It is traditionally accepted that acquisition of vision must occur in the first year or two of life, before the critical period for vision has elapsed. We were fascinated, therefore, by your News Feature “Look and learn” (Nature 441, 271–272; 2006) reporting acquisition of competent vision in Pawan Sinha's patient following almost 30 years of functional blindness, particularly as we have observed a similar phenomenon in the more limited domain of stereo-blindness.
Although stereo-blindness is infinitely less disabling than total blindness, it too is generally considered to be incorrigible if not treated in early childhood. Yet we have recently followed a 50-year-old stereo-blind woman who, with proper ocular alignment, has been able to achieve full stereopsis, including the perception of random-dot stereograms, after many decades of stereo-blindness (O. Sacks New Yorker 64–73; 19 June 2006).
A critical period for the development of many aspects of visual perception remains a valid concept, and every effort should be made towards early intervention when compromised vision is detected. Nonetheless, if there is any early vision at all — even such limited vision as Sinha's patient had, or the very small fusional area that severe esotropic strabismus may permit, as in our own patient — it seems that islands of cortical function may be established, which can be reactivated and enlarged even decades later, given the requisite optical or surgical help. In such a situation, there is apparently enough cortical plasticity still present in the adult brain to allow, in some people, a full visual recovery.