Discussion of jsiegle's comment: This paper provides a compelling demonstration that multisensory integration occurs in primary visual cortex. The authors record extracellularly from V1 neurons in mice running on a ball in the presence or absence of a virtual scene. They find that many neurons are strongly modulated by running speed, even in the dark. By breaking the correlation between actual running speed and the translational speed of the virtual environment, they can measure the relative contributions of movement and vision to a neuron's firing rate. Most neurons' responses were predicted by a linear combination of running speed and virtual speed, but some were influenced by running or vision alone. Given how dramatically and systematically movement alters visual responses, these experiments should encourage us to reconsider the role of primary sensory areas in perception.