Perception and production of inflections 1 Sentence-position effects ...

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Perception and production of inflections 14 To further validate the stimuli, we asked 5 native English-speaking adults (Mean age = 19.4; Range = 19:20) to decide whether the sentence stimuli were grammatical or ungrammatical. Sentence stimuli were presented in (a) un-altered and (b) low-pass filtered versions. Low-pass filtering eliminates most segmental information, particularly the presence or absence of the 3 rd person singular –s, while retaining the prosody (the rhythm, intonation, phrasing) of the sentence. Adults were expected to be at ceiling when tested with un-altered test sentences and to be at chance when presented with low-pass filtered sentences. To minimize recall bias, adults were tested on the low-pass filtered condition before being tested on the un-altered test stimuli. As expected, native English-speaking adults were at chance in the low-pass filtered condition (Mean percent correct = 52.5%; SD = 1.9), and identified grammatical and ungrammatical sentences almost perfectly when presented with un-altered test sentences (Mean percent correct = 98.2%; SD = 1.3). Thus, the grammatical and ungrammatical sentences differed in the presence/absence of the 3 rd person singular –s, but did not differ systematically on extraneous prosody. The duration of the 3 rd person singular –s and the preceding vowel duration are presented in Table 2. Recall that, due to final lengthening, both duration measures are expected to be longer sentence-finally compared to sentence-medially. For target sentences with verbs in medial position, the average duration of the 3 rd person singular –s was 125 ms (SD = 24), and the average duration of the preceding vowel was 290 ms (SD = 115). For target sentence with verbs in final position, the average duration of the 3 rd person singular –s was 202 ms (SD = 23), and the average duration of the preceding vowel was 471 ms (SD = 221). Paired t-tests were used to compare the duration of 3 rd person singular –s and the preceding vowel duration of target sentences with verbs in medial versus final position. As expected, 3 rd person singular –s was significantly longer in final position than in medial position, t(14) = -5.99, p = 0.001. Similarly,
Perception and production of inflections 15 the preceding vowel duration was significantly longer in final position than in medial position, t(14) = -4.44, p = 0.003. [insert Table 2 about here] Procedure Design Children participated in two tasks - the perception task (10 minutes), followed by the production task (20 minutes). For the perception task, half the children were tested on the sentence-final condition, and half were tested on the sentence-medial condition. All children participated in the production task. To obtain an estimate of each child’s language abilities and vocabulary size, parents were asked to fill out the short form of the MacArthur CDI - Vocabulary Checklist: Level II, Form A (Fenson, Pethick, Renda, Cox, Dale, & Reznick, 2000). MacArthur CDI scores (raw and percentiles) for each of the four groups of children are reported in Table 3. Each of the two groups of 22- and 27-month-olds tested on the sentence-medial and sentencefinal position were comparable in age, and raw and percentile CDI scores (p values’s > 0.4). [insert Table 3 about here] Perception task During testing, children sat on their parent’s lap in a dark room facing a TV monitor. Audio stimuli were played at a comfortable 77 dB SPL over Bose loudspeakers placed next to the TV monitor but behind a dark curtain. A Sony SuperExWave camera lens was placed below the monitor. A tester outside the room was able to record the infant’s gaze by watching the infant over a second TV monitor connected to the camera. During testing, the parent and tester listened to music over sound attenuating JTC Clearwater headphones so as not to influence the child's behavior.
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