The activation profile of these regions in functional MRI provides several important clues to the nature of the neural code of the sentences in these regions. Activation varies logarithmically with phrase size, both in intensity and phase. As the complexity of syntagms increases, we also observe an increasing delay in brain activation, particularly in the left inferior frontal region. This suggests that the network of brain areas performs computations over an increasing duration, probably extending beyond the end of sentence presentation. This systematic organization of activation delays is already present in babies a few months old. It suggests that a hierarchical succession of brain areas, with progressively slower integration constants, cascade linguistic signals from the primary auditory area. Different areas seem to be " tuned " to larger and larger linguistic structures. Access to the meaning of sentences and texts occurs at the highest level of this hierarchy, in the bilateral lateral frontal and inferior parietal regions, as well as in the precuneus.
To clarify the speed of sentence processing, we used functional MRI to study the brain activity evoked by listening to or reading sentences presented at an accelerated rate. The results suggest that only the most peripheral areas of this network can accelerate processing. On the contrary, the areas involved in syntagmatic processing seem to form a bottleneck whose slowness would explain the sudden loss of intelligibility of sentences beyond a threshold of the order of 100 milliseconds per word.
