[生物] typing LB 95~96惠珍 Typing / Done

LB095-097惠珍 done on Jan 5’09

The order of corrected neuronal events may yet be different from the three orders discussed so far, although we are entering a realm in which we are almost entirely reduced t speculation. The reason for suspecting a neuronal firing order that is different from the order of motor events is the anatomy of the peripheral nerves that innervate the respective muscle. Figure 3.8 shows diagrammatically the course of some of the nerves that are relevant to this discussion. There is considerable differences in the length of these nerves; notice especially the circuitous course of the recurrent nerve which is more than three times conduction time of impulses does not merely depend upon the distance of peripheral nerve that must be traversed but also upon the diameter of the nerve fibers (the smaller the fiber. the slower the condition). It is possible to measure the average length of the peripheral nerves and also to make microscopic studies of cross sections of these nerves and to measure the size of the diameters of fibers. This has been done (Auriti,1954; Krmpotic, 1958 and 1959), and it has been found that nerves involved in speech muscle innervation vary considerably in their composition of a nerve in terms of a caliber spectrum (a statistical frequency distribution of diameter sizes). Table 3.4 shows a few figures taken from the work of Krmpotic, indicating that there is always some overlap in the distributions but that there are nevertheless, marked differences among the various nerves. It is interesting that the the longest nerve, the recurrent, has statically the smallest fibers which thus aggravates the timing problem, introducing delays from two independent factors.

Kirmpotic, who has been specially interested in the problem of differential innervation time, has computed so-called neuromuscular indexes for all major muscles involved in speech. These indexes are simply the ratio of the average length of the nerve to the mean size of fiber diameter. Since there is still some uncertainty about the physiological interpretation of these determinations, we need not be concerned here with the details. Suffice it ti say that the anatomy of the nerves suggests that innervation time for intrinsic laryngeal muscles may easily be up to 30 msec longer than innervation time for muscles in and around the oral cavity.

Considering now that some articulatory events may last as short a period as 20 msec, it becomes a reasonable assumption that the firing order in the brain stem may at times be different from the order of events occurring at the periphery.


A concrete example will clarify the point. Let us take the words obtain and optimal. If we make spectrograms of these words and measure and durations of the first three phones, we get values such as these:
Duration of Duration of
First vowel the labial stop the aspiration
Obtain 90 msec 170 msec 50 msec
Optimal 110 msec 160 msec 20 msec

Among the acoustic cues for the discussion of certain voiced and unvoiced stops are the duration of the preceding vowel and the duration of the silence during which the lips are closed. Our perceptual acuity is highly sensitized to these temoral factores and , by the same token, our motor coordination is precise enough to times articulation with sufficient presision to bring about these distinctions. In the previous example the duration of the first vowel differs by only 20 msec and that of the labial stops by only 10 msec, and in the second instant, the duration of the aspiration (the only part of the /t/ we hear)lasts only 20 msec. Recent investigation carried out at the Haskins Laboratories have shown that a great variety of phonetic distinctions are entirely dependent upon timing factors of onset, duration, and cessation of voice where magnitudes well below 20 msec are od the essence. (See also the time relationships in the data reported on by Liberman, Delattre, Cooper, 1952; Schatz, 1954; and Liberman, Delatrre, Gerstman, and Cooper, 1956)

It is clear that there must be considerable precision in timing if laryngeal mechanisms are to integrated with oral ones. IN addition, there are hundreds of muscular adjustments to be made every second (that is a new neuromuscular event every few milliseconds) from which we begin to see the magnitude of the timing-ordering problem.

In view of the above we are hardly surprised that we may encounter patients with lesions in the central nervous system who have difficulty in keeping elements of speech and language in the right order. An important theoretical discussion on this subject was contributed by Jakobson and Halle(1956). Patients who have this problem speak very slowly and indicate to the examiner that speaking constitutes a great effort of concentration: the behave as if they had to “think of the right order.” Even so, they will constantly mix up individual sounds; is may become si, task may become taks, syllable may become syballel and they are very hard to understand.*Generally, they tend to anticipate sounds that should come at a later time. The difficulty may occur in every other word, since their rate of production is markedly slow down. The improper anticipation of sounds is usually nit the patient’s only problem of ordering. He will also show a pathological propensity for spoonerism, entire words and phrase s being switched around or produced in advance. The patient’s intention is frequently made known to the examiner because the patient is fully aware if his difficulty and can often make a fresh start and repeat the intended sentence once more and correctly . Ina sever cases the corrected sentence will show new mistakes.