Electropalatography (EPG)

EPG has been used in speech research for over twenty five years for analyzing connected speech processes such as coarticulation and assimilation. EPG has also been used to study speech related disorders such as cleft-palate and hearing impairment. The EPG consists mainly of two components, the artificial palate and an electric device detecting and displaying patterns of contact between the tongue and the palate during the process of speech. The subject wears an acrylic palate with silver sensors attached to the surface and connected with lead wires to a computer. When the tongue comes into contact with one of the sensors, a circuit is completed and the signals travelling through the lead wires are registered by the computer.

There are different EPG systems used today for speech research. For this study, the Articulate EPG palate was used. Due to the relatively high price of the articulate palate, in addition to the time-consuming process of having one made, only two participants were recorded for the EPG data; myself and another native speaker of TLA. The first Reading EPG palate made for myself was unsuccessful since it did not account for velar closures; thus, another Articulate EPG palate was ordered and used for this study. Both participants in the EPG data investigation used the Articulate palate.

There are a total of sixty two sensors embedded on the surface of the artificial palate. These sensors are arranged in eight horizontal rows each row having eight sensors except for the anterior row which has only six sensors. The first row corresponds to a line immediately behind the upper front teeth and the last row

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corresponds to a line between the hard and soft palate. The sensors within each row are evenly spaced but the distances between the rows vary for different parts of the palate, see Figure 3.1.

Figure 3.1 Articulate EPG palate used in this study

Authors have differed in the way that they divide up the artificial palate into different zones. Hardcastle (1989) divides the artificial palate into three zones: alveolar rows 1-3, palatal rows 4-5, and velar rows 6-8. On the other hand, Recasens et al. (1993) preferred a division consisting of two major zones: the alveolar zone from rows 1 to 4 and the palatal zone from rows 5 to 8. They then further divided the alveolar zone into two subzones: the front alveolar (rows 1 to 2) and the post-alveolar (rows 3 to 4). The palatal zone was also subdivided to create a pre-palatal zone (rows 5 to 6), a medio- palatal zone (rows 6 to 7), and a post-palatal zone (row 8).

In this study, the artificial palate was divided into three zones following Hardcastle (1989). Rows 1-3 correspond to the alveolar region and rows 6-8 the velar region (figure 3.2). This division was due to the fact that no palatal sounds were investigated with alveolar and velar stops constituting the main area of investigation.

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Figure 3.2 EPG palate showing rows corresponding to alveolar and velar contact regions EPG has many advantages over the other devices which have previously been used in articulatory studies. EPG allows the monitoring of changes in tongue-palate contact patterns of a speech segment over time at intervals of ten milliseconds. This method enables us to trace the developing pattern of contact between the tongue and the roof of the mouth: where, when, and how major constrictions start and end. It also enables the identification of the presence or absence of open oral tract during the production of stop consonant sequences signaling the presence or absence of vocalic elements during their production. As EPG allows continuous speech to be recorded, it offers a significant advantage for the study of connected speech. The most important advantage that the EPG has over acoustic analysis is its ability to measure the overlap duration between two adjacent segments. This proves useful not only in investigating the degree of gestural coordination but also the amount of overlap between stops under investigation. Furthermore, the EPG display also provides acoustic data analysis accompanying the EPG data display which is also used to facilitate durational

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measurements of segments. Furthermore, the acoustic analysis accompanying the EPG was also used to infer articulatory behaviour from the other acoustic data.

However, this device also has some limitations, one of these being the fact that it only records contact on the hard palate and as a result, the area of the palate where contact for /k/ occurs is sometimes incompletely recorded (Pouplier et al. 2010:624). Borden et al. (2003:224) also point out the fact that the artificial EPG palate lacks sensors at the most anterior part of the alveolar ridge, the teeth, in addition to the velum and therefore no there is no record of any articulatory contact for these regions. They also state that the uneven distribution of the sensors on the surface of the palate may provide a large amount of data where there is sensor concentration but less detail where there is lack of concentration of the sensors. In addition, the EPG cannot record bilabial closures since it does not cover the lips. However, this does not affect this study since only lingual-palatal stops were investigated. Nevertheless, even with these limitations, the EPG was very useful for investigating the articulatory aspects of speech sounds in this study.

The EPG was used to measure durations of singleton stops and stop clusters adjacent to the word boundary. ICI durations within coda, onset, and word boundary positions were also measured. In addition, the occurrence or absence of stop releases across the word boundary and the overlap period between two stops adjacent to the word boundary was also measured from the EPG data.

The duration of a stop HP was identified as the interval between the first EPG frame showing complete closure in the oral tract to the last EPG frame showing the same closure as seen in Figure 3.3. In this token, /dag#dam/, the duration of /g/ was measured starting from frame 184, which is the first frame showing a complete velar closure, to frame 198 which is the last frame with the same velar closure. Given that the duration of each EPG frame is 10ms, the total duration of singleton coda /g/ in this example was 15x10=150ms.

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Figure 3.3 EPG frames 184-198 corresponding to duration of velar /g/ in the token /dag#dam/ normal speech rate

In the case of ICI measurements, the ICI duration between two stop consonants C1C2 was identified as the interval between the first EPG frame showing an open oral tract following C1 closure to the last frame showing an open oral tract preceding the C2 closure. In the token /ʕagd#gdi:m/ Figure 3.4, a word boundary ICI can be seen between the alveolar and velar closures. This ICI begins from EPG frame 80 which is the first frame following the alveolar closure to frame 83 which is the last frame showing an open oral tract preceding the velar closure in frame 84. The duration of the ICI period in this example was measured as 4x10=40ms.

Figure 3.4 EPG frames 80-83 corresponding to #ICI duration the token /ʕagd#gdi:m/ normal speech rate

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As for the duration of stop clusters C1C2 in SF and SI positions, these were identified as the interval between the first frame showing complete closure for C1 to the last frame showing closure for C2 prior to the release. This is further highlighted in the SF cluster /gt/ in the token /wagt#tal/ Figure 3.5 where C1 is not released until after the formation of the C2 closure and therefore there is no ICI.

Figure 3.5 EPG frames 335-352 corresponding to duration of coda cluster /gt/ in the token /wagt#tal/ normal speech rate

The syllable-final cluster /gt/ in this example was measured as the interval between frame 335 showing the onset of velar closure of C1 to frame 352 being the last frame of the C2 alveolar closure. The syllable-final cluster /gt/ in this example was measured as 180ms. In cases where a release between the hold phases of two stops occurs resulting in an ICI, being a by-product of the cluster production, the ICI duration was included in the total duration of the cluster.

As for the case of measurements taken for overlap duration where the release of a stop is masked by the formation of a following stop, the duration of overlap was identified as the interval between the first frame where a second closure is formed while

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retaining a previous closure to the last frame where the second closure is released, i.e. the overlap of the hold phase of two gestures. These overlap durations can be seen in figure 3.5 and 3.6. In the token /fak#tal/ figure 3.6, the release of velar /k/ closure is masked by the formation of the alveolar /t/ closure resulting in a period of gestural overlap. The overlap period was measured as the interval between frame 294 exhibiting the occurrence of both an alveolar and velar closure and frame 297 which is the last frame exhibiting the double closure prior to the release. The overlap interval in this example was measured as 4x10=40ms.

Figure 3.6 EPG frames 294-297 corresponding to duration of word boundary overlap in the token /fak#tal/ normal speech rate

For each investigation, the EPG data was first analyzed in terms of the total productions of both speakers in order to find out the general pattern. Later, and in order to find out if any variability existed between speakers, the data for each speaker was analyzed separately.