Baggio, G. 1, 2 & Fonseca, A. 3, 4
1 SISSA International School for Advanced Studies,
2 Max Planck Institute for Psycholinguistics
3 Center of Mathematics, Computation and Cognition, ABC Federal University
4 Abdus Salam International Centre for Theoretical Physics
Understanding a word in context relies on a cascade of perceptual and conceptual processes, starting with modality-specific input decoding, and leading to the unification of the word's meaning into a sentence model. One critical cognitive event, turning a sensory stimulus into a meaningful linguistic sign, is the access of a semantic representation from memory. Considerable progress has been made in mapping semantic memory access in brain space and time, but little has been concluded from this body of work as to the changes that activating a word's meaning brings about in cortical dynamics. We recorded the EEG while participants read sentences that could contain a semantic violation, such as 'In July it is very cold outside'. We reconstructed the system's trajectories in phase space from individual EEG time series, and we applied three non-linear measures of predictability and complexity at either side of the semantic access boundary, defined as the onset of the N400 effect evoked by critical words (e.g. 'cold'). Semantic violations are associated with less predictable non-stationary signals preceding the N400 onset. This is not reflected in either ERP amplitude differences or EEG power modulations. Crucially, accessing the meaning of a semantically deviant word produces a phase transition to lower entropy states, in which cortical processing becomes more predictable and more regular, and in which information is generated at lower rates. Our study is a first step towards a quantitative complex systems approach to information flow through interfaces between sensory-motor and memory systems involved in language processing.