Brain activation and lexical learning: the impact of learning phase and word type.

This study investigated the neural correlates of second-language lexical acquisition in terms of learning phase and word type. Ten French-speaking participants learned 80 Spanish words-40 cognates, 40 non-cognates-by means of a computer program. The learning process included the early learning phase, which comprised 5 days, and the consolidation phase, which lasted 2 weeks. After each phase, participants performed an overt naming task during an er-fMRI scan. Naming accuracy was better for cognates during the early learning phase only. However, cognates were named faster than non-cognates during both phases. The early learning phase was characterized by activations in the left iFG and Broca's area, which were associated with effortful lexical retrieval and phonological processing, respectively. Further, the activation in the left ACC and DLPFC suggested that monitoring may be involved during the early phases of lexical learning. During the consolidation phase, the activation in the left premotor cortex, the right supramarginal gyrus and the cerebellum indicated that articulatory planning may contribute to the consolidation of second-language phonetic representations. No dissociation between word type and learning phase could be supported. However, a Fisher r-to-z test showed that successful cognate retrieval was associated with activations in Broca's area, which could reflect the adaptation of known L1 phonological sequences. Moreover, successful retrieval of non-cognates was associated with activity in the anterior-medial left fusiform and right posterior cingulate cortices, suggesting that their successful retrieval may rely upon the access to semantic and lexical information, and even on the greater likelihood of errors.

[Functional neuroimaging and the treatment of aphasia: speech therapy and repetitive transcranial magnetic stimulation]. / Imagerie fonctionnelle cérébrale et prise en charge thérapeutique de l'aphasie : orthophonie et stimulation magnétique transcrânienne répétitive.

Functional imaging has provided new evidence of the neurobiological impact of the treatment of aphasia, including speech therapy, through the alteration of the activated language neural network. In such a way, speech therapy has proved its impact. The role of each hemisphere is still very unclear. Some of the authors link the left-lateralisation of activations to the therapeutic improvement of language and the right-activated network to a maladaptative strategy, whereas others consider the latter as a useful compensatory network for speech disorders. Repetitive trans-cranial magnetic stimulation (rTMS), first used to determine cortical activity, is now used to directly interfere with cerebral activity. In the years to come, rTMS should be developed as an adjuvant therapy for aphasia.

Right hemisphere activation in recovery from aphasia: lesion effect or function recruitment?

BACKGROUND: Some neuroimaging studies have suggested that specific right hemispheric regions can compensate deficits induced by left hemispheric lesions in vascular aphasia. In particular, the right inferior frontal cortex might take part in lexical retrieval in patients presenting left-sided lesions involving the homologous area. OBJECTIVE: To address whether the involvement of the right inferior frontal cortex is either unique to recovering aphasic patients or present also in other circumstances of enrichment of lexical abilities, i.e., in non-brain-damaged subjects over learning of new vocabulary. METHODS: Ten post-stroke aphasic patients experiencing word finding difficulties were intensively trained to retrieve object names in French over a 4-week period. Twenty healthy subjects were similarly trained to name these items in either Spanish or English, i.e., foreign languages that they learned at school but did not master. By analogy to aphasic patients, healthy subjects had to work out the phonetic/phonologic representations of long-acquired but forgotten words. Brain activity changes were assessed in two H(2)(15)O PET sessions involving picture naming tasks that were performed before and after training. RESULTS: Comparable post-training performance and changes in regional cerebral blood flow including mainly the right insular and inferior frontal regions were found in both groups. CONCLUSION: Our results suggest that enhanced activities in right-sided areas observed in recovering aphasia is not the mere consequence of damage to left-sided homologous areas and could reflect the neural correlates of lexical learning also observed in control subjects.

Early head movements elicited by visual stimuli or collicular electrical stimulation in the cat.

During the course of previous recordings of visually-triggered gaze shifts in the head-unrestrained cat, we occasionally observed small head movements which preceded the initiation of the saccadic eye/head gaze shift toward a visual target. These early head movements (EHMs) were directed toward the target and occurred with a probability varying between animals from 0.4% to 16.4% (mean=5.2%, n=11 animals). The amplitude of EHM ranged from 0.4 degrees to 8.3 degrees (mean=1.9 degrees ), their latency from 66 to 270 ms (median=133 ms) and the delay from EHM onset to gaze shift onset averaged 183+/-108 ms (n=240). Their occurrence did not depend on visual target eccentricity in the studied range (7-35 degrees ), but influenced the metrics and dynamics of the ensuing gaze shifts (gain and velocity reduced). We also found in the two tested cats that low intensity microstimulation of the superior colliculus deeper layers elicited a head movement preceding the gaze shift. Altogether, these results suggest that the presentation of a visual target can elicit a head movement without triggering a saccadic eye/head gaze shift. The visuomotor pathways triggering these early head movements can involve the deep superior colliculus.