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The perception of the rhythmic quality of connected speech involves listeners tuning in to certain regulated features within the speech signal. Thus, Arvaniti (2009:59) defined rhythm as ‘the perception of series of stimuli as series of groups and repetitive patterns’. However, the link between perception and production is complex and not as straightforward as it seems because this regularity is not necessarily entirely a product of evenly-timed events within the signal (Terhardt and Schütte, 1976).

The fact that the alignment of perception and production is complex is illustrated by work on Perceptual-Centre or P-Centre. Morton, Markus and Frankish (1976:405) refer to P-Centres as ‘the psychological moment of occurrence of a word’.

The term P-centre was coined following an experiment conducted by Morton et al. (1976) where they presented a pair of sounds in alternations to two subjects; these sounds were controlled by a computer. The first sound has a fixed interval; the interval of the subsequent sound would be determined by the subjects. They would adjust the knob until they perceived that the pair of sounds occurred at equal intervals. From this experiment, Morton et al. (1976) found that the alignment of the moment of occurrence was different from the acoustic isochrony.

The schematic diagram in Figure 2.3 illustrates the marking of p-centre by the listeners, where the wavy line is the representation of a

waveform; the thick vertical lines are the acoustic onset of the intervals; the vertical serrated lines are the point on which the subjects perceived as the onset of the intervals; the horizontal serrated lines, which are labeled as P_x are the

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interval differences between the acoustic and perceived onset; t is the duration of the intervals marked by the subjects to be perceptually isochronous; and, _T is the relative P-centre distance between the two intervals (which is obtained by calculating the difference between P1 and P2 durations). The diagram shows that the onset of the dark bar is not aligned to the onset of t.

This led Morton et al. (1976) to propose that what the listeners judged as isochronous and regular was some form of perceptual correlate which does not map easily onto the acoustic signal; instead, subjects were found to diverge systematically from acoustic regularity. The pattern which can be observed, in this experiment, is for sequence interval /ba~ma/, the duration of P₁ for the initial bilabial plosive was shorter compared to the P2 of the following interval, the bilabial nasal. Meanwhile, for sequence interval /ma~ba/ the P₁ duration of the bilabial nasal was longer than that P2 duration of the following bilabial plosive.

Figure 2.3 The schematic diagram of P-Centre

T =^P2-P1

Following this, Fowler (1979) conducted two experiments to study the production and perception of P-centres. The first experiment focused on the production of evenly-timed syllable intervals. A male adult was instructed to produce sequences of nonsense sentences. Each of the sentences consisted of

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six monosyllables. The monosyllables rhymed with /ad/ but each had different initial consonants. These sentences were either homogeneous in composition like ‘mad mad mad mad mad mad’ or alternating in composition like ‘mad sad mad sad mad sad’. The subject was instructed to read these sentences in a slow rhythmic rate and by stressing each syllable. ‘Rhythmic’ in this experiment was referred to as ‘equality of intervals between syllable onsets’

(Fowler, 1979:378). Measurements were taken of the duration of the interstress intervals between acoustic onsets of the syllable as a function of their position in an utterance. The interstress interval was marked between the onset of the stressed vowel and the onset of the following stressed vowel. Fowler (1979) did not include the initial and final intervals in order ‘to avoid the contamination of the rhythmicity effects’ (1979:377) due to the effects of initial and final

lengthening. Thus, only the durational measurement, for both homogeneous and alternating utterances, of the interstress intervals between syllables 2 and 3, 3 and 4 and 4 and 5 were taken (in ms). The absolute mean differences for each of the successive interstress intervals were also calculated.

The results for the homogeneous utterances showed that the interstress intervals were produced in near isochrony as the mean deviation from

isochrony ranged between 19.7ms and 27.5ms. Subsequently, a one-way ANOVA also showed there is no significant effect of the interstress interval position on the vowel durations for the homogeneous utterances. This was because the durational differences between these intervals were rather small.

On the other hand, for the alternating utterances, the results indicated that there are big deviations from the isochrony, the mean deviations ranged from 18.4ms to 80.2ms. In fact, a one-way ANOVA showed a significant effect

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of the interstress interval positions on the durations, which suggested durational variability across the productions of the successive interstress intervals. A post-hoc analysis showed that interstress intervals with /s/ were significantly longer than interstress intervals with /m/.

Although the data were rather limited, Fowler (1979) concluded that when the subject in this experiment was asked to produce stress-timed

utterances for the alternating sentences, the subject deviated from acoustic isochrony in systematic ways. According to Fowler (1979), The deviations were apparent in intervals beginning with acoustically long-duration consonants such as /s/ and ending with short-duration consonants /m/ were longer than those of the intervals with the reverse type of consonants. This finding supports the observation made by Morton et al. (1976) earlier in which they suggest that the sound properties of the intervals somewhat contribute to the way in which the listeners aligned their P-centres.

The deviation seemed to support the notion brought forward by Morton et al. (1976) in which to hear an utterance as stress-timed, the listeners require the deviation from the acoustic isochrony just like the speaker in this experiment produced.

The results of the first experiment were further substantiated by the second experiment. The second experiment focused on the perception by subjects of stress-timed rhythm. There were 12 alternating stimuli used for this experiment (similar to that of the first experiment). Each of the stimuli

consisted of six monosyllables. These stimuli then were divided into two groups. The interstress interval duration within one group was manipulated and generated with very little standard deviation (12ms). While, the interstress

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interval duration of the stimuli in the other group were not controlled for and they have larger standard deviation (125ms). The unaltered stimuli were paired with the altered stimuli. Ten subjects were asked to identify which one of the utterances was ‘rhythmic’. The findings revealed that nine out of ten subjects chose the unaltered version of the six pairs to be rhythmic.

With regard to the first experiment, the speaker regulated something other than the acoustic components of the interstress intervals and, further to this, the second experiment showed that the listeners did not depend on the isochrony of the durations of the interstress intervals as a cue for rhythmicity.

These experiments did not establish what was being regulated by the speakers or what was being perceived as regular by the listeners. Moreover, Morton et.

al (1976) and Fowler (1979) suggested that P-centre markers were an abstract feature.

By using a similar stimuli described in Fowler (1979), studies like Patel, Lofqvist and Naito (1999) suggested cues such as the interval between syllable onsets, intervals between first formant amplitude slope maxima and between fundamental frequency amplitude slope maxima and intervals between articulator velocity maxima might be considered as P-centre markers. However, none of these features were found by Patel et al. (1999) to be acoustically isochronous. Perception of rhythm is clearly a complex matter. And it is nevertheless clear that languages can be differentiated by the rhythmic

characteristics. Notwithstanding that we do not understand the perceptual basis of these differences; researchers have over the decades try to capture them in a typology of rhythmic properties.

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