The 'visual' acuity of blind persons perceiving information through a newly developed human-machine interface, with an array of electrical stimulators on the tongue, has been quantified using a standard Ophthalmological test (Snellen Tumbling E). Acuity without training averaged 20/860. This doubled with 9 h of training. The interface may lead to practical devices for persons with sensory loss such as blindness, and offers a means of exploring late brain plasticity.