The effect of visual loss on the adult neocortex can have significant impact on the success of a visual implant. Recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its afferent input. The result of these changes may hamper the development of a visual prosthesis if visual sensation cannot be effectively restored by stimulation of the surviving elements of the visual pathway. In order to evaluate further the visual performance of the mammalian adult brain following visual loss, especially the dominant form of blindness in humans, namely loss of pattern vision, we examined the cortical evoked potential of adult mice following 7, 30 and 120 days of visual deprivation via bilateral eyelid suture. Cortical potentials were elicited with a flash visual stimulus or by electrical stimulation of the retina. We found that after 7 days deprivation there was a potentiation of the evoked response while at 30 and 120 days deprivation the visual evoked responses were significantly reduced. Increasing the visual stimulus intensity reduced the effects. The electrical evoked potential demonstrated a corresponding reduction in stimulus threshold at 7 days and a corresponding rise (40–50%) after 30 and 120 days. These findings suggest that the adult brain exhibited significant experience-dependent modifications following visual loss, and the impact depended on the duration of deprivation. Such reduction in visual responsiveness, especially with electrical activation, will need to be taken into account in the development of a visual implant.