Physica D 127 (1999) 250–266 Nonlinear analyses of interictal EEG map the brain interdependences in human focal epilepsy Michel Le Van Quyen a,b,∗ , Jacques Martinerie b , Claude Adam c , Francisco J. Varela b a H¨ ochleistungsrechenzentrum, Forschungzentrum J¨ ulich, D-52425 J¨ ulich, Germany b Laboratoire de Neurosciences Cognitives et Imagerie C´ er´ ebrale (LENA) (CNRS UPR 640 – University of Paris 6), Hˆ opital de la Salpˆ etri` ere, 47 Bd. de l’Hˆ opital, 75651 Paris Cedex, France c Unit´ e d’Epileptologie, Hˆ opital de la Salpˆ etri` ere, 47, Bd. de l’Hˆ opital, 75651 Paris Cedex, France Received 7 January 1998; received in revised form 11 May 1998; accepted 16 September 1998 Communicated by A.M. Albano Abstract The degree of interdependence between intracranial electroencephalographic (EEG) channels was investigated in epileptic patients with temporal lobe seizures during interictal (between seizures) periods. With a novel method to characterize nonlinear cross-predictability, that is, the predictability of one channel using another channel as data base, we demonstrated here a possibility to extract information on the spatio-temporal organization of interactions between multichannel recording sites. This method determines whether two channels contain common activity, and often, whether one channel contains activity induced by the activity of the other channel. In particular, the technique and the comparison with surrogate data demonstrated that transient large-scale nonlinear entrainments by the epileptogenic region can be identified, this with or without epileptic activity. Furthermore, these recurrent activities related with the epileptic foci occurred in well-defined spatio-temporal patterns. This suggests that the epileptogenic region can exhibit very subtle influences on other brain regions during an interictal period and raises the possibility that the cross-predictability analysis of interictal data may be used as a significant aid in locating epileptogenic foci. c 1999 Elsevier Science B.V. All rights reserved. Keywords: Temporal lobe epilepsy; Intracranial EEG; Epileptogenic focus; Nonlinear cross-prediction; Surrogate data PACS: 5.45.+b; 87.10.+e. 1. Introduction The existence of spatial patterns from neuronal ac- tivity is usually studied by mapping amplitudes or spectral powers in a given frequency band [1]. While such topographic maps represent the regional distri- ∗ Corresponding author. Address: Laboratoire de Neurosciences Cognitives et Imagerie C´ er´ ebrale (LENA-CNRS UPR 640), 47 Bd. de l’Hˆ opital, 75651 Paris Cedex 13, France. Fax: +33 01 44 243954; e-mail: lenalm@ext.jussieu.fr bution of the neural activities, they do not make full use of the information available in the multivariate data structure corresponding to the interdependences between spatially separated recording sites. This in- formation is essential for understanding how the brain integrates simultaneously distributed and divergent ac- tivities into globally coherent patterns [2–4]. In the present paper we propose an approach to study two related questions: (1) When specific patterns of neu- ronal activities occur simultaneously in two brain re- gions, do the two regions have close active couplings 0167-2789/99/$ – see front matter c 1999 Elsevier Science B.V. All rights reserved. PII:S0167-2789(98)00258-9