Chapter 2. The distribution of phonological information within Spanish words
2.7 The role of features: manner and place of articulation
This section tests a different way of calculating the information profiles. For the calculation of entropy, instead of using the finite set of segments, I now use features such as manner and place of articulation.
Manner of articulation speech features are best transmitted by the auditory channel, whereas place of articulation are best transmitted by the visual channel (Robert-Ribes, Schwartz, Lallouache & Escudier, 1998): in a noisy environment, seeing the speaker's face improves message intelligibility (Girin, Schwartz & Feng, 2001). Conflicting information from the two channels generate fused responses reflected, for instance, in the McGurk effect (McGurk & MacDonald, 1976): when presented simultaneously with the sound ‘ba’ and an image of a face pronouncing ‘ga’, people perceive ‘da’.
I compare the results of the last two sections with similar information profiles calculated with the finite sets of 17 ‘manner of articulation (plus vowel)’ and 19 ‘place of articulation (plus vowel)’ features (see Appendix A for full lists of features).
Phoneme
Fig 2.12. Information profiles of 6-segment tokens from the speech corpus, calculated using the finite scheme of phonemes, of manner of articulation features and of place of articulation features.
Manner and place of articulation information are distributed differently in the word. Figure 2.12 shows the profiles by phoneme and by manner and place of articulation of 6-segment long words from the corpus. Place of articulation is most informative in the word-initial position, where manner of articulation is relatively redundant in that position. The highest redundancy in the last segment is best captured by the phoneme analysis.
Figures 2.13 and 2.14 present a comparison of the information profile slopes and mean relative entropy generated by the segment finite set (the results already presented in § 2.5 and § 2.6) and by the manner and place of articulation finite sets.
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Figure 2.13. Values of the slopes (left) and of the mean Hrel (right) averaged over the four word lengths of the analysis by phoneme, by manner of articulation and by place of articulation using the assimilation transcriptions of the speech (adult) corpus and the child-directed corpus.
Figure 2.13 shows that manner of articulation yields the flattest contours, particularly for the tokens, both in the adult and the child-directed corpora.
In terms of average relative entropy, both manner and place of articulation behave similarly, with higher values than the phonemes, except in the adult types (lexicon), where the most efficient encoding seems to be attained with phonemes.
Manner of articulation information is more evenly spread across words than place of articulation, and in speech (tokens) this suggests that it is more immune to noise and could have an important role in auditory speech production and recognition. This means that manner of articulation encodes speech robustly in the absence of visual contact. In the lexicon (types), the even spread of manner of articulation information suggests that it produces a more efficient encoding and thus could have an enhanced role in the organization of the phonological mental lexicon. This is also supported by the fact that manner of articulation slopes are steeper in the types than in the tokens – manner of articulation might be encoding internal word structure in the types, but not in the tokens.
Place of articulation, providing visual information, shows much steeper slopes than manner of articulation, even steeper than phoneme slopes. This suggests that place of articulation is not an efficient dimension to organise the mental lexicon storage; however, the sharp difference of entropy between word-beginnings and endings is a good clue to speech segmentation (see Figure 2.12).
Summing up, while manner of articulation seems to encode auditory information more robustly, place of articulation encoding may be responding to the pressure to facilitate speech segmentation.
2.8 Conclusion
This chapter has examined the information profiles of words found in spoken language, a measure of how well different word systems are adapted to the informational requirements of their representational spaces. The information profile is calculated with a computationally inexpensive methodology that still finds reflections of the pressures acting on the distribution of phonological information within Spanish words. The consistency of the profiles’ behaviour over four independent word groups (words of length 4, 5, 6 and 7) supports the robustness of this method. The information profile calculated with different finite sets (segments and features) show comparable results, each reflecting different aspects of the phonological information structure of words.
The profile found in the adult lexicon supports the claim that it reflects phonological distributional features that allow an optimal strategy of storage in the brain. However, the features of the lexicon of child-directed speech do not respond in the same way. Caregivers’ speech is adapted to meet other critical demands that interfere with an efficient storage strategy at this early age.
The profiles of two different token sets (adult-directed and child-directed speech) show that they are equally well adapted to good communication over a potentially noisy medium.
The vocabulary employed with children has a more marked drop in entropy levels at the end of words, which could enhance word-boundary recognition and help with lexical acquisition. In adults, segmentation cues are clearer in the corpus, helping with speech stream segmentation, one of the crucial problems of language recognition.
Calculation of the information profiles generated by manner and place of articulation features suggests that while manner encodes a robust auditory representation of speech, place may serve as a cue for speech segmentation.
In conclusion, I have shown that information profiles of spoken words and of the lexicon are a useful tool to measure distributional aspects of large samples of language, and can be used to test and potentially falsify particular aspects of psycholinguistic theories about speech production and recognition, the mental lexicon, and lexical acquisition.