title: Donders is dead: cortical traveling waves and the limits of mental chronometry in cognitive neuroscience.
abstract: An assumption nearly all researchers in cognitive neuroscience tacitly adhere to is that of space-time separability. Historically, it forms the basis of Donders' difference method, and to date, it underwrites all difference imaging and trial-averaging of cortical activity, including the customary techniques for analyzing fMRI and EEG/MEG data. We describe the assumption and how it licenses common methods in cognitive neuroscience; in particular, we show how it plays out in signal differencing and averaging, and how it misleads us into seeing the brain as a set of static activity sources. In fact, rather than being static, the domains of cortical activity change from moment to moment: Recent research has suggested the importance of traveling waves of activation in the cortex. Traveling waves have been described at a range of different spatial scales in the cortex; they explain a large proportion of the variance in phase measurements of EEG, MEG and ECoG, and are important for understanding cortical function. Critically, traveling waves are not space-time separable. Their prominence suggests that the correct frame of reference for analyzing cortical activity is the dynamical trajectory of the system, rather than the time and space coordinates of measurements. We illustrate what the failure of space-time separability implies for cortical activation, and what consequences this should have for cognitive neuroscience.
authors.affiliation: Brain & Cognition Research Unit, University of Leuven, Leuven, Belgium. David.Alexander@ppw.kuleuven.be.
authors.forename: David M
authors.initials: DM
authors.lastname: Alexander
authors.affiliation: Brain & Cognition Research Unit, University of Leuven, Leuven, Belgium.
authors.forename: Chris
authors.initials: C
authors.lastname: Trengove
authors.affiliation: Brain & Cognition Research Unit, University of Leuven, Leuven, Belgium.|Kaiserslautern University of Technology, Kaiserslautern, Germany.
authors.forename: Cees
authors.initials: C
authors.lastname: van Leeuwen
mesh_terms: D001931:Brain Mapping; D058256:Brain Waves / Q000502:physiology*; D002540:Cerebral Cortex / Q000098:blood supply / Q000502:physiology*; D000066494:Cognitive Neuroscience / Q000266:history; D004569:Electroencephalography; D049672:History, 19th Century; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D015225:Magnetoencephalography; D008606:Mental Processes / Q000502:physiology*
keywords: Cortex; Difference method; Traveling waves