Aging modulates fronto-temporal cortical interactions during lexical production. A dynamic causal modeling study.

In this dynamic causal modeling (DCM) study, we evaluated the effect of age on the effective connectivity of a cerebral network involved in lexical production. Younger and older adults performed an object naming task during fMRI. The DCM was used to explore the interactions between four regions of interest: the occipital cortex, OC; the lateral temporal cortex, LTC; the medial temporal cortex, MTC; and the inferior frontal cortex, IFC. We mainly focused on the modulation of the fronto-temporal interaction, according to the hypothesis that aging requires strategies that modulate the access to the semantic knowledge, either through a neural reserve mechanism (increased MTC-LTC connectivity) or through a neural compensation mechanism (supplementary IFC-MTC connectivity). For younger adults, our results indicated a bi-directional interaction between the left IFC and LTC suggesting a typical activation related to lexico-semantic representations. For older adults, our results reveal the existence of bi-directional interaction between the IFC and MTC, but not between the IFC and LTC - which in turn suggests that older adults adapt a new strategy, via supplemental access to conceptual access and semantic retrieval processes. This neural compensation strategy would be facilitated by a top-down mechanism from the IFC to the MTC. We discuss our results in the context of the possible additional strategies used by older compared to younger adults, to retrieve and produce words.

Behavioral assessment of emotional and motivational appraisal during visual processing of emotional scenes depending on spatial frequencies.

Previous studies performed on visual processing of emotional stimuli have revealed preference for a specific type of visual spatial frequencies (high spatial frequency, HSF; low spatial frequency, LSF) according to task demands. The majority of studies used a face and focused on the appraisal of the emotional state of others. The present behavioral study investigates the relative role of spatial frequencies on processing emotional natural scenes during two explicit cognitive appraisal tasks, one emotional, based on the self-emotional experience and one motivational, based on the tendency to action. Our results suggest that HSF information was the most relevant to rapidly identify the self-emotional experience (unpleasant, pleasant, and neutral) while LSF was required to rapidly identify the tendency to action (avoidance, approach, and no action). The tendency to action based on LSF analysis showed a priority for unpleasant stimuli whereas the identification of emotional experience based on HSF analysis showed a priority for pleasant stimuli. The present study confirms the interest of considering both emotional and motivational characteristics of visual stimuli.

Neural dissociation of phonological and visual attention span disorders in developmental dyslexia: FMRI evidence from two case reports.

A dissociation between phonological and visual attention (VA) span disorders has been reported in dyslexic children. This study investigates whether this cognitively-based dissociation has a neurobiological counterpart through the investigation of two cases of developmental dyslexia. LL showed a phonological disorder but preserved VA span whereas FG exhibited the reverse pattern. During a phonological rhyme judgement task, LL showed decreased activation of the left inferior frontal gyrus whereas this region was activated at the level of the controls in FG. Conversely, during a visual categorization task, FG demonstrated decreased activation of the parietal lobules whereas these regions were activated in LL as in the controls. These contrasted patterns of brain activation thus mirror the cognitive disorders' dissociation. These findings provide the first evidence for an association between distinct brain mechanisms and distinct cognitive deficits in developmental dyslexia, emphasizing the importance of taking into account the heterogeneity of the reading disorder.

Catching falling objects: the role of the cerebellum in processing sensory-motor errors that may influence updating of feedforward commands. An fMRI study.

The human motor system continuously adapts to changes in the environment by comparing differences between the brain's predicted outcome of a certain behavior and the observed outcome. This discrepancy signal triggers a sensory-motor error and it is assumed that the cerebellum is a key structure in updating this error and associated feedforward commands. Using fMRI, the aim of the present study was to determine the main cerebellar structures that are involved in the processing of sensory-motor errors and in updating feedforward commands when simply catching a falling ball without displacement of the hand. Subjects only grasped the ball with their fingers when receiving it in their hand. By contrasting functional imaging signal obtained in conditions in which it was possible and impossible to predict the weight of the ball, we aimed to highlight sensory-motor error processing which we expected to be more marked in the conditions without prediction (less accurate feedforward process or more important feedback corrections) with respect to conditions with prediction (more accurate feedforward process or less important feedback corrections). When catching a falling ball and the possibility of prediction about the ball weight was manipulated, our results showed that both the right and left cerebellum is engaged in processing sensory-motor errors. It may also be involved in updating feedforward motor commands, perhaps on a trial by trial basis. In addition, when subjects were blindfolded, we observed a similar network but centered in a more anterior portion of the right cerebellum and we noted the presence of a cerebellar-thalamo-prefrontral network that may be involved in cognitive prediction (rather than sensory prediction) about ball weight.

Superior parietal lobule dysfunction in a homogeneous group of dyslexic children with a visual attention span disorder.

A visual attention (VA) span disorder has been reported in dyslexic children as potentially responsible for their poor reading outcome. The purpose of the current paper was to identify the cerebral correlates of this VA span disorder. For this purpose, 12 French dyslexic children with severe reading and VA span disorders and 12 age-matched control children were engaged in a categorisation task under fMRI. Two flanked and isolated conditions were designed which both involved multiple-element simultaneous visual processing but taxed visual attention differently. For skilled readers, flanked stimuli processing activated a large bilateral cortical network comprising the superior and inferior parietal cortex, the inferior temporal cortex, the striate and extrastriate visual cortex, the middle frontal cortex and the anterior cingulate cortex while the less attention-demanding task of isolated stimuli only activated the inferior occipito-temporal cortex bilaterally. With respect to controls, the dyslexic children showed significantly reduced activation within bilateral parietal and temporal areas during flanked processing, but no difference during the isolated condition. The neural correlates of the processes involved in attention-demanding multi-element processing tasks were more specifically addressed by contrasting the flanked and the isolated conditions. This contrast elicited activation of the left precuneus/superior parietal lobule in the controls, but not in the dyslexic children. These findings provide new insights on the role of parietal regions, in particular the left superior parietal lobule, in the visual attention span and in developmental dyslexia.

Polysyllabic pseudo-word processing in reading and lexical decision: converging evidence from behavioral data, connectionist simulations and functional MRI.

The cognitive mechanisms involved in polysyllabic pseudo-word processing -- and their neurobiological correlates -- were studied through the analysis of length effects on French words and pseudo-words in reading and lexical decision. Connectionist simulations conducted on the ACV98 network paralleled the behavioral data in showing a strong length effect on naming latencies for pseudo-words only and the absence of length effect for both words and pseudo-words in lexical decision. Length effects in reading were characterized at the neurobiological level by a significant and specific activity increase for pseudo-words as compared to words in the right lingual gyrus (BA 19), the left superior parietal lobule and precuneus (BA7), the left middle temporal gyrus (BA21) and the left cerebellum. The behavioral results suggest that polysyllabic pseudo-word reading mainly relies on an analytic procedure. At the biological level, additional activations in visual and visual attentional brain areas during long pseudo-word reading emphasize the role of visual and visual attentional processes in pseudo-word reading. The present findings place important constraints on theories of reading in suggesting the involvement of a serial mechanism based on visual attentional processing in pseudo-word reading.