One of the basic goals of cognitive psychology is the analysis of the covert processes that occur between stimulus and response. In the past 20-30 years, the tools available to cognitive psychologists have been augmented by a number of imaging techniques for studying the 'brain in action' in a non-invasive manner. These techniques have their strength in either temporal or spatial information, but not both. We review here recent advances of a new approach, the event-related optical signal (EROS). This method allows measurements of the time course of neural activity in specific cortical structures, thus combining good spatial and temporal specificity. As an example, we show how EROS can be used to distinguish between serial and parallel models of information processing.