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Evidence for Nonlinear Dynamics during Language Processing

INTRODUCTION Switching processes (bifurcations) and their path-dependency (hysteresis) are indicators for nonlinear behavior of dynamical systems. They have been reported for phonological processing by Raçzasek et al. (1999) and theoretically described by beim Graben et al. (2004) for syntactic sentence processing. Experimental evidence could be provided from ERP studies if the modulation of the P600 amplitude with stimulus frequency (Hahne & Friederici 1999) would appear as an averaging artifact using advanced data analysis techniques. This is addressed in the present study. Subjects were presented German sentences in subject-object and object-subject order with three different probability ratios (.75:.25; .5:.5; .25:.75). One half of the subjects read the objectsubject sentences with increasing probability, the other half with decreasing probability. Depending on the presentation paradigm, subjects continuously estimate a posteriori stimulus probabilities such that the a posteriori probability of object-subject sentences can be regarded as a control parameter of the experiment. According to the bifurcation hypothesis, one would expect that the P600 is elicited when the control parameter reaches a critical value. As a consequence, different portions of trials would contribute to the P600 among the stimulus frequency conditions. This would be reflected by differences in intertrial coherency, but not in amplitude differences. In addition, the hysteresis hypothesis predicts that subjects would react differently in the two presentation paradigms. The critical values of the control parameter would be different for increasing and decreasing a posteriori probabilities of the object-subject sentences. In order to dissociate between voltage amplitudes and intertrial coherency of the ERP data, the symbolic resonance analysis (SRA: beim Graben & Kurths 2003, Frisch & beim Graben 2005, beim Graben et al. 2005, Drenhaus et al. 2006) was employed.