photocell is more or less fixed. However, a major disadvantage is that due to the fact that the tube with the sensor is forced to hang against the wall of the pharynx, combined with the relatively small aperture of the photocell (ca. 20° for the BPY 61 Ш we used), it is not guaranteed that the recorded signal is a reliable representation of projected glottal area. In fact we often got waveforms which did not look like regular PGG waveforms. Some of the problems might be solved by replacing the photo sensor by a type with a larger aperture.
However, these were not commercially available. Therefore we decided to mount the photocell at the distal end of a tube. This improved the recordings in so far that most of the recorded waveforms now looked as expected; a disadvantage, however, is that it becomes more difficult to maintain the position of the sensor. Moreover, the free moving end of the tube caused more irritation than in case the end of the tube was swallowed.
In conclusion we may say that for ascertaining the integrity of both glottograms during the entire experiment, it is not sufficient to monitor the amplitudes only, at least not when the method is aplied we did. In order to enable the early detection of any unwanted distortions in
the recorded signals, it is necessary to continuously monitor these signals on an oscilloscope during the recording. Finally, we would like to note that interpreting the dc level of the PGG as a relative measure for glottal (rest) area is very tricky as long as photocells are used with a small aperture: the moving photo sensor might cause a significant sway of the dc level which has nothing to do with glottal area. Video registrations of the glottis while simultaneously using the light source for PGG recordings by means of a photo sensor at the outside of the subject's throat would be an attractive alternative. The video registration of the measurement situation might also be very helpful for the interpretation of (unexpected phenomena in other signals.