As Donald Rumsfeld memorably observed: "There are also unknown unknowns - the ones we don't know we don't know." While it is a truism that we do not know the things we are not conscious of, the implications of this statement are more than may meet the eye: such things can be "unknown unknowns" - things that we are not even aware that we should know. Certain patients with parietal lobe injuries suffer from contralateral neglect: they seem unaware of aspects of the world on the side opposite to their brain damage. Such a patient may deny that the opposite side of his body belongs to him, so that he shaves only half of his face. I started wearing glasses as a teenager, and still remember the surprise that distant views could contain so much detail. I had not previously missed this visual experience, as I did not know it existed; indeed, in my consciousness, it had not. In the field of neurology, it is well known that patients can lack awareness of a lost or missing aspect of normal perception. In contrast, we are all too aware of a misordered perception, such as double vision.
In Fixing My Gaze, the neuroscientist Susan Barry writes about her experience as a mature adult of gaining stereoscopic vision (the ability to see the world as three dimensional through integrating information from both eyes). The book is a joy to read, because it is suffused with the enthusiasm of new discovery. Topics are clearly explained so that the book should be easily accessible to a wide audience, while postscript notes provide references to the more detailed scientific background.
The brain allows us to adapt to our environment through learning from experience. Although there are still many unknowns (and, doubtless, many unknown unknowns), certain principles of such adaptation have been widely accepted. Thus, in general, the brain is most adaptable ("plastic") in early life and becomes less so later. The discovery that, for vision, certain types of adaptation appeared to be restricted to the first few years of life was important for the message that early intervention could be crucial to preventing lifelong loss. As Barry points out, it became the accepted view that if "squint" (strabismus, where both eyes do not point in the same direction) is not corrected in the first few years of life, the brain becomes permanently miswired so that the consequent visual impairment becomes irreversible.
However, we are all aware that as adults we remember new experiences every day, so at least parts of our brain remain adaptable throughout life. Barry points out that the adaptability of the mature brain is greater than a prevailing current view suggests. As she indicates, this is important not only for people like herself who may gain stereoscopic vision in later life but also for patients recovering from stroke.
It remains essential to treat visual defects in infancy: the young brain adapts much faster than the adult, and indeed measurable changes can occur in hours. But the adult brain remains plastic so that, with persistence (possibly over weeks and months), adaptation is more achievable than standard dogma suggests. Standard dogma says that unused visual pathways lose their anatomical connections. Barry's case, along with the others she quotes, suggests that these connections may be masked rather than lost - so leaving open the possibility of their being unmasked in adult life.
Barry describes her experiences of having a squint that was partially corrected in childhood but left her using first one eye then the other, rather than integrating the information from them both as happens in normal vision. She takes us through her childhood experiences and then recounts how, as an adult, she discovered vision therapy. This taught her how to bring together the information from both her eyes (producing binocular fusion) and, as a consequence, experience true stereoscopic vision.
She makes telling points about the importance of integrating the clinical work of optometrists and ophthalmologists, but it is the description of her changed perception that is most striking. We have multiple ways of knowing that the world is three dimensional (including occlusion of one object by another and an object's shading by light). For this reason, the loss of binocular vision is not considered a major impairment - although it may be more difficult to thread a needle, perception has been assumed to be largely normal. What Barry so eloquently establishes is how impoverished was her visual perception before she gained true stereoscopic vision, and how great was the perceptual enhancement when her previously "unknown unknown" became, instead, part of her everyday conscious experience. Reading her account may indeed be described as an eye-opening experience.
Fixing My Gaze: A Scientist's Journey into Seeing in Three Dimensions
By Susan R. Barry. The Perseus Book Group 2pp, £15.99. ISBN 9780465009138. Published 4 June 2009