Apparatus for monitoring activation in a heart comprises a probe (100), a plurality of electrodes (102) supported at respective electrode positions on the probe and each arranged to contact a respective detection position on the heart. Each of the electrodes (102) is arranged to detect electrical potential at the respective detection position during movement of a series of activation wavefronts across the heart and to produce a respective electrode signal. Processing means is arranged to analyse the electrode signals to: identify pairs of the electrode signals between which there is a degree of Granger causality; define a causality vector between the electrode positions of each of the pairs of electrodes; identify a potential driver location; and analyse the direction of a plurality of the causality vectors around the potential driver position to generate an indicator of the presence of a driver at the potential driver location.