gms | German Medical Science

The processing of relevant environmental sounds becomes increasingly specialized during early infancy. During language aquisition babys have to learn to segregate single speech units within the complex speech stream and learn to identify relevant information [Ref. 1]. Fast acoustic changes and short interruptions between the phonemes are helping to segregate these single speech units from each other. Psychoacoustic experiments showed that the gap duration which is required for the separation of two sounds is strongly age dependent. Infants of three months of age need longer gap durations than older infants of 12 months of age [Ref. 2]. The underlying neuronal maturation and specialization to relevant sounds is known to be correlated with changes in the cognitive components of the event related potentials (ERPs). Among other factors, the onset latencies of ERPs as response to short auditory stimuli decrease in the course of infant development [Ref. 3]. The aim was to find out how the ability to discriminate consonant-vowel sequences with different gap durations develops in the first year of life. Therefore, we examined the infants' ability to process syllables as separate units. Beside the general change in morphology of ERP components during the maturation we predicted to find correlates to the maturation of auditory system for the detection of gaps with different duration.

All experiments were part of a longitudinal study about healthy and normal hearing infants conducted within the German language acquisition study (GLaD, www.glad-study.de). Parents gave their written consent according to institutional informed consent procedures and accepted to participate with their children in a multiple set of examinations. The study was conducted in infants during their first year of life (in the age of 4 weeks, 6 months and 13 months).The processing of different phonemes was measured in dependence on different gap durations between two consonant-vowel items. ERPs were recorded to a phoneme change occurring during the iterative presentation of /da-da/, occasionally interrupted by a stimulus /da-ba/. The pre-attentive detection of the change is measured by the difference between the ERPs of the frequently presented standard stimulus and the rarely presented deviant stimulus.

The presented stimuli consisted of two consonant-vowel items (CVs) /da/ and /ba/ naturally produced by a male German speaker. The CVs were matched in peak amplitude and stimulus length (100 ms). The stimulus /da-ba/ was presented as the deviant with a probability of 0.2 interspersed with the standard stimulus /da-da/. The gap between the CVs was set either to 50 ms for the short gap condition or 150 ms for the long gap condition. The interstimulus interval was set to constant 750 ms. An electrode-cap (Easy Cap, Falk Minow) was used to fix 11 electrodes according to the international 10-20 electrode system.

The EEG was amplified with PORTI-32/MREFA (Twente Medical Systems) with a sampling rate of 250 Hz. After eliminating artefacts and filtering the EEG-signal, epochs of 850 ms from stimulus onset were averaged for standards and deviants in both conditions. The mismatch response was defined as the response to the average of standards subtracted from the response to the average of deviants. Using the maximal difference of the grand averages statistical analyses of the mean amplitude were carried out for all age groups in a time window around the maximal peak.

As demonstrated at electrode F3 in Figure 1 [Fig. 1], only infants of 13 months showed a mismatch response (MMR) of positive amplitude for the short gap condition in the time window of 200-300 ms after change onset (F_1.60 = 11.095, p = 0.001). The mismatch response for the long gap condition occurred again 200-300 after change onset and was significant in all age groups (4 weeks: F_1.101 = 5.458, p < 0.05; 5 months: F_1.102 = 5.710, p < 0.05; 13 months: F_1.62 = 19.438, p < 0.001).

In the present study, infants of the age of 13 months showed a positive mismatch response for both gap conditions, which indicates the detection of the change onset within the bi-syllabic stimulus. The positive amplitude is often found for infants and is defined as mismatch response which can be correlated to the adult mismatch negativity (MMN) in its functionality [Ref. 4]. In contrast to the adult MMNs the infants' MMR is instable in amplitude and shows a positive deflection for slight acoustic changes, which is also discussed to overlay the negative wave of adult MMN [Ref. 5]. Yet, it is not clear which of the infant ERPs are correlated to the adult ERPs, although there is some evidence for the explanation for ERPs of school age children up to adults ERPs [Ref. 6]. Younger infants showed this MMR of positive amplitude only for the long gap condition. The lacking MMR to the short gap condition can be explained by a longer processing time for the first syllable (mask effect). The auditory stream segregation which is important for the detection of sound patterns is highly dependent on the maturation of especially the central auditory system. Although changes in ERP may not be associated with clearly identifiable structural alterations [6] indicators of the electrophysiological methods can help to develop age dependent hearing tests, which can use the basic skills for language acquisition. The results showed that a short gap can only be detected in the later maturational stadium, whereas a long gap is detected already in the age of 4 weeks. The greater mask effect in younger infants is suggested to be induced by a slower neural processing in early infancy.

The authors thank Christina Rügen and Jördis Haselow for recording the ERP data. This study was supported by the German Research Foundation (DFG; 381) and Max Planck Institute of Cognitive Neuroscience in Leipzig.

The data characterising the developmental state of our subjects were kindly provided by Volker Hesse, head of the paediatric clinic of the Krankenhaus Lichtenberg, Berlin. He and his team took care for the somatic and neurological data of the children and he provided resources and manpower for recruiting subjects. Finally, we want to thank him for hosting our labs in the Lindenhof hospital.