A REVISIT TO THE APPLICATION OF MRI TO THE ANALYSIS OF SPEECH PRODUCTION – TESTING OUR ASSUMPTIONS

Magnetic Resonance Imaging (MRI) is well suited to study the three-dimensional shape of the vocal tract and tongue during sustained articulations. However, due to the very long acquisition times, the subject often has to sustain the articulation artificially for 30 seconds or more. Another drawback of MRI in speech production research is that the subjects have to be positioned in supine position lying on their back, due to the construction of the MRI scanner and antenna. This paper presents an evaluation of the effects of the supine position and the artificial sustaining based on MRI itself, as technical advances makes it possbile to collect full 3D data in 5 seconds and of the midsagittal plane at 9 images per second. The subject was further positioned facing downwards during one part of the acquisition in order to evaluate the gravitational effect. The paper also investigates if the coarticulation due to different vowel contexts found in artificially sustained consonants is equivalent to the coarticulatory effects obeserved in real time images. INTRODUCTION Ever since Baer et al. (1987) proposed to apply Magnetic Resonance Imaging (MRI) to analyze speech production, MRI has been used widely, e.g. by Story et al. (1996), Apostol et al. (1999) and Badin et al. (2002), to study the three-dimensional shape of the vocal tract or the tongue during speech – or rather during artificially sustained articulations. MRI is a tremendous tool in speech research as it is the only measurement method that can provide detailed 3D data of the entire vocal tract and tongue without radiation hazards for the subject. MRI has however great restrictions as well, mainly the – up till now – very long acquisition times, requiring the subject to sustain the articulation for over 30 seconds, e.g. 3.4 minutes for each image in Baer et al. (1991), or 43 seconds for the entire set of 54 images in Engwall (2002). The fact that the articulations were artificially sustained is often stated as a limit in papers on MRI acquisition of speech productions, but the authors have nevertheless always assumed that the measured articulations are more or less representative. Several studies have registered static consonants in different vowel contexts, and in Engwall (2002) it was shown that coarticulation could be found in the artificially sustained articulations. However, when the static MRI data was compared to real time EMA and EPG measurements, it was concluded that the artificially sustained articulations seemed to be hyperarticulated. Further, due to the construction of the MRI scanners, the subjects are positioned in supine position, without exception lying on their back, assuming that gravitational effects are neglible. However, several MRI studies, notably Badin et al. (2002) and Engwall (2002), have noted a backward displacement of the tongue and Tiede et al. (2000) also found postural effects between sitting and lying position using X-ray microbeam. The effects of the artificial sustaining and the supine position have thus been partially investigated already using other techniques (EMA, EPG, X-ray microbeam), but as the techniques are point-wise, so are the evaluations. This paper presents an evaluation based on MRI itself, as advances in the technique now allows us to collect 3D data in 5 seconds, i.e. during a normally sustained articulation, or images of the midsagittal plane in close to real time, at 9 images per second. Three issues are addressed in the evaluation: 1. Is the artificially sustained 3D articulation equivalent to one normally sustained? 2. Is a static coarticulated articulation equivalent to real time coarticulation? 3. Is the gravitational effect indeed neglible? DATA, SUBJECT AND CORPUS The data was collected at two seperate occasions, 2.5 years appart, but using the same reference subject, a male native speaker of Swedish, for all acquisitions. The first data set, set1, consists of a 3D set of 54 images collected in 43 seconds for each articulation, and a midsagittal set with an acquisition time of 11 seconds. The Proceedings of the 6th International Seminar on Speech Production, Sydney, December 7 to 10, 2003.