Non-motor basal ganglia functions: a review and proposal for a model of sensory predictability in auditory language perception.

While the primary function of the basal ganglia (BG) is linked to motor behaviour, several investigations of non-motor behaviour allocate cognitive and language-specific functions to the BG. What may such seemingly discrepant functions have in common? Some neurophysiologic theories of motor behaviour assign temporal sequencing, others the sequencing of general cognitive patterns to the BG. Turning to auditory language perception and syntax in particular, one may consider syntactic processing as a hierarchical sequencing phenomenon. Furthermore, previous data have shown that if events are predictable, the processing of successively following events in a sequence is facilitated. We propose that sequencing is closely linked to the perception of predictable cues (regular beats, meter, temporal chunks etc.). If this is the case, syntactic processing should rely on the extraction of predictable cues in auditory language perception. Consequently, dysfunctional extraction of such cues in BG patients should then lead to secondary deficits in syntactic processing as evidenced in recent behavioural and electrophysiological evidence (ERP). The fact that such "secondary syntactic deficits" can be compensated by external and speech inherent predictable cues permits two conclusions: (i) syntactic deficits in BG patients are epiphenomenal, and (ii) sequencing dysfunctions of the pre-supplementary motor area (SMA)-BG circuit may be compensated by increased influence of the cerebellar-thalamic-pre-SMA pathway. In the current review we elaborate on this possibility drawing comparisons to similar proposals in motor and language production.