Visual experience begins at eye opening, but current models consider cortical circuitry to be resistant to experience-dependent competitive modification until the activation of a later critical period. Here we examine this idea using optical imaging to map the time course of receptive field refinement in normal mice, mice in which the contralateral eye never opens and mice in which the contralateral eye is silenced. We found that the refinement of ipsilateral eye retinotopy is retarded by contralateral deprivation, but accelerated by silencing of the contralateral eye. Patterned visual experience through the ipsilateral eye is required for this acceleration. These differences are most obvious at postnatal day 15, long before the start of the critical period, indicating that experience-dependent binocular plasticity occurs much earlier than was previously thought. Furthermore, these results suggest that the quality of activity, in terms of signal to noise, and not the quantity, determines robust receptive field development.