Frequency

The stimuli consisted of a series of tones each of duration 200ms played at 80.95dB. The minimum frequency was 250Hz (reference tone) and the maximum frequency 279.82 Hz. The frequency intervals between tones were of incremental intervals of 0.0513 semitones. The on-screen picture was of three elephants and the children were asked to identify which made the different, highest sound.

3.2.2 Intensity

The stimuli consisted of a series of tones each of duration 200ms and a frequency of 250Hz. The minimum intensity was 61.472dB and the maximum intensity was 80.95dB (reference tone). The intensity intervals between tones were of incremental intervals of 0.5128dB. The on-screen picture was of three cartoon mice and the children were asked to identify which one made the different, quietest sound.

3.2.3 Rise Time

The stimuli consisted of a series of tones, each of duration 800ms played at 80.95dB at a frequency of 531.25Hz. The minimum rise time was a 15ms slope (reference tone) and the maximum was a 300ms slope. The fall-off was consistent at 50ms. The minimum rise time of 15ms was always used as the reference tone and incremental intervals were set at 7.0377ms. The on-screen picture was of three cartoon dinosaurs and the children were asked to identify which one made the sound with the different, gentlest beginning.

3.2.4 Duration

The stimuli consisted of a series of tones played at 80.95dB with a frequency of 250Hz. The minimum duration was 400ms (reference tone) and the maximum duration was 595ms. The duration intervals between tones were of incremental intervals of 5.1282ms. The on-screen picture was of three cartoon sheep and the children were asked to identify which one made the different, longest sound.

3.3 Estimating Thresholds

The Dino program uses a staircasing procedure in order to estimate threshold. Trials begin with the maximum difference (i.e. between level 1 and level 40 of 40 possible levels) and initially uses a two- up, one-down procedure (i.e. two correct answers at a given difference pairing, and the program moves to a closer (harder to discriminate) difference pairing; one incorrect answer, and the program moves to a more distant (easier to discriminate) difference pairing). After the fourth reversal, this becomes a three-up, one-down procedure. Initially, pairings move by eight levels in each stepchange

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(e.g. move from 1:40 to 1:32), following the fourth reversal this is progressively halved to a four, two then one level difference, in order to home in on the most accurate estimation. The final threshold figure is taken as the mean level from the fourth reversal.

3.4 Results

A series of t-tests were carried out to determine any between-group differences on the Acoustic Processing measures (Table 3-1).7

Table 3--3-1 Results of t-tests by Group for Acoustic Threshold tasks

Task AMC DLD df t p Mean (SD) Mean (SD) Rise Time (ms)a _{127.529 (81.81) 202.651 (65.86) 36 -2.922 .006} Frequency (semitones)a _{0.668 (0.537) 1.391 (0.497) 34 -3.978 .000} Duration (ms)a _{83.626 (44.065) 124.333 (51.97) 36 -2.571 .014} Intensity (dB) -3.038 (1.516) -3.421 (1.576) 35 .734 .468 Note: a) AMC < DLD

The DLD group had significantly higher thresholds (i.e required a greater difference to discriminate between stimuli) than the AMC group for RiseTime, Duration and Frequency. There was no group difference for Intensity.

The DLD group therefore had poorer sensitivity to three of the four acoustic cues to stress than the AMC group.

3.5 Discussion

The pattern of results obtained is similar to that found in previous studies, with the DLD group impaired in discrimination of Rise Time and Duration. This participant group also had particular difficulties with the discrimination of Frequency. Although frequency discrimination was preserved in the previous DLD sample studied by Richards & Goswami (2015), impaired frequency perception has been reported in other studies of DLD children (e.g. Cumming, Wilson, & Goswami, 2015) and so this result is not without precedent.

7 _{Three scores were not recorded by the software – two Frequency scores (one AMC child, one DLD child), one}

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In comparison with their age-matched peers therefore, the children with DLD had difficulties in discriminating fine-grained changes to Rise Time, Frequency and Duration – all of which are significant acoustic cues to the detection of stress and thereby language rhythm. We therefore expected that this poorer acoustic sensitivity would relate to the experimental tasks probing aspects of language rhythm.

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4 Entrainment

Before treating each experimental task individually in the following chapters, we will discuss a common factor which runs throughout the set of experimental tasks, which is the concept of entrainment.

4.1 Introduction to Entrainment

Entrainment was central to the construction of the experimental task battery, and was included across tasks as a support mechanism. We expected children with DLD to perform more poorly than the AMC controls in each of our experiments and we hypothesised that their difficulties would be related to poor processing of the acoustic cues to speech rhythm in the form of the amplitude envelope.

As a practising Speech and Language Therapist, however, I was also interested to find out whether there was a means of supporting the children in completing these tasks. Could there be a way in which rhythm could be used to provide a framework which would facilitate task completion?

Dynamic Attending Theory (DAT) and neural oscillation research suggested that prior entrainment to a rhythmic structure may have a facilitating effect on language processing.