From acoustic segmentation to language processing: evidence from optical imaging

Abstract

During language acquisition in infancy and when learning a foreign language, the segmentation of the auditory stream into words and phrases is a complex process. Intuitively, learners use "anchors" to segment the acoustic speech stream into meaningful units like words and phrases. Regularities on a segmental (e.g., phonological) or suprasegmental (e.g., prosodic) level can provide such anchors. Regarding the neuronal processing of these two kinds of linguistic cues a left-hemispheric dominance for segmental and a right-hemispheric bias for suprasegmental information has been reported in adults. Though lateralization is common in a number of higher cognitive functions, its prominence in language may also be a key to understanding the rapid emergence of the language network in infants and the ease at which we master our language in adulthood. One question here is whether the hemispheric lateralization is driven by linguistic input per se or whether non-linguistic, especially acoustic factors, "guide" the lateralization process. Methodologically, functional magnetic resonance imaging provides unsurpassed anatomical detail for such an enquiry. However, instrumental noise, experimental constraints and interference with EEG assessment limit its applicability, pointedly in infants and also when investigating the link between auditory and linguistic processing. Optical methods have the potential to fill this gap. Here we review a number of recent studies using optical imaging to investigate hemispheric differences during segmentation and basic auditory feature analysis in language development.

Keywords: NIRS; acoustic segmentation; infants; language acquisition; optical imaging.

Figure 1

Principles of non-invasive optical imaging: Optical probes are fixed to the head defining the sampling volume (pink ellipse). Measuring changes in attenuation at two or more wavelengths allows to calculate changes in the concentration of HbO, HbR. An increase in HbO and/or a decrease in HbR indicate cerebral activation. Physiologically this pattern results from an increase in regional cerebral blood flow (rCBF) consisting of an increase in blood flow velocity (rCBFv) and volume (rCBV). The increase in rCBF overcompensates the demand in oxygen (Fox and Raichle, 1986). An increased “washout” of HbR results from increased rCBFv. This decrease in HbR is the major source of fMRI BOLD-contrast increases (Steinbrink et al., 2006). The change in total hemoglobin concentration (HbT = HbR + HbO) correlates with blood volume (CBV), which can be assessed by positron emission tomography (PET). Notably the assessment of electrophysiological markers of activation by EEG or MEG (right side of graph) is undemanding as demonstrated in a neonate in Figure 2E.

Figure 2

Examples of optical imaging results in infants and neonates. (A–C) Time-courses of HbO and HbR in three different age groups. Note that the response pattern is similar in all three studies though magnitude differs. (A) Neonates’ response to structured noise. Here the statistics on HbR↓ yielded a lateralized response (adapted with permission from Telkemeyer et al., 2009). (B) Response to sentences in 10-month-old infants. In this study HbO↑ yielded the effects shown in (D) (adapted with permission from Homae et al., 2007). (C) Response to hummed sentences in 4-year-old children. HbO↑ and HbR↓ showed statistically significant results (adapted with permission from Wartenburger et al., 2007). (D) Topographical information is available when using probe arrays (24 volumes over each hemisphere; adapted with permission from Homae et al., 2007). The results show the differential activation to normal vs. flattened speech in two age groups (10 and 3 months). Note that the lateralization changes sides (red: lager response amplitude for flattened speech than normal speech; blue: normal > flattened; results are based on HbO↑). (E) Simultaneous assessment of EEG and optical imaging in a neonate as used in (Telkemeyer et al., 2009). (Permission from the infant's parents to show the picture was obtained.)

Figure 3

Sketch of temporal frames relevant for speech perception. The smallest linguistically relevant segmentation deals with the differentiation of phonemes which rely on transitional differences in the range of 10 ms. Syllables and words form the nucleus of a lexico-semantic decoding of speech. Syntax and prosody act on the level of phrases and sentences which may have a duration of several seconds. The speaker's intention (pragmatics) evolves over largely different temporal frames ranging from single vocalizations to tales and stories. Note that the components below the time line (blue) are available also to the prelinguistic infant due to their acoustic prominence. The other components (red) are fully available only when a certain language competence is reached. In the language-competent adult, all levels strongly interact.

Figure 4

Optical imaging and EEG results from a series of studies on the processing of phonotactics. Phonotactically legal were contrasted to illegal pseudowords (e.g., /brop/ vs. /bzop/). In adults the EEG showed a characteristic N400-effect mostly at the central electrode positions. Optical imaging showed a clear lateralization of the activation to the left hemisphere (Rossi et al., 2010). Preliminary data in infants suggest that the ERPs (frontal negativity) robustly indicate the tuning into the native phonotactic rules by the age of 6 months. In both 3- and 6-month-old infants, optical imaging showed a bilateral differential activation with respect to the phonotactic legality (Rossi et al., in preparation). The results are in line with a gradual evolution from acoustic change detection to a more linguistic analysis guided by the knowledge on legal word-onsets in the native language.

Figure 5

Lateralization of language processing. The “classic” left-hemispheric language areas are essential for syntactic and semantic analysis. The dual pathway model (Friederici and Alter, 2004) predicts a left-lateralized processing of segmental and a right-lateralized analysis of suprasegmental information (e.g., sentential prosody). Since segmental and suprasegmental features develop along different temporal frames (see Figure 3) lateralization of auditory analysis with regard to spectral/temporal or fast/slow modulations may constitute some of the lateralization (Poeppel et al., 2008). Note that lateralization is relative and all theories include a close interaction between both hemispheres (Friederici et al., 2007). The stronger engagement of the left hemisphere for syntactic and semantic analysis (also over longer temporal windows) constitutes the critical role of the left hemisphere for intact language functions.