MEG correlates of learning novel objects properties in children

The power of children's sleep - Improved declarative memory consolidation in children compared with adults

Post-learning slow wave sleep (SWS) is known to support declarative memory consolidation. As SWS is more abundant in young population, we suggested that sleep-dependent memory consolidation processes could occur at a faster pace in school-aged children. After learning new associations between non-objects and their functions, retrieval performance was tested in 30 children (7–12 years) and 34 adults (20–30 years) during an immediate (IR) and a delayed retrieval (DR) session separated by either a Sleep or a Wake condition. Sleep led to stabilized memory retrieval performance only in children, not in adults, whereas no age-related difference was observed after a similar period of wakefulness. Hence, our results suggest more efficient sleep-dependent declarative memory consolidation processes in children compared with adults, an effect potentially ascribed to more abundant and deeper SWS during childhood.

The consonant/vowel pattern determines the structure of orthographic representations in the left fusiform gyrus

Recent findings demonstrated readers' sensitivity to the distinction between consonant and vowel letters. Especially, the way consonants and vowels are organised within written words determines their perceptual structure. The present work attempted to overcome two limitations of previous studies by examining the neurophysiological correlates of this perceptual structure through magnetoencephalography (MEG). One aim was to establish that the extraction of vowel-centred units takes place during early stages of processing. The second objective was to confirm that the vowel-centred structure pertains to the word recognition system and may constitute one level in a hierarchy of neural detectors coding orthographic strings. Participants performed a cross-case matching task in which they had to judge pairs of stimuli as identical or different. The critical manipulation concerned pairs obtained by transposing two letters, so that the vowel-centred structure was either preserved (FOUVERT-fovuert, two vowel letter clusters) or modified (BOUVRET-bovuret). Mismatches were detected faster when the structure was modified. This effect was associated with a significant difference in evoked neuromagnetic fields extending from 129 to 239 msec after the stimulation. Source localization indicated a significant effect in the visual word form area around 200 msec. The results confirm the hypothesis that the vowel-centred structure is extracted during the early phases of letter string processing and that it is encoded in left fusiform regions devoted to visual word recognition.

Objective and subjective measures of cross-situational learning

Statistical learning is often considered to be automatic and implicit, but little is known about the extent to which the resulting representations are available to conscious awareness. In the present study, we focus on whether the knowledge acquired in statistical learning of word-referent pairs is available to conscious control. Using a cross-situational learning paradigm, adult participants were first exposed to a set of pictures associated with auditorily presented words. Immediately thereafter, they were exposed to a second set of word-picture pairs. After the exposure phase, learning and conscious accessibility to the acquired knowledge were measured by using an adaptation of the Process Dissociation Procedure (Jacoby, 1991): two recognition tasks that only differed by instructions. In the Inclusion task, participants were instructed to accept all the correct associations (either from the first or the second set) and reject all the incorrect associations. In the Exclusion task, they had to accept all the correct associations from one of the sets and reject both the correct associations from the other set as well as all incorrect associations. Moreover, binary confidence judgments were recorded after each trial. Results show that participants were able to control the acquired knowledge. However, confidence judgments revealed that participants correctly identified the learned associations even when they claimed to guess, suggesting that cross-situational learning involves a mixture of both conscious and unconscious influences.

Impaired sleep-related consolidation of declarative memories in idiopathic focal epilepsies of childhood

OBJECTIVE

Declarative memory is consolidated during sleep in healthy children. We tested the hypothesis that consolidation processes are impaired in idiopathic focal epilepsies (IFE) of childhood in association with frequent interictal epileptiform discharges (IEDs) during sleep.

METHODS

A verbal (word-pair association) and a nonverbal (2D object location) declarative memory task were administrated to 15 children with IFEs and 8 control children 6-12 years of age. Patients had either centrotemporal (11 patients) or occipital (4 patients) IEDs. All but 3 patients had a history of unprovoked seizures, and 6 of them were treated with valproate (VPA). The learning procedure (location of object pairs presented on a grid; association of word pairs) was executed in the evening. Retrieval was tested immediately after learning and on the next morning after a night of sleep. Participants were tested twice, once in natural home conditions and one month later in the unfamiliar conditions of the sleep unit under EEG monitoring.

RESULTS

Overnight recall performance was lower in children with IFE than in control children on both tasks (ps<0.05). Performance in home conditions was similar to that in hospital conditions. Higher spike-wave index (SWI) during nonrapid eye movement (NREM) sleep was associated with poorer performance in the nonverbal task (p<0.05). Valproate treatment was not associated with overnight recall performance for both tasks (ps>0.05).

CONCLUSION

Memory consolidation is impaired in IFE of childhood. The association between higher SWI during NREM sleep and poorer nonverbal declarative memory consolidation supports the hypothesis that interictal epileptic activity could disrupt sleep memory consolidation.

Atypical right hemisphere specialization for object representations in an adolescent with specific language impairment

Individuals with a diagnosis of specific language impairment (SLI) show abnormal spoken language occurring alongside normal non-verbal abilities. Behaviorally, people with SLI exhibit diverse profiles of impairment involving phonological, grammatical, syntactic, and semantic aspects of language. In this study, we used a multimodal neuroimaging technique called anatomically constrained magnetoencephalography (aMEG) to measure the dynamic functional brain organization of an adolescent with SLI. Using single-subject statistical maps of cortical activity, we compared this patient to a sibling and to a cohort of typically developing subjects during the performance of tasks designed to evoke semantic representations of concrete objects. Localized patterns of brain activity within the language impaired patient showed marked differences from the typical functional organization, with significant engagement of right hemisphere heteromodal cortical regions generally homotopic to the left hemisphere areas that usually show the greatest activity for such tasks. Functional neuroanatomical differences were evident at early sensoriperceptual processing stages and continued through later cognitive stages, observed specifically at latencies typically associated with semantic encoding operations. Our findings show with real-time temporal specificity evidence for an atypical right hemisphere specialization for the representation of concrete entities, independent of verbal motor demands. More broadly, our results demonstrate the feasibility and potential utility of using aMEG to characterize individual patient differences in the dynamic functional organization of the brain.