Newcastle University

Because our eyes see things from a slightly different view point (try closing each eye separately) our brains can use this difference (called ‘disparity’) to judge depth or the distance an object is from us. A distinctive feature of this stereoscopic vision is that we are far better at judging the relative disparity between two features (is one closer than the other?) than judging the absolute depth of an isolated object (how far is something from us?).  Within our brains visual information is broadly divided into two pathways – a "what pathway" which is involved with identifying objects and which travels along the base of the brain (a so-called ventral stream), and a "where pathway" which is involved in the location of objects and travels along the top of the brain (a so-called dorsal stream).  It is currently unclear how each of these two pathways contributes to the disparity which enables stereoscopic vision. To examine this question Dr Jenny Read and her group, working with colleagues at Durham and Oxford, undertook a detailed examination of the stereoscopic vision of a patient (called DF), particularly her ability to see things in depth.  In 1988 DF suffered carbon monoxide poisoning, which caused damage to both sides of her brain in an area called the lateral occipital cortex.  This is a major component of one of the two visual pathways - the ventral stream. Whilst not blind, DF has profound deficits in interpreting the images she sees – for example, she is unable to recognize objects by shape or even to report correctly whether a line is vertical or horizontal.  In this study the team found that, despite the severe damage to cortical visual areas, DF's ability to see things in stereo was strikingly unimpaired. She is actually better than many control subjects at judging whether an isolated object appears near or far.  However, DF is poor at using relative disparity between two objects to determine which is closer. They suggest that these results indicate that the ventral stream may play a critical role in enabling healthy observers to extract fine depth information from the relative disparities of objects within the visual scene. DF uses her intact “where pathway” to determine how far an object is, despite not necessarily being able to recogniseit.

Stereoscopic vision in the absence of the lateral occipital cortex. Read JCA, Phillipson GP, Serrano-Pedraza I, Milner AD, Parker AJ (2010) PLoS One 5(9):e12608 (Link to the article)