Stereoscopic vision in the absence of the lateral occipital cortex

"D.F." is a famous neuropsychological patient who experienced carbon monoxide poisoning in an accident many years ago. This left her with long-term damage to a number of different brain areas, visible on magnetic resonance imaging. As a consequence, she has a remarkably specific visual impairment known as visual form agnosia. Although she can see colours and movement, and she can navigate the world visually without bumping into things, she cannot recognise objects visually. This is not because her vision is blurred; she has good acuity. She simply perceives objects as a meaningless jumble of elements. Fortunately for science, DF is a highly intelligent woman who is interested in her unusual brain injury, and over the past three decades she has devoted untold hours of her time to working with psychologists in order to map out the precise nature of her visual impairment. David Milner, emeritus professor at Durham, has worked extensively with DF along with his long-time colleague Mel Goodale. I highly recommend their two books, "The visual world in action" (the longer and more detailed) and "Sight Unseen" (slimmer and accessible for lay readers). On this project, I collaborated with David and with Andrew Parker from Oxford, along with my postdoc Ignacio Serrano-Pedraza and PhD student Graeme Phillipson, in order to examine DF's stereo vision.

It was already known that DF retained stereo vision, but that her stereoacuity was somewhat impaired. Standard clinical measures of stereo vision ask subjects to use stereo disparity to tell which of two surfaces was the closer. People with normal vision are incredibly precise on making such relative disparity judgments. DF could do the task, but only when the separation between the two surfaces was relatively large. Andrew had recently written a very insightful review on the neuronal basis of stereo vision in Nature Neuroscience. Based on the physiological and imaging literature Andrew reviewed, and the location of DF's most severe cortical damage, we were not surprised that DF was impaired on standard relative disparity tasks, but predicted that she would not be impaired on an absolute disparity task, i.e. judging the depth of an isolated object.

To our delight, this prediction was initially borne out. On DF's first two visits, she performed as well as age-matched controls on an absolute disparity task, but whereas they performed much better when a reference surface converted the task to a relative-disparity task, DF did not. However, this initially clean story was complicated when, after testing DF on a variety of other stimuli, she subsequently improved on the relative disparity task. It was as if we had provided training which enabled her to use information she had previously been blind to. For example, maybe her attention was not automatically drawn to the boundary demarcating the two surfaces, as controls' is, but with training she learnt where the informative region of the stimulus was. However, our results did show clearly that there is a big difference between relative disparity between adjacent surfaces (which was compromised by DF's cortical lesion) and relative disparity between transparent surfaces in relative (which DF sees perfectly). This neuropsychological evidence agrees with the predictions from physiology.

This was my first neuropsychological study, and it was a very interesting experience for me. While it is inevitably difficult working in an "n=1" situation, I found it fascinating gaining insight into DF's visual world, seeing first-hand the problems faced by someone who has a completely unique visual experience and thus no words to communicate it. I would like to record my gratitude to DF for bearing with me so patiently!

ReadEA10.pdf
File Size1.7 MiB
DateJanuary 17, 2012
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AuthorRead JCA, Phillipson GP, Serrano-Pedraza I, Milner AD, Parker AJ