2.1 The rhythmic grouping literature

The most widely cited historical antecedents for modern ITL studies are Bolton (1894) and Woodrow (1909), who tested the preferences of adult American English speakers who were asked to group patterned sequences of tones or clicks in which intensity and duration were systematically varied. Their early findings provided the foundation for the generalizations in (1). In addition to studying intensity and duration varied singly, Bolton (1894) investigated the effect of varying them orthogonally. He found that when listeners segmented sequences that alternated a long soft tone with a short loud one, listeners preferred the grouping in which the loud tone came first and the long came last. This grouping is consistent with both the Loud-First and Long-Last principles of the ITL. However, for sequences in which a short soft sound alternated with a louder sound that was twice as long, listeners preferred iambic groupings. In a contest, therefore, the dominant influence was duration. Unfortunately, Bolton does not support his generalization quantitatively with a detailed statistical analysis, as is standard today; nor does his study answer the question of whether incrementally increasing a sound’s intensity while keeping its duration constant would modulate the effect of the more dominant feature.

In more contemporary ITL studies that serve as updated methodological antecedents for the current research, adult participants are presented with streams of sounds in which different values for acoustic intensity or duration are alternated at fixed ratios, typically in a binary … ABABA … pattern. Participants are tasked with indicating whether they think these sequences break most naturally into AB or BA pairings. Studies using this or a similar method have consistently associated a loud-first grouping bias with varied intensity, as predicted by the ITL. These include studies in which non-speech sounds (tones or beeps) have been used to test English speakers (Rice, 1992); English and Japanese speakers (Iversen et al., 2008; Kusumoto & Moreton, 1997); and English and French speakers (Hay & Diehl, 2007). Bell (1977) reports a loud-first preference for small groups of subjects representing five languages (Bengali, English, French, Persian, and Polish) who were asked to group tones of different intensities that alternated in a ternary pattern. Three key studies for speech in which listeners segmented alternating sequences of syllables also report loud-first preferences for English and French speakers (Hay & Diehl, 2007); French and German speakers (Bhatara et al., 2013); and Zapotec and English speakers (Crowhurst & Teodocio, 2014).

Findings for duration in both the speech and non-speech studies cited above have been more mixed. A long-last grouping bias has been consistently reported for English speakers (Crowhurst & Teodocio, 2014; Hay & Diehl, 2007; Iversen et al., 2008; Kusumoto & Moreton, 1997), French speakers (Bhatara et al., 2013; Hay & Diehl, 2007), and German speakers (Bhatara et al., 2013). Vos (1977) reports a long-last grouping preference among Dutch speakers when the interval between non-speech tones was held constant. However, two nonspeech studies have failed to find a consistent grouping preference with Japanese speakers (Iversen et al., 2008; Kusumoto & Moreton, 1997). Interestingly, Iversen et al. (2008) report not merely inconsistent, but conflicting findings among groups of Japanese speakers they tested. One group (26% of their Japanese participants) showed a clear long-last grouping bias. A second group (nearly 45%) had a clear preference for long-short groupings. Iversen et al.’s remaining Japanese participants showed no consistent preference for either grouping.² A more nuanced set of outcomes for duration among speakers of Zapotec and English is reported by Crowhurst and Teodocio (2014), the antecedent for the current research, described in Section 2.2.

2.3 Relevant prosodic characteristics of Spanish and English

Phonetic research has shown that measures of intensity and duration are closely associated with the stress contrast. Stressed syllables are reliably longer than unstressed syllables in many languages, and listeners use this information to identify them in speech.³ Measures of intensity are less reliably associated with stress across languages, at least for unaccented stressed syllables (Ortega-Llebaria & Prieto, 2011), although intensity may be important in combination with duration (e.g., Sluijter et al., 1997; Turk & Sawusch, 1996). These generalizations hold true for the languages under study here: stressed syllables are lengthened in both Spanish (Ortega-Llebaria & Prieto, 2007; 2011) and English (e.g., Beckman & Edwards, 1994; Campbell & Beckman, 1997), although the contrast is more pronounced in English due to vowel reduction in unstressed syllables (e.g., Beckman & Edwards, 1994; Lindblom, 1963; Ortega-Llebaria et al., 2007; Ortega-Llebaria & Prieto, 2011). Stressed syllables in higher-level prosodic constituents (pitch accents) tend to be longer than lower-level stressed syllables in English (Beckman & Edwards, 1994). There is less evidence that this is so in Spanish (Ortega-Llebaria & Prieto, 2007; 2011). Evidence associating greater intensity with stress in Spanish and English has been more conflicting. While Ortega-Llebaria and Prieto (2007) concluded that spectral tilt was a reliable correlate of stress in Spanish (though less robust than duration, and in deaccented syllables), Ortega-Llebaria and Prieto (2011) did not find this to be the case. For American English, Sluijter and van Heuven (1996) report that spectral balance differences but not overall intensity differences mark unaccented stressed syllables. Campbell and Beckman (1997) did not find this difference; they associate spectral balance differences with stress-related vowel reduction (a point Ortega-Llebaria and Prieto, 2011, have made for Catalan). However, Kochanski et al. (2005) report that overall vowel intensity differences reliably mark word-level stress contrasts in some UK dialects of English. In perception, Ortega-Llebaria et al. (2007) report that their Spanish-speaking subjects relied most on duration to distinguish unstressed from (unaccented) stressed syllables, but that discrimination was better when differences in overall intensity were also present. Numerous studies have found length to be the more reliable perceptual cue to stress in English as well (e.g., Beckman & Edwards, 1994; Campbell & Beckman, 1997; Kochanski et al., 2005; Turk & Sawusch, 1996). Turk and Sawusch (1996) found that intensity differences can help listeners identify stressed syllables, but not in the absence of duration cues.

In addition to its close association with stress, increased duration is associated with constituent-final syllables in both English and Spanish. In English, preboundary lengthening has been observed across accentual contexts and at multiple levels of the prosodic hierarchy (Beckman & Edwards, 1994; Byrd, 2000). Beckman and Edwards (1994) and Turk and Shattuck-Huffnagel (2000) report a degree of preboundary lengthening as low in the hierarchy as the prosodic word, although effects are more pronounced at higher levels. For Spanish, Rao (2010) reports preboundary lengthening effects at the phonological and intonational phrase levels in the productions of speakers from Cuba, Ecuador, and Spain. In a recent tri-language study, Prieto et al. (2010) found evidence for preboundary lengthening at the ends of intermediate (i.e., phonological) and intonational phrases in Spanish, Catalan, and English. These authors report that both preboundary lengthening effects and stress-related duration differences are greater in English than in Spanish and Catalan.

Most theoretical accounts of stress posit a word-final, trochaic main stress foot in both Spanish and English (see Harris, 1983, for Spanish; Hayes, 1982; 1985, for English). However, while precise quantitative data is not available, a case can be made that they differ in the frequency with which stress occurs on word-final syllables. Domahs et al. (2014) report that primary stress fell on the penult in 61.1%; on the antepenult in 27.4%; and on the final syllable in 11.5% of polysyllabic words in a 1,160-item sample of the English lexicon. However, the prevalence of stress on final syllables in English is higher than 11.5%, given that heavy final syllables have secondary stress in words with nonfinal main stress (e.g., sálivàte, níghtingàle), a class that notably includes all compounds in which the second element is a monosyllable (e.g., sídewàlk, búbblegùm). Nor did the sample analyzed by Domahs et al. (2014) include stressed monosyllables, which have final as well as initial stress. These accounted for 11.4% of items in a 33,060-word lexical sample extracted from the Shorter Oxford English Dictionary and analyzed by Cutler and Carter (1987). There is also reason to believe that the percentage of stressed, word-final syllables in connected speech is higher than in the lexicon. The speech that English speakers hear contains a disproportionately large number of stressed monosyllables: information in Cutler and Carter (1987, p. 138, calculated from Table VII) indicates that in a 187,699-word sample of spontaneous British English conversation, 59.4% of the 76,963 tokens of lexical words were stressed monosyllables whereas only 28.2% were polysyllables with initial primary stress. The prevalence of stressed monosyllables in speech is important, because they combine with preceding unstressed function words, prepositions, or pronouns to form higher-level prosodic constituents that may be as short as a foot (e.g., D’she GO? They’re HERE!) Therefore, while word-initial stress is very common in English and plays a role in speech segmentation (Cutler & Butterfield, 1992), stress on domain-final syllables in speech is also quite common. As stress-related and preboundary lengthening effects are additive (Beckman & Edwards, 1994), it seems likely that longer syllables in domain-final positions are natural for English speakers.

Stress on domain-final syllables is arguably less common in Spanish than in English, both in the lexicon and especially in connected speech. Estimates place primary stress on penultimate syllables in over 80% of lexical words (Morales-Front, 1999; Quilis, 1993) or 74%, weighted for frequency (Sebastian & Costa, 1997). Nonfinal stress also occurs in a minority of lexical words with antepenultimate stress (e.g., Sábado ‘Saturday’, próximo ‘next’). While again, precise estimates for actual speech were not available, the morphological characteristics of Spanish suggest that the frequency of stress on final syllables is less common in speech than in the lexicon, and is less common than in English overall. First, due to Spanish’s robust inflectional system, relatively few major class words are monosyllabic, in proportion to polysyllables, even in isolation. Most nouns and agreeing adjectives end in unstressed class-marking vowels (cf. English cát, Spanish gáto; all stressed vowels are given diacritic accents for clarity). Second, while uninflected consonant-final words tend to have final stress (Lipsky, 1997; e.g., ciudád ‘city’), as do a small minority of vowel-final words (e.g., menú ‘menu’), these take the plural allomorph –es, which adds an unstressed syllable (e.g., ciudádes ‘cities’, menúes ‘menus’). Third, while infinitive verbs have final stress, regularly conjugated finite verbs generally have nonfinal stress (with exceptions in the future and preterite tenses). Fourth, some encliticised verbs with antepenultimate (or even preantepenultimate) stress occur frequently in discourse (e.g., fíjate ‘look; think about it!’, pásale ‘come in’, recuérdate ‘remember!’). Finally, because stressable words may be deaccented in sentential contexts, some word-final syllables that might be stressed in isolation are unstressed in speech (Hualde, 2007).

Summarizing, increased length on constituent-final syllables can have more than one source (stress and preboundary lengthening), and duration contrasts are saliently marked in English. Final syllables in Spanish are less likely to be longer because final stress is less prevalent and duration contrasts are less salient than in English. For these reasons, it is reasonable to anticipate that Spanish speakers might have a lower expectation than English speakers of stress-related duration cues in final positions, and I expected this difference to be reflected in the outcomes of rhythmic grouping studies in which speech-based stimuli were used.

2.4 Hypotheses

Expectations for the current studies were to some extent determined by the findings of prior studies. Comparable ITL studies that have tested intensity-based rhythmic grouping preferences have confirmed the Loud-First Principle (see Section 2.1), and I expected to replicate these findings in the current research in both language groups. This prediction is operationalised as the Incremental Loud-First hypothesis in (2a). Two outcomes reported in Crowhurst and Teodocio (2014) were also important in setting expectations. The first was their finding that when intensity disparities were incrementally increased, intensity effects were greatest at lower levels, and were attenuated at the highest levels of the scale. This expectation is stated as the Intensity Attenuation hypothesis in (2b). Second, their findings indicated that intensity effects were consistently more pronounced than effects of duration, leading to the Intensity Wins hypothesis in (2c).

The hypotheses of greater importance for the theoretical questions of interest were associated with duration. The basic question driving the research was whether language-specific differences in the distribution of longer syllables would be reflected in the decisions of Spanish and English speakers when segmenting duration-varied sequences. Multiple studies have confirmed a long-last grouping preference for English speakers, and I expected the same outcome here. The operationalization of the Long-Last Principle adopted here is stated as the Incremental Long-Last hypothesis in (3b). It was argued in Section 2.3 that Spanish speakers should have a weaker association between increased length and constituent final syllables than English speakers, and I expected that this difference should be reflected in listeners’ duration-based rhythmic grouping preferences. Accordingly, one of the study’s primary hypotheses was that effects related to varied duration should be weaker in the Spanish than in the English group. This expectation is stated in hypothesis (3a). Expectations as to the nature of any duration-based grouping preferences for the Spanish group were less clear; because distributional patterns associated with duration are less convergent in Spanish than in English (both languages have preboundary lengthening, but stress-related lengthening is associated more consistently with nonfinal syllables in Spanish than English), there were competing predictions for the Spanish group. If the Spanish speakers’ rhythmic grouping perceptions were more strongly influenced by boundary-related duration cues, then the outcome should confirm the Incremental Long-Last hypothesis in (3b). Alternatively, if their preferences were more strongly shaped by stress-related duration patterning, they should prefer long-short groupings, as hypothesized in (3c). Finally, the possibility of a third outcome was considered: as duration cues related to stress and prosodic boundaries conflict more in Spanish than English, the Spanish speakers might not show a clear grouping preference when duration was varied. This would also be consistent with the hypothesis in (3a).

3.2 Results

The raw data obtained in the experiment are presented in Table A in Appendix B. This table indicates the absolute number and proportion of baga and gaba responses by manipulation level for each language group. The design provided for a maximum of 6,156 observations. There were 83 missing responses (1.3% of the total possible), so that a total of 6,073 data points were included in the analysis. Trends for the Spanish-speaking (solid lines, squares) and English-speaking (dashed lines, circles) groups are charted in Figure 2.

Figure 2

Proportion of baga responses in Experiment 1 in the (a) duration condition (as a function of increasing magnitude of duration, or MOD) and (b) intensity condition (as a function of increasing magnitude of intensity, or MOI).

4.2 Results

The raw data obtained in Experiment 2 are presented in Appendix B, Table B. The design of the stimulus set provided for 7,332 potential observations (2,964 for the English group and 4,368 for the Spanish group). Of this maximum, there were 126 missing responses (1.75% of the total possible), so that a total of 7,206 data points were included in the analysis (2,937 and 4,269 for English and Spanish, respectively).

4.2.1 Basic trends

Trends observed in the singly-varied conditions are plotted in Figure 3. Figure 3a shows that in the English group (dashed lines, circles), MOD and baga responses were again negatively correlated, consistent with a long-last preference. The correlation was more consistently linear than in Experiment 1, confirming the Incremental Long-Last hypothesis, (3b). However, the trend was also weaker in Experiment 2. In the Spanish group (solid lines, squares), increasing MOD had no clear effect on listeners’ grouping choices, apart from an increase at MOD 2. That the influence of varied duration was stronger in the English than the Spanish group was again consistent with hypothesis (3a).

Figure 3

Proportion of baga responses as a function of (a) MOI and (b) MOD in conditions where intensity and duration were varied singly in Experiment 2.

Plots for the intensity-varied condition appear in Figure 3b. Apart from a decrease at MOI 2 in the Spanish group, MOI was again positively correlated with baga responses in both languages, consistent with a preference for loud-first groupings. The functions were S-shaped, indicating that the greatest effects of varied intensity were again clustered at the centre of the scale in both language groups, as predicted by the Intensity Attenuation hypothesis, (2b). The functions for MOI in both language groups were more regular than in Experiment 1 and with the exception noted, outcomes were consistent with the Incremental Loud-First hypothesis (2a), within the limits predicted by Intensity Wins, (2c). The effect associated with Intensity was again stronger in the Spanish group, as predicted based on the outcome of Experiment 1: the proportion of baga groupings was lower at MOIs –2 and –1, and higher at MOIs 1 and 2, than in the English group. In sum, outcomes for the singly-varied conditions in Experiment 2 on the whole replicated the main tendencies observed in Experiment 1; however, effects associated with varied duration were smaller in Experiment 2 compared with the first study (absent in the Spanish and weaker in the English group).

Trends in the co-varied conditions are plotted in Figures 4 and 5. In Figure 4, the response data for the co-varied conditions is arranged to show the effect of manipulating Duration on outcomes for the more robust predictor, Intensity. Black dotted lines represent the baseline trends in the singly-varied intensity condition. MOI is represented on the x-axis, and MOD in the vertical dimension by the stacked circles at MOI 0. Blue lines plot trends associated with varied intensity when ga was longer than ba (positive MODs), and red lines plot outcomes when ba was longer than ga (negative MODs). Dashed lines represent a 60 ms, and solid lines a 120 ms disparity. Figures 4a and 4b show that the dominant effect in both co-varied conditions was one of intensity: increases in MOI increased baga responses regardless of whether a duration disparity was also present. The interaction between intensity and duration is seen most clearly in Figure 4a, the graph for English, because differences in the baseline duration condition allowed the plotted lines to be well differentiated. Here, we see that introducing a duration disparity weakened the correlation between MOI and baga responses; this is revealed in the flatter slopes of the intensity functions.

Figure 4

Proportion of baga responses as a function of increasing MOI (x axis) by MOD (vertical) in Experiment 2. Lines: dotted = baseline intensity (MOI 0); dashed = 6 dB disparity; solid = 12 dB disparity; red = positive MODs (ba is longer); blue = negative MODs (ga is longer).

Figure 5

Proportion of baga responses as a function of increasing MOD (x axis) by MOI (vertical) in Experiment 2. Lines: dotted = baseline intensity (MOD 0); dashed = 60 ms disparity; solid = 120 ms disparity; red = positive MOIs (ba is louder); blue = negative MOIs (ga is louder).

The Competing condition is represented by points labeled with squares (the blue lines at negative MOIs and the red lines at positive MOIs), and the Co-operating condition by the points labeled with triangles (the blue lines at positive MOIs and the red lines at negative MOIs). In Figure 4a, we see that in the English group, the lines representing the 60 and 120 ms disparities diverge in the Competing condition, and converge in the Co-operating condition. This indicates that in the Competing condition, as a syllable got longer, there were fewer loud-first (long-short) and more long-last groupings (more baga responses when ga was longer, fewer baga responses when ba was longer) as the same syllable got louder. In the Co-operating condition, by contrast, lengthening the softer syllable produced more long-short groupings, regardless of how loud the short syllable was. An explanation for this finding is suggested in the general discussion.

The information presented in Figure 4 is rearranged in Figure 5 to show the effect of Intensity on Duration. In the English group, we see that effects of duration were most robust in the co-varied conditions when ba was louder than ga (the red lines), and when ga was 12 dB louder than ba (blue, leaving aside the point representing MOD 0). In the Spanish group, the influence of duration in the co-varied conditions was clear only when ba was 6 dB louder than ga (dashed red line). Otherwise, varied duration had no consistent influence on the Spanish speakers’ grouping choices.

5.1 Intensity and duration taken separately

At the most general level, outcomes for intensity in Experiments 1 and 2 showed that the presence of an intensity disparity between alternating syllables, whatever its magnitude, increased responses representing loud-first syllable pairings relative to a control sequence in which no disparity was present. This overall preference, expressed as more baga responses when ba was louder and fewer when ga was louder, was significant in both language groups, in both experiments, and in all conditions in which intensity was varied. The basic finding is consistent with the basic prediction of the Loud-Soft Principle of the ITL in (1).

More granular predictions were made by the Incremental Loud-Soft and Intensity Attenuation hypotheses in (2a) and (2b) (see Section 2.4). The Incremental Loud-Soft hypothesis predicted a positive association between increases on the MOI scale and baga responses, creating the expectation of a monotonic function. According to the Intensity Attenuation hypothesis, changes between levels were not expected to be proportional. Rather, the effect of increasing the intensity disparity by the same constant (an increment of 3 dB in Experiment 1, and 6 dB in Experiment 2) would be greatest between MOI 0 and MOIs –1 and 1 and should taper off at successively higher levels. This means that the function associated with intensity should be curvilinear at each end of the scale, producing an overall S-shaped function. These hypotheses were generally borne out, with the exception of several unanticipated decreases in baga responses (to be discussed momentarily): in the baseline intensity condition in Experiment 2, the increase from 0 to 6 dB of disparity produced proportionally more loud-first groupings than the increase from 6 dB to 12 dB of disparity, with one exception in the Spanish group. A similar pattern was found in Experiment 1. The Intensity Attenuation hypothesis received weaker support in the co-varied conditions in Experiment 2, due to interactions with duration. This outcome was predicted and is discussed later in the section.

Focusing on the baseline intensity conditions, at least two interesting differences were observed across language groups and across experiments. First, the magnitude of the fixed effect of Intensity was greater in Experiment 2 than in Experiment 1. The ORs associated with Intensity indicate that a unit of MOI increase doubled the odds in Experiment 1 (OR = 2.0), and nearly tripled the odds (OR = 2.82) in Experiment 2 of a baga response overall. Second, the effect of Intensity was greater in the Spanish than in the English group in both experiments. In Experiment 1, the ORs for Intensity were 2.92 for Spanish and 2.01 for English, and in Experiment 2, the ORs were 8.61 for Spanish and 2.84 for English. These measures reveal large differences across experiments: the odds of a baga response were approximately a third higher in the Spanish group than in the English group in Experiment 1; but the odds were three times greater in the Spanish group in Experiment 2. The reason for the between-group difference is unclear, but it suggests that the Spanish speakers were more sensitive to intensity variations than the English speakers tested. A similar between-language difference in a cross-linguistic ITL study was observed by Bhatara et al. (2013). These investigators report a loud-first grouping preference among both monolingual German and French speakers they tested, but this bias was more robust among the German speakers.

The between-experiment differences in the effect of intensity in the current research may be attributable to differences in the design of the stimulus set: in Experiment 1, singly-varied intensity sequences were presented together only with singly-varied duration sequences, whereas in Experiment 2, co-varied sequences were added to the mix. It is difficult to identify other factors that might have contributed to the observed outcome: within each language group, participants were drawn from the same populations and were tested in the same locations; the experimenters, procedures, and equipment were the same. Given these factors, the most reasonable interpretation of the finding may be that listeners’ perception of the simpler singly-varied intensity sequences was heightened in comparison with the more complex co-varied sequences.

The S-shaped function associated with intensity, especially in the singly-varied conditions, may have an auditory explanation. When listeners with normal hearing are asked to rate the loudness of a pure tone on a scale from “soft” to “too loud”, loudness growth is roughly linear up to a subjective “loud” rating, at about .4 subjective loudness units per dB (Allen et al., 1990; Olsen et al., 1999). After about 85 dB, the slope is steeper, about 1 subjective loudness unit per dB (Olsen et al., 1999, p. 303). However, Cox et al. (1997) report that loudness growth is different for speech than for tones. These authors conducted a study of loudness growth effects among normally hearing subjects who were asked to subjectively rate the loudness of speech stimuli varying in mean-level intensity (dB HL). These investigators report that at intensity levels characteristic of speech, the loudness growth function was more curvilinear than for tones: increasing intensity (dB HL) was associated with increasingly smaller increases in subjective loudness units between approximately 20 and 70 dB HL. (After approximately 70 dB, the relationship between unit increases in intensity and subjective loudness began to reverse.) In light of Cox et al. (1997), the “tapering off” of loud-first judgements in the current studies suggests that equal increments in the intensity of the more intense syllables may not have produced equal differences in the perception of their loudness.

The outcomes of particular interest for the main research questions were those associated with varied duration. In the singly-varied duration conditions in Experiments 1 and 2, significant and straightforwardly interpretable outcomes for duration were found only in the English group. With one exception (at MOD 2), the presence of any duration disparity in Experiment 1 was associated with more long-last groupings compared with the control sequence, MOD 0 (more baga responses when ga was longer and fewer when ba was longer). This general result was significant and provided overall support for the Long-Last Principle in (1). At a more granular level, the Incremental Long-Last hypothesis in (3b) predicted that proportional increases in MOD would be associated with decreases in baga responses. However, this hypothesis was not fully supported in Experiment 1 because changes in baga responses between MOD levels were often not proportional and there were irregular deviations from the expected linear function (see Figure 2a). In contrast to Experiment 1, the negative trend associated with duration for singly-varied sequences in Experiment 2 was linear and therefore more consistent with the Incremental Long-Last hypothesis. However, the size of the effect was smaller: a unit of MOD increase reduced the odds of a baga response by 44% on average (OR .56) in Experiment 1, whereas in Experiment 2, the odds were only 34% lower (OR .66).

In the Spanish group, there was a small but significant negative association between MOD and baga responses in Experiment 1. However, changes between levels were very small and only one difference between individual MOD levels (between MODs 0 and 3) was significant, in the direction predicted by the ITL. That the negative correlation was significant suggests a marginal trend supporting the Incremental Long-Last hypothesis. This was the outcome predicted by the ITL, and not what we would expect if listeners were attending to increased duration as a stress-marking cue. However, because the changes were so small, it is difficult to be confident of this interpretation. In Experiment 2, increasing MOD did not affect listeners’ responses in the singly-varied duration condition, with one exception: an increase in baga responses at MOD 2, consistent with a long-short grouping bias, was in the direction expected if listeners were attending to varied duration as relating to stress. It is difficult to assign a meaningful interpretation to this result in the absence of a clear trend, and given that outcomes across the two experiments were in conflict.

5.2 Intensity and duration together

An interesting outcome, consistent across experiments and across language groups, was that the fixed effect of Duration was less robust than that of Intensity. The ORs associated with Duration were .56 in Experiment 1 and .66 in Experiment 2, overall, and fixed effects of Duration were more robust in the English group.

Outcomes in the co-varied conditions in Experiment 2 provide two important new pieces of information. First, just as the fixed effect of Intensity was more robust than the fixed effect of Duration (the singly-varied sequences), Intensity was also the more dominant predictor of listeners’ responses in the co-varied conditions. These outcomes confirmed the Intensity Wins hypothesis in (2c). Perhaps the most interesting outcome was that even though its influence was less robust, manipulating duration reduced the size of the intensity effect: When there was a fixed duration disparity between alternating syllables, increasing intensity produced fewer responses representing loud-soft groupings (more baga groupings when ga was longer, fewer when ba was longer), and this effect generally strengthened as the duration disparity increased. This finding was consistent with the Incremental Long-Last hypothesis. Based on the outcomes of Experiment 1, this result in the English group was not surprising. However, it was also true in the Spanish group, even though the fixed effect of Duration was not robust in Experiment 1 and not significant in Experiment 2. Surprisingly, given the differences in the fixed effects, the effect on Intensity of increasing the duration disparity was comparable in the two language groups in the co-varied condition (see the ORs associated with the interaction Duration.disp*Intensity in Table 11).

The finding that increasing the duration disparity lessened the effect of manipulating intensity on listeners’ responses meant different things in the Competing and Co-operating conditions. In Figures 4 and 5, the Competing condition is represented by points labeled with squares (the blue lines at negative MOIs and the red lines at positive MOIs), and the Co-operating condition by the points labeled with triangles (the blue lines at positive MOIs and the red lines at negative MOIs). Trends are clearest in Figure 4a; there, functions associated with intensity are well differentiated due largely to differences in the baseline duration condition (the stacked circles at MOI 0). Taking Figure 4a as the point of reference, it can be seen that the functions associated with co-varied sequences converge with the baseline intensity function in the Co-operating condition, and diverge in the Competing condition. In the Co-operating condition (where one alternating syllable was long and the other loud), the interaction between the predictor variables meant that increasing the duration disparity produced increases in long-short responses. As noted in Section 2.3 (the discussion of Crowhurst and Teodocio, 2014), a possible explanation for this superficially incongruent effect, given listeners’ responses to varying duration in other conditions, may be related to the way in which the auditory system integrates intensity over time. When sounds of the same intensity have different lengths, the longer sound is perceived to be louder (see Gordon, 2005, and references cited there). This suggests that there may have been a tendency for listeners to perceive longer, low-intensity syllables as louder than the high-intensity short syllables in the Co-operating condition. It should be noted that the same auditory explanation might be proposed for Crowhurst and Teodocio’s (2014) finding of a long-last bias among Zapotec speakers for singly-varied duration sequences. However, that Crowhurst and Teodocio found different grouping biases in the Zapotec and English groups in their singly-varied duration condition when the same stimuli were used may indicate that listeners from different language backgrounds may respond differently, even at a purely auditory level.

For the theoretical questions explored in the current study, the outcome in the Competing condition is more interesting. Here, it can be seen in Figure 4 that the effect of increasing the duration disparity was to decrease responses representing loud-first groupings as the intensity disparity increased. In other words, increasing a duration disparity increased long-last groupings in both language groups. In the Spanish group, this effect was observed only when duration and intensity were varied together (in competition). Implications of the outcome in the Competing condition for the issues identified in Section 1 and Section 2 are discussed in the following section.

On the whole, generalizing over the two experiments, the Incremental Loud-First and Intensity Attenuation hypotheses were confirmed to an extent that permits a degree of confidence in the relationships they assert. However, as discussed in the results sections for Experiments 1 and 2, several irregularities in the form of unanticipated deviations from the expected (curvi)linear relationships were observed in both language groups in one or both studies. The functions associated with duration in Experiment 1 were not monotonic, as expected, and changes between levels were not proportional, most dramatically in the English group. Such irregularities are not unique to the studies reported here; they have also been found in prior ITL studies that have tested participants using sequences of synthesized syllables (Bhatara et al., 2013; Hay & Diehl, 2007), and sequences produced from naturally recorded speech (Crowhurst & Teodocio, 2014). On the whole, irregularities of this nature appear to have been more prevalent in the series of studies that have based stimuli on natural speech (Crowhurst & Teodocio, 2014, and the research reported here). This difference may reflect a cost of using stimuli prepared from resynthesized natural speech, as opposed to synthesized sequences. It is not possible to manipulate naturally produced speech to precisely control all aspects of stimuli, and this limitation may account for a degree of noise in the study’s outcomes. Another source of noise may have been linked to the way in which stimuli were presented to subjects. Other prominently published studies in which subjects heard stimuli in free field over loudspeakers (e.g., Iversen et al., 2008; Sluijter et al., 1997) have achieved clear results; however, a legitimate concern for some research would be that participants inevitably hear stimuli at different sound levels depending on their position in the room. The rationale for presenting stimuli in this way was that the issue of interest was whether listeners’ response patterns, based on a binary decision, would generally reflect the order of steps on an ordinal scale; the dependent measure was not a continuous variable that measured the finer details of listeners’ responses. I also considered that listeners naturally hear speech in free field, not through headphones. However, concerns about how the mode of presentation might affect listeners’ responses could be avoided in future studies by presenting sequences in a more controlled manner under laboratory conditions.

Another factor that may have contributed to the irregularity noted was that in the current studies, intensity-varied and duration-varied sequences were not segregated into different blocks as has been done in prior ITL studies, with the exception of Crowhurst and Teodocio (2014). In the current series of studies (the two reported here, together with Crowhurst & Teodocio, 2014), control, intensity-varied, and duration-varied sequences were presented to study participants in the same blocks because the investigator reasoned that this would provide a stronger test of the ITL. That is, we may be more confident that a loud-first or long-last bias is meaningful if it is observed under conditions in which listeners could be distracted by more variation. Another way in which the investigator worked to increase the rigor of the test was in designing the task in a way that did not direct listeners to think in terms of the relative amplitudes, durations, or prominence in general of the syllables in the alternating sequences they were hearing. Orienting subjects to the segmental properties of the alternating syllables (which do you hear – baga or gaba groupings?) provided an indirect measure of listeners’ perceptions, while directing their attention away from the acoustic parameters being manipulated. However, the very indirect framing of the task that was intended to provide a stronger confirmation of the ITL’s basic predictions may also have contributed to irregularity in listeners’ response patterns.