(A) Under certain conditions in the heart, spiral waves of electrical excitation are emitted by a ‘rotor’ of self-sustained electrical excitation. Hussaini et al. have used computer simulations to explore how a rotor in a two-dimensional model of heart tissue responds to various patterns of low-intensity illumination. (B) Upon illumination with a spatial gradient (here, the intensity increases from left to right), the rotor moves in the direction of higher light intensity. (C) Partial illumination of the model tissue with constant sub-threshold intensity light also causes the rotor to move towards the illuminated area.