Implicit learning of artificial grammatical structures after inferior frontal cortex lesions

Objective. Previous research has associated the left inferior frontal cortex with implicit structure learning. The present study tested patients with lesions encompassing the left inferior frontal gyrus (LIFG; including Brodmann areas 44 and 45) to further investigate this cognitive function, notably by using non-verbal material, implicit investigation methods and by enhancing potentially remained functions via dynamic attending. Patients and healthy matched controls were exposed to an artificial pitch grammar in an implicit learning paradigm to circumvent the potential influence of impaired language processing.

Methods. Patients and healthy controls first listened to pitch sequences generated within a finite-state grammar (exposure phase) and then performed a categorization task on new pitch sequences (test phase). Participants were not informed about the underlying grammar in either the exposure phase or the test phase. Furthermore, the pitch structures were presented in a highly regular temporal context as the beneficial impact of temporal regularity (e.g. meter) in learning and perception has been previously reported. Based on Dynamic Attending Theory, it was hypothesized that a temporally regular context helps developing temporal expectations that, in turn, facilitate event perception, and thus benefit artificial grammar learning.

Results. Behavioral and electroencephalography results suggest preserved artificial grammar learning of pitch structures in patients and healthy controls. For both groups, analyses of event-related potentials revealed a larger early negativity (100-200 msec post-stimulus onset) in response to pitch violations in ungrammatical sequences than in new grammatical sequences.

Conclusions. These findings suggest that (i) the LIFG does not play a necessary role in the implicit learning of artificial pitch grammars, and (ii) the use of non-verbal material and an implicit task reveals cognitive capacities that remain intact after LIFG lesions. These results provide grounds for training and rehabilitation that is learning of non-verbal grammars that may impact the relearning of verbal grammars.