Out of sight but not out of mind

Not Exactly Rocket Science discusses the case of a man who experiences the world as a blind man, but who is able to navigate through rooms despite having no conscious visual experience.

TN was a doctor before two successive strokes destroyed his ability to see. The first one severely damaged the occipital lobe on the left side of his brain, which contains the visual cortex. About a month later, a second stroke took out the equivalent area on the right hemisphere [see MRI scan image on the left]. TN is one-of-a-kind, the only known patient with damage like this in the entire medical literature. The fibres that connect the occipital lobes on the right and left halves of the brain have also been severely damaged and tests reveal that no blood flows between these disconnected areas.

Alan Pegna from the University of Bangor in Wales was the first to study TN’s abilities after he was recovering from this second stroke in a Swiss hospital. Pegna was the first to discover TN has an ability called blindsight, that allows him to unconsciously detect things in his environment without any awareness of doing so. He could correctly guess the emotions playing across the faces of other people. And as he did so, his right amygdala – an area of the brain involved in processing emotions – became active.

Blindsight is a condition where, after brain injury, patients lack conscious visual experience but can perform some visual tasks successfully despite thinking they are just guessing.

The first case of blindsight was reported in 1974 by neuropsychologist Larry Weiskrantz, although as with the majority of blindsight cases the patient wasn’t completely blind – in this case it was only for vision on the left hand side.

However, the patient was still able to reliably point to the locations of lights flashed up in the area, despite having no conscious experience of seeing them.

Despite damage to the cortical visual areas in the occipital lobe, it is likely that the earlier subcortical areas, such as the lateral geniculate nucleus (LGN) and the superior colliculus, allow for decision making on the basis of unconscious visual information.

In the new study, the patient is able to use this to avoid obstacles in his path, despite not being conscious of ‘seeing’ them.

This patient has a complete blindness and can complete relatively complex visual tasks without conscious awareness, making him one of the most interesting cases to come to light.

As well as being brilliantly written, the post is illustrated with a video of him avoiding obstacles in a room while walking. Impressive stuff.

Link to NERS post on blindsight case.
Link to study.
Link to DOI entry for same.

One Comment

  1. bowser
    Posted December 26, 2008 at 3:45 am | Permalink

    In some aspects, a case such as this isn’t unusual. A portion of the neural projections from the lateral geniculate do not go to visual cortex, rather they follow the tectum-pulvinar route to the cerebellum, where their input is used to guide motor behavior. Clearly, the patient here is relying on that pathway to perform navigational tasks. There are other visual inputs to the limbic system, which would explain the ability to recognize emotions. If I remember correctly these go to both the amygdala and the hippocampus. What makes this patient unusual is the extent of the damage to visual cortex. The scans above don’t indicate the elevation they’re at, but I’d suspect it must be lower down near the level of the corpus callosum. If the damage were higher up in the occipital lobe, and/or going into the parietal lobe, then navigational/spatial reasoning abilities would probably be affected as well. It will be interesting to see how progress on the study of this patient goes.


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