During recording of the stimuli, a beat of 750ms between stressed syllables was induced in the speaker via headphones using a metronome setting of 80bpm. There was therefore a constant inter- stressed syllable interval of 750ms. The timing of this was verified and adjusted as necessary using Audacity software. The entraining beat also had an inter-stress interval of 750ms with 250ms intervals between any intervening weak beats (corresponding to unstressed syllables). Two ‘sets’ of entraining beats were given before each stimulus. In order to separate the two sets, a ‘blank’ beat was inserted between the sets. This was the temporal equivalent of inserting an extra ‘phantom’ inter-stress interval, meaning that the second set of entraining pulses remained consistent with the 750ms pulse.
Procedure
Two stimulus lists were created of 12 items each. Children listened to each list in either an entrained or unentrained version (for example, they might listen to List 1 Entrained and List 2 Unentrained). List and version were counterbalanced across participants as was order of presentation. Children completed each version as part of the wider task battery within a session. The complementary Figure 5-2 Soundwave of an example entrained three item stimulus - 'snow, men and spades'
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version was then completed in a separate session. The younger, language-matched children (YLC group) completed an Unentrained version of the task only910.
Children had already completed the PEPS-C chunking input task in a previous session, so were familiar with the concept underlying the task. For each trial, a picture was displayed on the laptop, with the screen divided in two. One set of pictures was displayed on each side of the screen. The left-hand side always displayed ‘three item’ responses, the right-hand side always displayed ‘two item responses’. The ‘A’ and ‘L’ buttons on the keyboard were used as the response buttons, indicated by a plain orange sticker.
Children were told that if they thought the words matched the left-hand picture set, they should press the left-hand button, and if they thought the words matched the right-hand picture set, they should press the right-hand button (the experimenter demonstrated by pointing). For entrained tasks, children were told they would hear some beeps first which were there to help them hear the pattern of the words. Children completed two practice items with experimenter feedback before proceeding to the experimental items.
9 This counterbalancing of Entrained and Unentrained lists across participants and sessions was repeated for all the
Experimental Tasks.
10 Whilst it would have been theoretically interesting to include Entrained tasks for the YLC group as well, we were mindful
of the testing burden for these younger children. In the light of results obtained for AMC and DLD groups, it was therefore decided no to gather additional Entrained data for the YLC group.
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Results
5.5.1 Scores
Each child’s score was calculated as the number of responses correctly identifying the stimulus. The maximum score for each list was 12.
A one-way ANOVA (DV-Score) was carried out to see if the groups differed in their accuracy levels (see Table 5-1). The AMC group was more accurate in their responses than both the DLD (p = .027) and YLC groups (p = .001), whilst there was no significant difference between the DLD and YLC groups.
Table 5--5-1 Results of one-way ANOVA for Score
Note: a) AMC > DLD b) AMC > YLC , c) Welch’s F used due to significant Levene’s test
Our prediction that the DLD group would be less accurate than typically developing children in detecting the word boundaries demarcating lists into groups of two or three items was therefore supported by the results.
5.5.2 Directions of bias
5.5.2.1 d’It was interesting to know whether there was a systematic tendency among the groups to prefer a particular response. To that end a measure of d’ was taken using 3-item-Target, 3-item Response as the hit rate, and 2-item-Target, 3-item-Response as the false alarm rate.
Table 5-2 Results of one-way ANOVA for d'
Note: a) AMC > DLD b) AMC > YLC , c) Welch’s F used due to significant Levene’s test
The AMC group had a significantly higher d’ score than the DLD group (p = .026) and the YLC group (p = .001) whilst the DLD and YLC groups did not significantly differ on their d’ score. This indicates that the responses of the DLD and YLC groups were more consistently biased than those of the AMC group.
Score (max 12) AMC Mean (SD) DLD Mean (SD) YLC Mean (SD) df F p Scorea,b 11.182 (1.56) 9.143 (2.74) 8.545 (2.365) 2, 29.060 10.522c .000 d’ AMC Mean (SD) DLD Mean (SD) YLC Mean (SD) df F p d’a,b 2.396 (.716) 1.452 (1.262) 1.184 (1.09) 2, 29.023 10.54c .000
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5.5.2.2 Preference for two or three item responses
In order to discover in which direction this bias lay (towards ‘two items’ or towards ‘three items’) a calculation was made of the number of times each child pressed the ‘two item’ and ‘three item’ response buttons (no bias would result in a score of 6 for each).
Table 5-3 Mean number of responses of 'two items' and 'three items'
A 3 x 2 repeated-measures ANOVA (Group [AMC, DLD, YLC] x Number of Items [two, three]) revealed a significant overall effect of item number (F(1, 55) = 31.606, p =.000, with children being
significantly more likely to choose the ‘two item’ response. There was also a significant group*item number interaction, F(2) = 4.562, p = .015, indicating that the pattern of responses varied
significantly between the groups.
Response AMC Mean (SD) DLD Mean (SD) YLC Mean (SD) ‘two items’ 6.455 (1.654) 7.857 (2.35) 8.182 (2.039) ‘three items’ 5.545 (1.654) 4.143 (2.35) 3.818 (2.039)
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Figure 5-4 Graph showing the mean number of responses by group for two- and three-item responses. The solid black line indicates where a ‘no response bias’ line would fall.
Figure 5-4 shows the mean number of responses for each item number and illustrates the nature of the interaction. All groups have a tendency to prefer ‘two items’ as a response, however for the AMC group this is small and non-significant. On the other hand, the bias for the DLD and YLC groups towards ‘two items’ is clear.
The repeated-measures ANOVA was then conducted for each group individually. There was no significant difference between the number of 2-item and 3-item responses for the AMC group, F(1, 21) = 1.661, p = .211, however there was a significant difference for the DLD group, F(1, 13) = 8.753,
p = .011, and the YLC group, F(1, 21) = 25.2, p = .000, with significantly more ‘2-item’ responses than
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Children from the AMC group were therefore able to accurately distinguish stimuli containing two or three items. Children from the DLD and YLC groups were less accurate and their errors displayed a preference for responding that stimuli were composed of two items.
5.5.3 Reaction Times
Reaction Times (RTs) were recorded by the Presentation software as the time between the onset of the stimulus playback and the response button being pressed in units of a tenth of a millisecond (0.0001s).
A further adaptation was made in order to avoid individual, unrepresentative scores affecting the final results. For each child, the mean and standard deviation of their RTs were calculated. Any individual time which lay outside 3 standard deviations of the mean was discarded and the child’s mean time re-calculated11. In practice, only for one child did this make a difference to their final
mean RT.
5.5.3.1 Effect of Group on Reaction Times
A one-way ANOVA (DV – RT) was conducted in order to see if the groups differed in their RTs. There were no significant group differences in the RTs, despite a general tendency for the DLD and YLC responses to be slower than the AMC group.
Table 5-4 Results of one-way ANOVA for Reaction Times
Speed of reaction therefore did not vary between the participant groups.
Effect of Entrainment
We predicted that listening to an entraining beat would have a facilitating effect on task performance, resulting in higher scores in the Entrained version of the task.
11 This procedure was carried out for each experiment in which Reaction Time was recorded.
AMC Mean (SD) DLD Mean (SD) YLC Mean (SD) df F p Reaction Time (.0001s) 40268 (8988) 47540 (18300) 52095 (24765) 2 2.264 .114
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