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Oscillatory activity in human electro- or magnetoencephalogram has been related to cortical stimulus representations and their modulation by cognitive processes. Whereas previous work has focused on gamma-band activity (GBA) during attention or maintenance of representations, there is little evidence for GBA reflecting individual stimulus representations. The present study aimed at identifying stimulus-specific GBA components during auditory spatial short-term memory. A total of 28 adults were assigned to 1 of 2 groups who were presented with only right- or left-lateralized sounds, respectively. In each group, 2 sample stimuli were used which differed in their lateralization angles (15° or 45°) with respect to the midsagittal plane. Statistical probability mapping served to identify spectral amplitude differences between 15° versus 45° stimuli. Distinct GBA components were found for each sample stimulus in different sensors over parieto-occipital cortex contralateral to the side of stimulation peaking during the middle 200–300 ms of the delay phase. The differentiation between "preferred" and "nonpreferred" stimuli during the final 100 ms of the delay phase correlated with task performance. These findings suggest that the observed GBA components reflect the activity of distinct networks tuned to spatial sound features which contribute to the maintenance of task-relevant information in short-term memory.
Recent proposals suggest that timing in acquisition, i.e., the age at which a phenomenon is mastered by monolingual children, influences acquisition of the L2, interacting with age of onset of bilingualism and amount of L2 input. Here, we examine whether timing affects acquisition of the bilingual child’s heritage language, possibly modulating the effects of environmental and child-internal factors. The performance of 6- to 12-year-old Greek heritage children residing in Germany (age of onset of German: 0–4 years) was assessed across a range of nine syntactic structures via the Greek LITMUS (Language Impairment Testing in Multilingual Settings) Sentence Repetition Task. Based on previous studies on monolingual Greek, the structures were classified as “early” (main clauses (SVO), coordination, clitics, complement clauses, sentential negation, non-referential wh-questions) or as “late” (referential wh-questions, relatives, adverbial clauses). Current family use of Greek and formal instruction in Greek (environmental), chronological age, and age of onset of German (child-internal) were assessed via the Questionnaire for Parents of Bilingual Children (PABIQ); short-term memory (child-internal) was measured via forward digit recall. Children’s scores were generally higher for early than for late acquired structures. Performance on the three early structures with the highest scores was predicted by the amount of current family use of Greek. Performance on the three late structures was additionally predicted by forward digit recall, indicating that higher short-term memory capacity is beneficial for correctly reconstructing structurally complex sentences. We suggest that the understanding of heritage language development and the role of child-internal and environmental factors will benefit from a consideration of timing in the acquisition of the different structures.
The role of gamma oscillatory activity in magnetoencephalogram for auditory memory processing
(2010)
Recent studies have suggested an important role of cortical gamma oscillatory activity (30-100 Hz) as a correlate of encoding, maintaining and retrieving auditory, visual or tactile information in and from memory. It was shown that these cortical stimulus representations were modulated by attention processes. Gamma-band activity (GBA) occurred as an induced response peaking at approximately 200-300 ms after stimulus presentation. Induced cortical responses appear as non-phase-locked activity and are assumed to reflect active cortical processing rather than passive perception. Induced GBA peaking 200-300 ms after stimulus presentation has been assumed to reflect differences between experimental conditions containing various stimuli. By contrast, the relationship between specific oscillatory signals and the representation of individual stimuli has remained unclear. The present study aimed at the identification of such stimulus-specific gamma-band components. We used magnetoencephalography (MEG) to assess gamma activity during an auditory spatial delayed matching-to-sample task. 28 healthy adults were assigned to one of two groups R and L who were presented with only right- or left-lateralized sounds, respectively. Two sample stimuli S1 with lateralization angles of either 15° or 45° deviation from the midsagittal plane were used in each group. Participants had to memorize the lateralization angle of S1 and compare it to a second lateralized sound S2 presented after an 800-ms delay phase. S2 either had the same or a different lateralization angle as S1. After the presentation of S2, subjects had to indicate whether S1 and S2 matched or not. Statistical probability mapping was applied to the signals at sensor level to identify spectral amplitude differences between 15° and 45° stimuli. We found distinct gamma-band components reflecting each sample stimulus with center frequencies ranging between 59 and 72 Hz in different sensors over parieto-occipital cortex contralateral to the side of stimulation. These oscillations showed maximal spectral amplitudes during the middle 200-300 ms of the delay phase and decreased again towards its end. Additionally, we investigated correlations between the activation strength of the gamma-band components and memory task performance. The magnitude of differentiation between oscillatory components representing 'preferred' and 'nonpreferred' stimuli during the final 100 ms of the delay phase correlated positively with task performance. These findings suggest that the observed gamma-band components reflect the activity of neuronal networks tuned to specific auditory spatial stimulus features. The activation of these networks seems to contribute to the maintenance of task-relevant information in short-term memory.