The research conducted within the scope of this doctoral thesis supports and complements the findings of previous studies and furthermore shows that the Rhythm Rule plays an important role in stress-timed languages like German and English. With regard to the acoustic correlates of the Rhythm Rule, the production data reveal that there are various possibilities in producing rhythmically regular structures, either by levelling or shifting secondary stress. Moreover, the implementation is also language-dependent. For German, syllable duration was determined as the decisive factor. In order to obtain a perceptible shift, the potentially clashing syllable is shortened and the duration is transferred to a following stressable syllable, leading to a real reversal of prominence. For English, on the contrary, reversing the prominence of F0 and an additional shortening of a potentially clashing syllable leads to perception of stress shifts.
The overall data reveal neuronal reflections of rhythmical processes during language processing. They confirm that rhythmically regular structures are advantageous as regularity is an important factor for the ability to build up predictions about the prosodic structure of the following speech signal. The conducted ERP data also give an important insight into the weighting of lexical violations and rhythmical deviations. Normally, deviations from lexical stress lead to higher processing costs. However, no reflections of these costs are found when these shifts are rhythmically licensed. In contrast, compliance with lexical stress leads to more costly processing if this adherence leads to a deviation from rhythmic regularity. Thus, deviation from lexical stress seems to be acceptable when this results in a harmonious rhythmical structure. Regarding the different types of rhythmic irregularities, either containing an additional lexical stress deviation or not, the ERP studies on German were able to show that stress clashes and stress lapses are in fact processed differently as they are reflected by two distinct negative components, due to the double violation (lexical and rhythmical deviations) in stress lapse structures versus rhythmical deviation only in stress clash structures.
39
The results of the ERP study on English compounds shed further light on the lexical status of potential targets of the Rhythm Rule. The finding of an N400 effect for stress shifted words, reflecting higher costs in lexical retrieval, are in opposition to the proposition that these words might not possess lexical default stress but context-dependent stress. Moreover, the results from this study further show that these shifts have to be rhythmically licensed in order to be acceptable. This is in line with the results from the German ERP studies, complementing and supporting the importance of rhythmical compliance in stress-timed languages.
Overall, the results of the ERP and the reaction time studies could show that the processing of rhythmically irregular structures is associated with higher costs as they are processed differently from well-formed structures. This is not only true for salient violations but also for rather subtle and allowed rhythmical deviations, even in natural contexts. These findings contradict the proposition that rhythmical regularity as well as the Rhythm Rule can be ascribed to a purely perceptual repair phenomenon. The compliance with rhythmical predictions and thus the application of the Rhythm Rule is beneficial and desirable for language processing. The results therefore show that the phenomenon of rhythmically induced stress shifts plays indeed an important role in the processing of English and German and that English as well as German listeners are sensitive to rhythmic deviations even if these are allowed forms in the respective language.
Due to the optionality of the Rhythm Rule, the analysis of more natural production data is of particular interest for future investigation. By inspecting spontaneous and thus more natural speech with regard to rhythmic regularity, the articulatory reality of the RR might be illuminated. To this end, a corpus study on the audio edition of the Spoken British National Corpus (Coleman et al., 2012) would enable us to gain deeper insight in which way and how regularly the Rhythm Rule is actually applied in the English language.
Since rhythm is a key element in language as well as in music and both domains follow the Principle of Rhythmic Alternation, future studies are planned to further highlight the relationship between music and language with regard to rhythmic processing. As a first step, a follow up study on the German stimuli (i.e.
phrasal verbs & compounds) with musicians as participants will be realised in the near future (in cooperation with Sonja Kotz, Richard Wiese and Ulrike Domahs).
40
Based on the findings by previous studies which showed an increased sensitivity of musicians to even subtle deviations in rhythmic and metric structures in comparison to non-musicians (cf. Koelsch et al., 2002; Schön et al., 2004; Tervaniemi et al., 2009; Marie et al., 2011), this study should deliver a finer-grained picture of the processing of these subtle rhythmic deviations in language processing. It can further give a deeper insight into the influence of musical expertise on language processing, i.e. the transfer of training effects on musical rhythm to language prosody.
With regard to the processing of stress clash structures in German, it was assumed that their particular difficulty might arise from the tension of lexical stress compliance and the concurrent rhythmical deviation. It was hypothesised that stress clashes are not directly and consciously recognised as deviations, leading to higher processing costs. Due to the fronto-central occurrence of the early negative component for stress clashes, it was assumed that they are kept longer in the auditory working memory for inspection and evaluation, which is described to be located in the fronto-central area (e.g. Kaiser & Lutzenberger, 2004; Eulitz & Obleser, 2007).
Support for this assumption comes from a study using the functional magnet resonance imaging (fMRI) method on different types of deviations from the prosodic foot structure in German nouns (Domahs et al., 2013b). It showed that mild prosodic violations lead to a stronger activation in frontal areas such as the inferior frontal gyrus (IFG), reflecting longer retention in the working memory due to higher demands for deviation detection in comparison to more severe violation types (Domahs et al., 2013b). In order to further test this localisation hypothesis for subtle rhythmic deviations, a follow up study using the fMRI technique has just recently been conducted (in cooperation with Katerina Kandylaki, Arne Nagels, Tilo Kircher, Ina Bornkessel-Schlesewsky, Ulrike Domahs and Richard Wiese). The high spatial resolution of this method makes it possible to review this hypothesis and to further investigate the connection between the pronounced ERP effects in certain regional areas and the actual localisation of rhythmical processing in the brain. This way, the two distinct processes reflected by different ERP components for stress clashes and stress lapses can be further disentangled and located.
41