What, when, where in the brain? Exploring mental chronometry with brain imaging and electrophysiology

Most models of information processing in the brain assume that cognitive tasks can be broken down into components that are solved in sequential steps, from perception through a number of cognitive comparisons (e.g., memory) or transformations (e.g., language) to the preparation and execution of a motor response. The relative and absolute timing of these stages and the degree to which they occur parallel or in serial order has traditionally been investigated with behavioural methods, mainly reaction times. This classical mental chronometry has over the past decades been complemented with neurophysiological approaches, using event-related potentials (ERPs), functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS). This review highlights the contributions and limitations of these techniques and focuses on recent advances in the combination of ERPs and fMRI to elucidate working memory and number processing. It will be argued that similar design principles as established for behavioural studies of mental chronometry should apply to neurophysiological investigations as well.