THE TOPOGRAPHY OF NONLINEAR CORTICAL DYNAMICS AT REST, IN MENTAL CALCULATION AND MOVING SHAPE PERCEPTION

Differential cortical activation by cognitive processing was studied u sing dimensional complexity, a measure derived from nonlinear dynamics that indicates the degrees of freedom (complexity) of a dynamic syste m. We examined the EEG of 32 healthy subjects at rest, during a visual ly presented calculation task, and during a moving shape perception ta sk. As a nonlinear measure of connectivity, the mutual dimension of se lected electrode pairs was used. The first Lyapunov coefficient was al so calculated. Data were tested for non-linearity using a surrogate da ta method and compared to spectral EEG measures (power, coherence). Su rrogate data testing confirmed the presence of nonlinear structure in the data. Cognitive activation led to a highly significant rise in dim ensional complexity. While both tasks activated central, parietal and temporal areas, mental arithmetic showed frontal activation and an act ivity maximum at T3, while the moving shape task led to occipital acti vation and a right parietal activity maximum. Analysis of mutual dimen sion showed activation of a bilateral temporal-right frontal network i n calculation. The Lyapunov coefficient showed clear topographic varia tion, but was not significantly changed by mental tasks (p<.09). While dimensional complexity was almost unrelated to power values, nonlinea r (mutual dimension) and linear (coherence) measures of connectivity s hared up to 37% of variance. Data are interpreted in terms of increase d cortical complexity as a result of recruitment of asynchronously act ive, distributed neuronal assemblies in cognition. The topography of n onlinear dynamics are related to neuropsychological and neuroimaging f indings on mental calculation and moving shape perception.