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Language difficulties of dyslexic subjects may result partly from a basic deficit in processing rapidly changing sensory inputs. In this fMRI study, we compared brain activities in adult dyslexics and controls during implicit categorical perception of phonemes with normal and slowed down stimuli. Perception of phonemic contrasts activated a frontal parietal network (Broca's area and the left supramarginal gyrus) in which the frontal component was down-regulated by slowed speech in controls and enhanced in dyslexic subjects. No modulation by speech rate was observed in the left supra-marginal gyrus. Enhancement of activity in Broca's area for slowed speech in dyslexic subjects might represent a neural basis of the improvement of performance that has been observed after remediation using this type of stimuli.

The understanding and the prediction of the clinical outcomes of focal or degenerative cerebral lesions, as well as the assessment of rehabilitation procedures, necessitate knowing the cerebral substratum of cognitive or sensorimotor functions. This is achieved by activation studies, where subjects are asked to perform a specific task while data of their brain functioning are obtained through functional neuroimaging techniques. Such studies, as well as animal experiments, have shown that sensorimotor or cognitive functions are the offspring of the activity of large-scale networks of 2/44 anatomically connected cerebral regions. However, no one-to-one correspondence between activated networks and functions can be found.Our research aims at understanding how the activation of large-scale networks derives from cerebral information processing mechanisms, which can only explain apparently conflicting activation data. Our work falls at the crossroads of neuroimaging interpretation techniques and computational neuroscience.Since knowledge in cognitive neuroscience is permanently evolving, our research aims more precisely at defining a new modeling formalism and at building a flexible simulator, allowing a quick implementation of the models, for a better interpretation of cerebral functional images. It also aims at providing plausible models, at the level of large-scale networks, of cerebral information processing mechanisms in humans.In this paper, we propose a formalism, based on dynamic Bayesian networks, that respects the following constraints: an oriented, networked architecture, whose nodes (the cerebral structures) can all be different, the implementation of causality -the activation of a structure is caused by upstream nodes' activation -, the explicit representation of different time scales (from one millisecond for the cerebral activity to many seconds for a PET scan image acquisition), the representation of cerebral information at the integrated level of neuronal populations, the imprecision of functional neuroimaging data, the nonlinearity and the uncertainty in cerebral mechanisms, and brain's plasticity (learning, reorganization, modulation). One of the main problems, nonlinearity, has been tackled thanks to new extensions of the Kalman filter. The capabilities of the formalism's current version are illustrated by the modeling of a phoneme categorization process, explaining the different cerebral activations in normal and dyslexic subjects.3/44