Authors:

GARAGNANI, M., Wennekers, T. & PULVERMULLER, F.

Reference:

Year of publication:

2006

CBU number:

6310

Abstract:

During speech production, two different areas of the brain becomesimultaneously active: primary motor cortex (responsible for soundarticulation) and auditory cortex (stimulated by the produced sounds). Wepostulate that during language acquisition, the repeated simultaneousactivation of these two areas leads, through Hebbian learning, to theformation of strongly-associated cell assemblies, which are: (a)distributed across cortical areas, (b) word-specific, and (c) activatedeven by only partial (e.g., auditory) stimulation. In order to test thevalidity of this hypothesis, we: (1) simulated learning of neuralassemblies in a biologically-motivated computational model of the leftperisylvian language cortex; (2) observed the functional characteristicsof the resulting model, and (3) explored its neurophysiological propertiesand compared them with real experimental data.Six cortical areas were simulated: (i) primary auditory cortex, (ii)auditory belt and (iii) parabelt areas (Wernicke areas), (iv) inferiorprefrontal and (v) premotor cortex (Broca areas), and (vi) primary motorcortex. Each area (layer) consisted of a 25x25 grid of graded-responseneurons. The network included random recurrent, forward and backwardlinks; recurrent connection probabilities decreased with increasingcortical distance following a Gaussian distribution; between-area linkswere topographic and followed the same probability distribution;non-adjacent layers had no connections. Local and global inhibitionmechanisms controlled activity within each layer.The model was confronted with simultaneous patterns of activations in bothits auditory (1) and motor (6) layers, as one would expect it in earlyspeech production. During training, the synaptic weights of all excitatorylinks were modified according to the co-variance learning rule [Sejnowski,1977]. We observed formation of cell assemblies, different assembliesresponding selectively to different input patterns. We also observed thepresence of spontaneous rhythmic activity in the network. During thetesting phase, the following functional characteristics emerged: when onlythe auditory layer was stimulated with one of the learnt patterns (words),we observed that (a) the relevant assembly was immediately and stronglyactivated in that layer and started to oscillate; (b) the rhythmicactivity of the assembly in that layer spread to the remaining layers; (c)presentation of new, previously unseen "pseudo-patterns" (pseudo-words)produced, on average, a smaller response in the network than that producedby words. These results are consistent with recent evidence obtained fromEEG recordings of the brain activity during speech-listening tasks, andhelp explain neurophysiological observations about cortical processing ofwords and pseudo-words in terms of a clearly-defined neurobiological modelof language.