 Relative Voicing Fraction on the underlyingly voiceless plosives in our data: /t/ and /k/. We proceeded by sequential position (1, 2, 3, 4). For each plosive, we fitted a linear mixed effects model containing random intercepts for Speaker and Repetition and fixed effects for the Plosive Voicing Fraction values of immediately adjacent plosives, ICI Site levels of immediately adjacent ICIs, and the identity of the plosive (/t/ or /k/). We removed any non-significant predictors, and then assessed, through inspection of alternative models' AIC values, whether replacing fixed effects by interactions improved model fit, and whether adding random slopes for Speaker and Repetition did. The latter was not the case for any of our models, and the random effect for Repetition did not contribute significantly to model fit in any. We also assessed whether replacing Plosive Voicing Fraction values of immediately adjacent plosives by corresponding Plosive Underlying Voicing improved model fit; this was to allow for the possibility that underlyingly voiced and voiceless plosives condition adjacent plosives differently, even if their own surface voicing is identical. In fact, this replacement reduced fit in all of our models. This is consistent with the prevalence of surface voicing throughout our plosive sequences and makes the identification of possible spreading 'triggers' rather hard. We return to this issue below; first we report the final models for /t/ and /k/ voicing fractions and present corresponding conditional inference regression trees for visualization. We do not report modelling for /t/ and /k/ in sequential position 4 (-CC#CC-) because the model for position 3 (-CC#CC-) is clear on the relationship between the third and fourth hold phases. We modelled the voicing of /t/ and /k/ in position 1 across two, three, and four-plosive sequences (N = 545). A pertinent distinction is between two and four-plosive sequences on the one hand and three-plosive sequences on the other: In the latter (-VC#CCV-, -VCC#CV-), the plosive hold phase is separated from the following plosive by a predicted epenthesis site; in the former (-VC#CV-, -VCC#CCV-) it is not. An associated interaction between ICI Site and the voicing of the following plosive might provide evidence for predicted epenthesis sites and other sites influencing the extent of [voice] spreading differently. In fact, the best fit linear mixed effects model has fixed effects only: for the following ICI Site (F(3,537) = 34.43, p < 0.001) and for Plosive Voicing Fraction for the following hold phrase (F(1,538) = 25.06, p < 0.001). Inspection of level comparison coefficients suggests that in relation to ICI Site, the main explanatory factor is whether the plosive is part of a coda cluster (-VCC#CV-, -VCC#CCV-), or followed by a word boundary (-VC#CV-, -VC#CCV-). This grouping of ICI sites is reflected in a conditional inference regression tree model, as seen in Figure 13. (We residualized Plosive Relative Voicing Fraction for Speaker prior to tree modelling, as above.) The tree model also confirms that in both contexts there is the same positive relationship between the voicing fractions of the first and second hold phases.
For the purpose of modelling the voicing of /t/ and /k/ in position 2, we restricted our attention to three and four-plosive sequences (N = 420). For two-plosive sequences (-C#C-), the relationship between the two plosives is clear from the preceding analysis, and there is no third plosive to include in the model. The preceding and following ICI sites are logically related, such that modelling requires only one associated variable; we will refer to this as ICI Sites (levels '4_5' for -C#CC-, '6_7' for -CC#C-, and '8_9' for -CC#CC-). In relation to the preceding ICI, sites 4 and 6 are predicted epenthesis sites; site 8 is not. In relation to the following ICI, site 9 is a predicted epenthesis site; sites 5 and 7 are not. We were again particularly interested in whether these groupings had any relevance for the relationships between the three successive plosives in hold phase voicing.
Again, however, the best fit linear mixed effects model has fixed effects only, for ICI Sites (F(2,412) = 42.96, p < 0.001), Plosive Voicing Fraction for the preceding hold phase (F(1,398) = 37.84, p < 0.001), and Plosive Voicing Fraction for the following hold phase (F(1,414) = 8.72, p = 0.003). Inspection of level comparison coefficients suggests that the grouping by ICI Sites separates -C#CC-and -CC#C-on the one hand from -CC#CC-on the other. In the former, the hold phase is preceded by a predicted ICI site, while in the latter, it is followed by one. In the latter context, hold phase voicing is less extensive. The same pattern is visible in a conditional inference regression tree model, as shown in Figure 14. The tree model further suggests that the effect of the voicing fraction of the following hold phase is only observed when that hold phase is not preceded by a predicted epenthesis site. If we interpret the positive correlation between the Plosive Voicing Fraction values for the second and third hold phases in terms of (leftward or rightward) spreading, this pattern might provide evidence for the predicted epenthesis site 9 (-CC#CC-) limiting the extent of this spreading. We should note that site 9 is a word boundary site, and we have seen evidence for word boundaries constraining hold phase voicing relations in the analysis for sequential position 1 above. We will return to this below. For the purpose of modelling the voicing of /t/ and /k/ in third position, we restricted our attention to four-plosive sequences only (N = 248). Here (-CC#CC-), the hold phase is preceded by a predicted epenthesis site and word boundary; the following ICI site is not a predicted epenthesis site. The best fit model has fixed effects for both Plosive Voicing Fraction for the preceding hold phase (F(1,245) = 7.69, p = 0.005) and Plosive Voicing Fraction for the following hold phase (F(1,245) = 40.49, p < 0.001). The latter effect is the stronger of the two, consistent with the observation above that the correlation of voicing fraction values across ICI site 9 is relatively weak.

ICI and hold phase voicing
We now return to our earlier observation that while ICI sites 4, 6, and 9 are not distinguished by the occurrence of fully voiced ICIs between fully voiceless hold phases, they are associated with more substantial voicing in contexts with relatively little voicing. We illustrated this with the density scatters in Figure 7, which show a relative scarcity of data points in the bottom left corners for sites 4, 6, and 9. Another look suggests that more generally, low values for Mean Plosive Voicing Fraction are comparatively rare for these sites. This is primarily due to hold phases following sites 4, 6, and 9 having relatively high voicing fractions, as shown in Figure 15. (The effects of sites 4 and 6 are also reflected in the split for ICI Sites-'4_5' and '6_7' versus '8_9'-in the tree model in Figure 14.) As we pointed out above, while we would not expect epenthetic vowels to block [+voice] spreading, it is possible for robustly voiced ICIs to give rise to voicing tails into subsequent underlyingly voiceless hold phases, thereby contributing to the spread of surface voicing. To check whether rightward spread of voicing from voiced ICIs might provide a reasonable account for the data pattern described here, we quantified rough hold phase 'voicing contours' for partially voiced instances of /t/ and /k/ (that is, instances with voicing fractions between 0.1 and 0.9, in line with Davidson, 2016), by subtracting the voicing fraction for the second half from that for the first half. On the resulting scale, a value of 1 means all voicing is located in the first half of the hold phase; zero means the (partial) voicing is evenly dispersed across the hold phase; and -1 means all voicing is in the second half. We then modelled the resulting variables, which we will call Plosive2 Contour (for /t/ and /k/ in sequential position 2) and Plosive3 Contour (for /t/ and /k/ in sequential position 3) to assess the relevance of the preceding ICI site.
Mixed effects modelling of Plosive2 Contour with Speaker as a random intercept and preceding ICI Site as a fixed factor reveals a significant effect of the latter (F