Experiment 2b Perception

In this part of the experiment, examples of each speakers speech was used in a dynamic rhythm setting task to determine whether the same patterns of interval types observed in production were replicated when the speech items were presented to subjects in a perception experiment. That is, are the perception intervals predicted by the production intervals.

5.3.1 Method

Examples of each speaker’s (one)s and (two)s were selected from the digitized sequences used in Part a. This selection was only constrained by the speech items being those that were in the sequences used to determine the production intervals. No systematic criteria were possible with such as collection of natural, varying speech. One example of each speech item was used. Figure 5.2 shows the oscillograms of each item, for each speaker. Each was amplified to a maximum of 10V peak to peak. This was to control for the amplitude of the stimuli presentations. The stimuli were put into an SFS file, and transferred over to the audio lab, along with a reference sound (50ms of signal correlated noise, with a 10ms ramped onset and offset).

There were thus nine stimuli, a (one) and (two) from each speaker, and a reference noise. These were used in a rhythm setting experiment. For each

speaker, their (one) and (two) tokens were compared to each other. This was to see if, in a rhythm setting experiment, with a standardized rate of presentation, the same pattern of (one) - (two) intervals as was found in the production part of this experiment, would be evident, implying differing P centres. Three subjects performed the experiment, with six trials in each experimental block.

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op i 0(1 1(0111 ' Oil

o p ied from <twos>

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rd o ie d from <lwos>!

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copied from <twos>

Figure 5.2 Oscillograms o f examples o f "ones" and "twos" from speakers PH, SHS,

LCE, SH, WC, DG and

SM

f

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Chapter 5 - Experiment 2

The subjects were required to set the signals such that it sounded as though the speaker were ‘counting evenly’. Thus, as was described in the Methodology chapter, subjects were instructed to set the speech items to a perceptually isochronous rhythm. The A - A interval was 1250ms. Any deviation from the final settings of [A-A]*0.5 (that is, 625ms) would indicate that the speakers ones and twos had different P-centres from each other, and the relative sizes of the intervals would show patterns similar to those in perception if the perception and production relationship does hold generally.

5.3.2 Subjects

Four subjects took part in this experiment. All were naive to the aims of the experiment. As described in Chapter 4 all had completed practice trials to ensure that they could perform the dynamic rhythm setting tasks.

5.3.3 Results

Table 5.3 shows the means and standard deviations of the intervals set for each combination of stimuli, that is, for each interval type for each speaker.

Mean intervals (ms)

Speaker One - Two Two - One

SHS 624.27 (14.74) 622.27 ( 17.22) PH 675.67 (17.21) 585.40 (11.75) LCE 615.42 (11.04) 631.07 (8.61) SM 593.67 (17.74) 657.56 (19.50) DG 612.29 (8.24) 639.06 (12.91) WC 614.59 (13.13) 633.00 (15.66) SR 611.76 (11.00) 635.83 (17.96)

The pattern of different one-two and two-one intervals shown in the production data in Table 5.1 is shown again in Table 5.3 above. For example, the intervals are different for the speech of speakers PH and SM, and the variation of interval size is in opposite directions.

The speech of PH, which exhibited the largest differences in interval size in the production data, is here set by subjects to have the largest deviations from absolute isochrony (625ms).

The speech of SM, as irUhe production data, shows the next largest deviation from isochrony. The (two - one) interval is still the larger.

The speech of LCE shows slightly different (one - two) and (two - one) intervals when set to a rhythm by subjects; this precise pattern was not found in the production data. The difference is very small.

Subject SHS shows, as in the production data, no difference between (one - two) and (two - one) intervals.

The speech of subjects DG WC and SR all showed smaller (one -two) intervals and larger (two - one) intervals in the production sequences; this pattern of intervals is maintained in the settings made for all three subjects in the perception task.

Thus the within subjects examination of the intervals shows a similar pattern of results in the production data. Figure 5.3 shows these perception intervals graphically.

The statistical significance of these differences in interval size was investigated in two ways. The first test was to establish that across the speakers, there were

Chapter 5 - Expehment 2

differences in the (one - two) and (two - one) intervals set by the subjects - that is that perceptual isochrony was not based upon physical isochrony (as the perception results implied). The second test was to confirm the hypothesis that there are differences between the speakers in the patterns of intervals that result in perceptual isochrony (again based on the production data).

Figure 5.3 mean Intarval (mm) 7 0 0 r - ipatkmr a QM • t#m a tv« • on*

Figure 5.3 Mean (one-two) and (two-one) intervals for each speaker in perception experiment

5.3.2.1 Interval differences

The intervals set by the subjects for each "one" "two" stimulus were regressed against speaker, subjects, and the interval type; (one - two) or (two - one). This was performed with multiple linear regression, fitting the equation:

y = c + a ( x j + ^(x^)

The regression fit had the equation:

Intervals set = 617 - 0.733(speaker) + 7,95(interval type)

The only significant predictor was interval, (t = 2.15, df = 1, 203, p < 0.05). The predictor speaker was not significant (p>0.05). To test for subject differences, the predictor subjects was entered as a dummy variable (ie.

variable y (x 3 ) entered into the above model); this was not significant

(p>0.05).

This regression shows that if the raw (one - two) and (two - one) intervals for each speaker are considered, then the only significant factor is the nature of the interval - that is the (one - two) and (two - one) intervals are different in size. This suggests that perceptual isochrony is not based upon physical isochrony. This corresponds to the results of the production experiment.

5.3.2.2 Speaker differences

The above multiple regression of raw intervals against interval type, speaker and subject showed no effect of speaker differences. As speaker differences were apparent in the production data, this issue was considered further.

The deviations from physical isochrony for each raw interval setting were calculated. Thus for each (one - two) and (two - one) interval, for each speaker, the size of the difference from [A - A]*0.5 (625ms) was found. This leads to values that range around zero (with zero representing no difference from physical isochrony). All the values were rendered absolute to give a value that expressed, for each speaker, the amount of physical deviation needed from physical isochrony to lead to perceptual isochrony. This measure was used to control for the fact that the physical shift could be in either direction. This absolute deviation gave a measure of the deviations set by subjects to acheive

Chapter 5 - Experiment 2

perceptual regularity. A comparison of these values across the subjects would give an indication of whether the subjects differed significantly in the deviations they set across the speech items from the different speakers. The table 5.4 below give the absolute deviations of the intervals set with speech items from each speaker.

Table 5.4 shows, as the means of the interval show, that the speakers differ in the amount of deviation from physical isochrony needed in order to achieve perceptual isochrony. The statistical significance of this observation was tested by regressing all the absolute deviations against interval type, speaker and subject. A linear regression was used.

speaker mean and SO of absolute

deviations (ms) SHS 12.733 (9.19) PH 45.133 (15.53) LCE 10.000 (7.16) SM 34.000 (16.65) DG 14.133 (9.60) WC 13.900 (10.69) SR 11.367 (9.52)

Table 5.4 mean and SD.s of absolute deviations from isochrony for (one - two)

and (two - one) inten/als for each speaker

The absolute deviations from isochrony were regressed against the predictors interval type, speaker and subject, using multiple regression, fitting the equation: