How to measure the information gained from one symbol

Information theory provides a powerful framework to analyse how neurons rep resent sensory stimuli or other behavioural variables. A recurring question regards the amount of information conveyed by a specific neuronal response . Here we show that the commonly used definition for this quantity has a se rious flaw: the information accumulated during subsequent observations of n eural activity fails to combine additively. Additivity is a highly desirabl e property, both on theoretical grounds and for the practical purpose of an alysing population codes. We propose an alternative measure for the informa tion per observation and prove that this is the only definition that satisf ies additivity. The old and the new definitions measure very different aspe cts of the neural code, which is illustrated with visual responses from a m otion-sensitive neuron in the primate cortex. Our analysis allows additiona l interpretation of several published results, which suggests that the neur ons studied are operating far from their information capacity.