Experiment 1: Testing the Switch Procedure

tions will be discussed here. After introducing the method and procedure applied in the infant tests, section 6.2 presents the results of the infant con-trol test (Experiment 1). Section 6.3 deals with some methodological issues and introduces a new perspective on data collected with help of the Switch Procedure. Subsequently, this new perspective will be applied to the data gathered in Experiment 1, resulting in two additional analyses (section 6.4).

Finally, section 6.5 closes the present chapter with a summary and some concluding remarks. Infants’ sensitivity to laryngeal and length contrasts in stops will be addressed in chapters 7 and 9.

6.2 Experiment 1: Testing the Switch Procedure

As became clear above, the Switch Procedure is a habituation paradigm applicable to subjects of a broad age range. It is based on the assumption that, following a habituation phase, subjects will differ in their reactions to familiar and novel stimuli (‘same’ and ‘switch’ trial, respectively). A pre-and a posttest control for infants’ general attention to the task (see below for a more detailed description).

The rationale behind Experiment 1 was to assure the appropriateness of the paradigm for revealing infants’ speech contrast discrimination skills.

Although the Switch Procedure is an established method in infant speech perception research (see, e.g., the multitudinous studies by Janet Werker and her colleagues, in particular Stager & Werker 1997; Werkeret al.1998a;

Pater et al. 2004, but also more recently by others as, e.g., Curtin et al.

2007; Altvater-Mackensen & Fikkert 2010; Fikkert 2010; see also chapter 4), evidence for successful operation at the Baby Speech Laboratory at the University of Konstanz was desired.

The present thesis is mainly concerned with infantsdiscrimination skills and aims to avoid increasing task demands by enticing subjects to pro-cess auditory stimuli as meaningful signifiers associated to specific objects.

Therefore, a simplified variant of the Switch Procedure with a checkerboard pattern as visual stimulus was applied (see below and appendix A). Since developmental aspects were not a crucial issue in the present control exper-iment, the age range of the participants was confined to 6 to 8 months, a period in early infancy that is associated with very good perception skills (see chapter 4). Furthermore, only infants from a German language back-ground participated in the control test. In order to demonstrate that the Switch Procedure reliably shows infants’ recognition of changes in the speech signal, a phonetically highly dissimilar contrast was chosen for the control

test. It was assumed that, if the method – the checkerboard variant of the Switch Procedure – worked well, 6- to 8-month-old infants should easily distinguish two very different sound sequences.

6.2.1 Method 6.2.1.1 Participants

16 German infants aged 6 to 8 months (mean age 6 months, 28 days; 7 girls) were included in the study. All subjects were without apparent health problems. At the time of testing, they lived in Konstanz and the vicinity.

A language questionnaire was used to ensure that the infants were exposed to their mother tongue at least 90% of the time. In addition, seven more infants were tested. Their data had to be excluded from the analysis because they started crying (n= 1), they were too fussy (n= 3), did not habituate (n = 1) or due to experimenter error or equipment failure (n = 2). This amounts to a drop-out rate of 30.4%. All infants were rewarded for their participation with either a T-shirt or a bib of the Baby Speech Lab or with a small toy or a book and parents were reimbursed for parking fees or bus tickets.

6.2.1.2 Stimuli

Infants were tested on their ability to discriminate the pseudo-words pata ["p^ha:t^ha] and medo ["me:to]. They were spoken by the same speaker who also recorded the stimuli for the pilot perception tests presented in section 5.3. Using a Sennheiser MD 421 U microphone, 80 productions ofpata and 74 productions of medo were recorded as .wav-files onto a portable stereo audio recorder (TASCAM HD-P2) at a sampling rate of 44,100 Hz. Of each pseudo-word, seven items were selected. UsingPraat (Boersma & Weenink 2007), they were scaled for intensity at a 70 dB level and ordered in a way that the firstpata was similar in its intonation contour to the firstmedoand so on. The seven items of the respective pseudo-words were concatenated with an ISI of 1.5 s and the same amount of silence was put at the beginning of each trial. The duration of the single medo items was approximately 0.7 s each, the completemedo-trial had a length of 15.6 s. With a duration of approximately 0.6 s each, the pata items were slightly shorter, resulting in an overall trial length of 14.5 s. Infants were balanced across conditions (habituation onmedovs. habituation onpata) and test order (‘same’ before

‘switch’ trial vs. ‘switch’ before ‘same’ trial).

6.2 Experiment 1: Testing the Switch Procedure 147 An additional nonsense VCV-sequence was recorded for the pre- and posttest. For this purpose, the same speaker who spoke the test stimuli recorded 80 instances of [i"lo].¹ Seven of the [i"lo]-sequences were selected, which differed in intonation but were similar in voice quality. Each item was scaled for intensity at a level of 70 dB. A sequence of seven items was created by concatenating the items with an ISI of 1.5 s and a 1.5 s silent interval was also added at the beginning of the trial, resulting in a trial length of approximately 14 s.

As a visual stimulus a black-and-white checkerboard pattern with some of the originally white squares coloured in blue, red, and yellow was used.

During presentation the coloured squares were changed randomly. This stimulus was used for the habituation as well as for the test trials. The visual stimulus for the pre- and posttest was a coloured toy waterwheel (‘spinner’). On the video, the base of the waterwheel remained stationary while the wheel rotated clockwise.² Additionally, a blinking red spot was used as an attention-getter to focus infants’ gaze to the screen between the individual trials.

6.2.1.3 Auditory Stimulus Conditions

The contrast to be discriminated was the same for all participants – pata versus medo. One half of the infants heard pata in the habituation phase and as ‘same’ trial and medo was presented as ‘switch’ (condition ‘pata’).

For the other half of the subjectsmedo was the habituation and ‘same’ trial stimulus andpata functioned as ‘switch’ (condition ‘medo’). A preselected order was assigned for infants’ participation making sure that male and female participants were evenly distributed across conditions; also, test order – ‘same’ trial before ‘switch’ trial (sa/sw), ‘switch’ trial before ‘same’ trial (sw/sa) – were counterbalanced across subjects.

1The same pre-/posttest items were intended to be used in the experiments on infants’

sensitivity to laryngeal and length contrasts presented in chapter 7 (Experiment 2) and in chapter 9 (Experiment 5). Similar to the stimuli used in the pilot perception test with adults (see section 5.3), the stimuli in Experiments 2 and 5 consisted of the sound sequence schwa plus labial stop plus /a/. For the pre- and posttest, the sound sequence[i"lo] was chosen since it differs from the test stimuli in Experiments 2 and 5 in vowel and consonant quality but has the same iambic stress pattern.

2Thanks are due to Paula Fikkert, Nicole Altvater-Mackensen and the Baby Research Center in Nijmegen, who were so kind as to provide a copy of the visual stimuli they obtained from Janet Werker for their own experiments.

6.2.1.4 Equipment and Apparatus

All tests were conducted at the Baby Speech Lab at the University of Kon-stanz. In a dimly lit sound-attenuated room infants sat on their parents’

laps facing a 120 cm diagonal flat screen monitor (NEC LCD Monitor Mul-tiSync LCD 4610) at a distance of approximately 130 cm, framed by a three-sided experimental booth. The audio-stimuli were presented from speakers (Mackie HR 624, high resolution studio monitors) at the side walls of the experimental booth, directly to the right and to the left of the screen. Ap-proximately 20 cm below the screen a digital video camera (Panasonic NV-GS500) was hidden behind a black cloth revealing only the lens to record infants’ head- and eye-movements. To prevent parents from hearing the stimuli and inadvertently influencing the child during the test, parents were asked to wear closed-eared headphones (Beyerdynamic DT 770) over which male vocal music was played. The experimenter, who ran the test invisibly for parent and child from behind the experimental booth, wore identical headphones with the same music playing during the test, in order to avoid unintended influence on the control of the experiment. Thus, the experi-menter did not know whether the trials played were habituation trials or test trials. The experiments were run with the Habit X software (Cohen et al.2004). The infants’ head- and eye-movements were monitored on the computer screen behind the experimental booth and were recorded directly onto a Macintosh computer (Mac Pro, Dual-Core Intel Xeon, Mac OS X, Version 10.4.10) with the iMovie program. To compute the infant looks in the Habit program, the experimenter pressed a designated key on the computer keyboard when the infants looked at the monitor. It was released as soon as the child turned his/her gaze away from the screen. The video recordings were used for offline coding afterwards. A schematic depiction of the laboratory set-up is presented in figure 6.1.

6.2.1.5 Procedure

Prior to the experiment, infants were given some time to accommodate to the location. In the meantime the experimenter explained the procedure to the parents. The parents filled in a questionnaire regarding the language background of the child and signed a consent form, in which the proce-dure was explained again. Both the consent form and the questionnaire are attached in appendix G.

As indicated above, infants were tested using a simplified variant of the Switch Procedure (cf. appendix A). Since the present experiments focus

6.2 Experiment 1: Testing the Switch Procedure 149

Figure 6.1: Schematic depiction of laboratory set-up for experiments using the Switch Procedure.

on infants’ discrimination abilities and not on their word-learning skills a checkerboard pattern was used as a visual stimulus instead of an object, which might be linked to a label (the auditory stimulus) by the older infants (see Stager & Werker 1997 and chapter 4). The procedure functions as fol-lows. A preset criterion determines when the habituation phase ends. Each trial begins when the infant fixates on the flashing red spot that serves as attention-getter and is run until the end of the trial, irrespective of whether the infant remains attentive for the complete trial duration or not (fixed trial length as opposed to infant-controlled trial length). On the first trial, subjects are presented with the pretest stimulus, the sound sequence [i"lo]

paired with the ‘spinner’. During the habituation phase, the checkerboard pattern is shown and repetitions of only one auditory stimulus are played to the infant. Looking time is calculated online. When the total looking time across a four-trial block decreases to the preset criterion (65% of the first four-trial block), the habituation phase ends. The maximum number of habituation trials to reach criterion is 28 trials. The habituation phase is succeeded by two test trials. One trial is a ‘switch’ trial in which the checkerboard pattern is presented together with a new auditory stimulus.

The other test trial is a ‘same’ trial, that is, the combination of checker-board and sound stimulus that occurred already in the habituation phase is

presented once more. It is expected that if infants are sensitive to the sound contrast, attention should increase during the ‘switch’ trial and be low in the ‘same’ trial, resulting in longer looking times during the ‘switch’ than during the ‘same’ trial. If, however, infants fail to recognise the stimulus change, there should be no significant difference in looking time for ‘same’

and ‘switch’ trial, respectively. Finally, a posttest trial is presented, identi-cal to the pretest, in which the nonsense label[i"lo]paired with the ‘spinner’

appears again. If infants are still involved in the experiment, their looking time recovers to near pretest level during this final trial.

The whole experiment lasted approximately 5 to 10 minutes, depending on how fast infants reached the essential criterial decline in looking time during the habituation phase. For further analyses, all video recordings were re-coded offline. Using the software Supercoder (Hollich 2005), a trained coder determined whether infants did or did not look at the screen for each single frame (frame rate: 30 frames per second). The looking time data obtained in this way were entered into statistical analyses (see below).

6.2.2 Results

First, infants’ looking times during pre- and posttest trials were compared.

The pre- and posttest are conducted to ensure that the infants’ attention or absence of attention during the test trials is caused by the test stimuli and not by other factors such as fatigue. If the looking times during the posttest reach a level similar to that of the pretest, it can be assumed that the infant is still involved in the experiment and that a preceding drop in looking time during the habituation and test phase is owing to the specific stimuli and not to a general loss of interest in the test. All 16 subjects showed high attention during both pre- and posttest. Nonetheless, looking times that diverged more than two standard deviations from the mean of all subjects were disregarded, leading to the exclusion of 3 of 32 observations (9.4%).³

Infants’ looking times in the pre- and posttest trials were compared in an analysis of variance (ANOVA) with residual maximum likelihood (REML) estimation, performed with the statistical softwareJMP, Version 5.0.1.2 on a Macintosh computer. The level of significance is 5%. This model and

3There are two observations per subject. In this case, it is one for the pretest and one for the posttest. Since the analysis is based on a between-subjects design and investigated infants’ behaviour as a group, it was possible to exclude single observations without having to discard the whole subject data. The model applied (see below) is able to handle such missing values without problems. Many thanks to Henning Reetz and Mathias Scharinger for their helpful comments regarding the statistical analysis.

6.2 Experiment 1: Testing the Switch Procedure 151 the 5%-level of significance were applied to all following analyses.⁴ Look-ing time was the dependent variable and the factors subject (as random factor) and test (pretest| posttest) were added as independent variables.

The analysis yielded no main effect. Looking times during pre- and posttest did not differ significantly [F(1,12) = 0.09, p > .05, M_pre = 15.50, SD = 0.59, M_post= 15.43, SD = 0.76], indicating that infants were equally atten-tive at the beginning and at the end of the session and that they did not lose interest in the experiment in general. Thus, decreasing attention during the habituation and the test phase can be attributed to a loss of interest in the specific (auditory) stimuli.⁵

A further analysis was conducted to statistically demonstrate that ha-bituation worked successfully, that is, that the looking times dropped sig-nificantly during the habituation phase. For this purpose, the mean looking time of the first four habituation trials was compared to the mean looking time of the last four habituation trials. 2 of 32 observations (6.3%) were disregarded in this analysis due to looking times diverging more than two standard deviations from the group mean looking times to the first and the last habituation block, respectively. An ANOVA was performed with look-ing time as the dependent variable and the factors subject (as random factor) and block (first | last) as independent variables. A main effect of block [F(1,13) = 204.43, p < .0001, M_{f irst} = 13.30, SD= 1.73, M_last = 7.11, SD= 1.01] confirmed that looking times decreased significantly during the habituation phase, indicating successful habituation.

The main analysis addressed infants’ looking behaviour during the test trials in order to reveal whether infants succeeded in discriminating the contrast in the auditory stimuli. No outliers had to be excluded for this analysis since none of the observations exceeded±2 standard deviations of the mean looking times. Looking time was the dependent variable and the factorssubject(as random factor),condition(pata|medo),trial(same

|switch) and test order (sa/sw |sw/sa) were entered in an ANOVA as well as the following interactions:⁶

4p-values≤.05 are indicated by one asterisk (∗), two asterisks are assigned top-values

≤.01 (∗∗) and three asterisks indicatep-values≤.001 (∗∗∗).

5Above, the trial length of the pre- and posttest was indicated to be 14 s while the actually measured looking times exceed 15 s. TheHabit X software (Cohenet al. 2004) does not allow to set maximum trial length individually for each trial type. Since the medo-sequence had a length of 15.6 s, the maximal trial length was set to 16 s. This implies that the checkerboard pattern remains on the screen for a short time after the audio-signal stopped. Therefore infants’ looking times may exceed the auditory trial length.

6Such a model specification, which includes all possible interactions except crossings with the random factorsubject, will be referred to as ‘full-factorial’ below.

• condition× trial

• condition× test order

• trial×test order

• condition× trial×test order.

There was a main effect of trial [F(1,9) = 5.49, p < .05, Mswitch = 10.61, SD = 3.95, M_same = 8.00, SD = 4.21]. Infants looked significantly longer to the ‘switch’ than to the ‘same’ trial, suggesting that the German 6- to 8-month-olds were able to distinguish pata from medo. There were no other main effects and no interactions. Figure 6.2 provides a graph of infants’ looking times during ‘switch’ and ‘same’ trials.

Figure 6.2: German 6- to 8-month-olds’ looking behaviour in Experiment 1 (pata vs.medo).

6.2.3 Discussion

The results of Experiment 1 confirm the expectation that infants easily discriminate a linguistic contrast when the stimuli are phonetically very dissimilar. After having decreased in the habituation phase, attention re-ascended significantly during the ‘switch’ but not during the ‘same’ trial,