Hierarchical and linear sequence processing: an electrophysiological exploration of two different grammar types.

The present study investigated the processing of two types of artificial grammars by means of event-related brain potentials. Two categories of meaningless CV syllables were applied in each grammar type. The two grammars differed with regard to the type of the underlying rule. The finite-state grammar (FSG) followed the rule (AB)n, thereby generating local transitions between As and Bs (e.g., n=2, ABAB). The phrase structure grammar (PSG) followed the rule AnBn, thereby generating center-embedded structures in which the first A and the last B embed the middle elements (e.g., n=2, [A[AB]B]). Two sequence lengths (n=2, n=4) were used. Violations of the structures were introduced at different positions of the syllable sequences. Early violations were situated at the beginning of a sequence, and late violations were placed at the end of a sequence. A posteriorly distributed early negativity elicited by violations was present only in FSG. This effect was interpreted as the possible reflection of a violated local expectancy. Moreover, both grammar-type violations elicited a late positivity. This positivity varied as a function of the violation position in PSG, but not in FSG. These findings suggest that the late positivity could reflect difficulty of integration in PSG sequences.