BITE-BLOCK SPEECH IN THE ABSENCE OF ORAL SENSIBILITY

The ability of a patient suffering from loss of oral sensibility to produce acoustically accurate vowels in the presence of a bite-block, both with and without additional auditory masking, was examined. The results indicated that in the absence of oral afferent information articulatory compensation was forced to rely on auditory feedback. INTRODUCTION Bite-block experiments have been a popular means of investigating the articulatory system's compensatory abilities, especially regarding the speed with which compensation is achieved and the necessity for various forms of feedback. Lindblom, Lubker & Gay (1979) reported for isolated vowels almost perfect articulatory compensation for the presence of a 22 mm bite-block, even when formant measurements were made at the first glottal pulse. The question of whether production of bite-block vowels suffers when sensory information from the oral region is suppressed was addressed by Lindblom, Lubker & McAllister (1977) and Gay & Turvey (1979). The former reported distorted formant values when the bite-block condition was combined with anesthesia of the oral mucosa; the latter also reported distortion, but only when sensory deprivation also included temporo-mandibular nerveblock. The results of these two experiments were interpreted by Perkell (1979) as demonstrating the motor system's dependence on afferent information to mark out an orosensory frame of reference. In Gay & Turvey (1979) one subject was able to approach normal formant values over the course of several syllables, presumably by using auditory information. This led to Kelso & Tuller's (1983) logical extension of the paradigm, with auditory information now being eliminated as well through masking with white noise. For their 5 subjects, including, remarkably, Gay & Turvey's subject just mentioned no significant vowel distortion was found, even under these more difficult conditions. These results thus cast doubt on Perkell's concept of an orosensory frame of reference underlying compensatory behaviour. Using a different paradigm (unexpected electrical stimulation of orbicularis oris) Linke (1980) has reported undisturbed spontaneous speech but reduced compensatory abilities in a patient suffering from absence of trigeminal afferent information bilaterally following surgical treatment for trigeminal neuralgia. These conflicting results impelled us to perform a bite-block experiment with a patient from our clinic who showed substantial deficits in oral sensibility. SUBJECT Three years prior to the experiment reported here the patient (male, aged 29, native German speaker with some Bavarian dialectal influence) suffered closed-head trauma and whiplash injury to the cervical cord in a sporting accident. For about the first month afterwards he was only capable of monosyllabic utterances, but subsequently his articulatory abilities recovered rapidly, being essentially normal six months after the accident. Substantial sensory deficits for the oral region were observed immediately after the accident, with no signs of subsequent improvement. Immediately prior to the experiment we examined the patient's oral sensibility in detail. In all speech structures where detailed testing was possible, namely lower and upper lip, tongue tip and blade, and mucosa of the oral cavity, thresholds for light touch, twopoint discrimination, temperature and vibrotactile perception were raised so substantially as to be unmeasurable with our custom-developed equipment for assessment of oral sensibility. No forms at all could be recognised in a 12-form test of oral stereognosis. Less formal testing techniques also revealed substantial deficits in the pharyngeal region. As far as the speech system is concerned, the sensory deficits of our patient were thus probably more severe than those of Linke's patient and possibly also than those of the subjects in Gay & Turvey (1979), Kelso & Tuller (1983), and Lindblom et al. (1977). It is perhaps also relevant to point out that in contrast to these subjects the sensory deprivation no longer constituted a novel experience for our patient. Regarding the patient's articulatory abilities we have mentioned above that they recovered quickly, and at the time of the experiment he had for a considerable period no longer been considered dysarthric. Intentional mobility of the tongue for non-speech tasks had remained impaired, however (e.g. moving the tongue along the outer surface of the upper lip on command); yet it is important to note that the patient described by Linke showed very similar problems while also apparently having undisturbed speech articulation.