Degeneracy in the neurological model of auditory speech repetition
In the neurological model of language, repeating heard speech involves four left hemisphere regions: primary auditory cortex for processing sounds; Wernickes area for processing auditory images of speech; Brocas area for processing motor images of speech; and primary motor cortex for overt speech articulation. Previous functional-MRI (fMRI) studies confirm that auditory repetition activates these regions. Here, we used dynamic causal modelling (DCM) to test how the four regions interact with each other during single word and pseudoword auditory repetition. Contrary to expectation, we found that, for both word and pseudoword repetition, the effective connectivity between Wernickes and Brocas areas was predominantly bidirectional and inhibitory; activity in the motor cortex could be driven by either Wernickes area or Brocas area; and the latter effect varied both within and between individuals. Such variability speaks to degenerate functional architectures that support auditory repetition and may explain resilience to functional loss after brain damage.