In vowel perception, the so-called "context effect" is caused by the existence of short-term memory (STM) of the preceding and following vowels. That is, in the case of identifing a vowel in a context, shift of the phoneme boundary occurs because of the contrast effect caused by STM. The purpose of this paper is to investigate how the boundary shift is influenced by the difference in the context of a vowel. Here, the phoneme boundary is quantitatively determined by performing psychological experiments using V_1CV_2 and V_1V_2 syllables. The results obtained are summarized as follows: (1) STM of the preceding vowel gradually decays with time. Accordingly, the contrast effect on the following vowel decreases. That is, the boundary shift becomes smaller with the decay of STM. Thus, the retention curve of STM was obtained estimating the amount of boundary shift of V_2 influenced by V_1 in identifing V_1V_2 (Fig. 2). The individual difference of STM appears in the retention curve obtained from the stimulus continuum. (2) In case of identifing V_1bV_2, the phoneme boundary of V_2 shifts temporarily towards V_1 because of the contrast effect caused by STM of V_1 (Fig. 6). In addition, the boundary shift depends on the difference in the quality of V_1. The amount of boundary shift increases monotonously as V_1 is displaced from the phoneme boundary. However, the amount decreases or saturates when V_1 comes to the range of another vowel (Fig. 7). Furthermore, in order to investigate the two-dimensional boundary shift, the phoneme boundaries on /e/-/a/-/u/ ranges were represented in F_1-F_2 space. When V_1 is /e/ and V_2 is the vowel in the vicinity of the boundary of /u/, the contrast effect mainly occurs between /e/ and /a/. Consequently, the boundary of /u/ shifts towards the low F_1(Fig. 8). From the viewpoint of the distinctive feature, /e/ is more similar to /a/ than to /u/. The acute of the distinctive feature is extracted form V_1 and V_2. Thus, the acute of V_1 is contrasted with that of V_2. (3) The effect on the difference in the position of vowel was investigated using V_1bV_2 and V_2bV_1 syllables. The boundary shift of V_2 in V_2bV_1 is more marked than that in V_1bV_2(Fig. 9). That is, the effect on V_1 in V_2bV_1 is more remarkable than that in V_1bV_2. (4) The effect of C in identifing V_1CV_2 tends to reduce the contrast effect occuring between V_1 and V_2, if C bears the distinctive feature relating to the contrast between V_1 and V_2 (Table 2).
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