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William Saletan clears up some confusion I had over the Terry Schiavo autopsy report.

Most reports claimed that Schiavo was blind. I had assumed this meant massive damage to her visual cortex. Instead:

According to Terri Schiavo’s autopsy report , her “lateral geniculate nucleus (visual) demonstrated transneuronal degeneration with gliosis.” Or, as the medical examiner put it in plainer English, “Her vision centers of her brain were dead. Therefore, Mrs. Schiavo had what’s called cortical blindness. She was blind, could not see.”

The lateral geniculate nucleus, or LGN, is a sort of waystation between the eyes and the visual cortex. Cortical blindness usually involves damage to the visual processing areas rather than the LGN. If damage is constrained to the visual cortex, then a subject will exhibit a phenomenon of perception called blindsight. Patients will perform above chance in guessing the quadrant of a visual stimulus (usually a bright light). They may even be able to point in the general direction the stimulus. Yet, patients will have no conscious perception of the light.

These behavioral experiments told us that blindsight exists. Later neurophysiological study has revealed its mechanism in the brain. There is an extra pathway out of the LGN that avoids the visual cortex. Its much smaller, and thus went undetected for sometime. This pathway leads to what are often considered more primitive areas of the brain associated with movement, position and basic orientation.

Its possible that this extra pathway survived Terry Schiavo’s stroke less scathed than the other. If so, it could explain her scant ability orienting toward voices that fundamentalists read so much into. Leave no doubt, blindsight is famously unconscious. As the autopsy shows, Terry Schiavo had no higher brain function, and certainly no conscious perceptual ability.

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