Sight Unseen: An Exploration of Conscious and Unconscious Vision

This slim volume is based on the authors’ investigations over a period of 15 years of a patient (“Dee Fletcher”) who developed visual form agnosia following a freak accident in which she suffered carbon monoxide poisoning. Specifically, DF has lost certain perceptual abilities, namely identifying shape and form, although she can still perceive colour and the fine detail of surfaces (visual texture), yet her visuomotor (“vision for action”) control is strikingly preserved. The neuroanatomical substrate of this pattern of deficits is selective damage to the ventral stream of visual processing, specifically the lateral occipital area, whilst the dorsal stream is left intact (the deficits are the inverse of those seen in patients with optic ataxia). The authors take DF as the starting point for an exposition on the workings of the two visual systems, originally postulated by Mishkin & Ungerleider, summarizing animal work and functional imaging studies as well as neuropsychology. The conclusions which emerge are that visuomotor control is viewpoint-dependent (egocentric), uses real-world metrics, and has a very short time constant, whereas visual perception is object-based, relational, and has an indefinitely long time constant. Moreover, the workings of the former are not available to consciousness, whereas the latter are (hence the subtitle of the book). Despite these polarities, and the possible implication of Cartesian dualism, nonetheless the two systems interact seamlessly. It is a fascinating tale and well-told. Although obviously of most appeal to those with an interest in cognitive neurology, this book may nonetheless be read with profit by any neurologist with an interest in how the brain works. The lack of a bibliography of papers referred to in the text is, however, a significant omission.