We form our conscious sense of vision using the occipital lobe of our cerebrum, the uppermost portion of the brain that has increased in size during mammalian (and independently in bird) evolution. Other vertebrates rely more heavily on other regions of the brain, especially the midbrain, to process sight. We still use a region in the midbrain, the superior colliculi, for visual reflexes and tracking objects with our eyes.
A paper in Current Biology provides the best evidence to date that functionally blind humans can use these more ancient brain regions to “see” their environment – an ability that is called blindsight. The subject in this study, TN, damaged both primary visual cortices during two successive strokes. Lack of activity in these brain regions was confirmed by brain imaging. Amazingly he was still able to navigate an obstacle course, clearly sensing objects in his way.
Proving a negative is always difficult, so the authors go to some effort to convince us that TN truly cannot consciously see. They also argue against the possibility that he is using echolocation to sense the objects in his path and note a previous study on a blind monkey that was able to similarly navigate obastacles. While the authors of this new paper did not identify the brain regions used by TN in the video above, one possibility is that he is relying on his more ancient vertebrate midbrain.
The same authors previously showed that TN could also respond to human facial expressions – for example a scary looking face would produce electrical activity in the fear regions of the amygdala. In this case it was not the midbrain, but deeper regions of the cerebrum being used. Both of these studies highlight the complex ways that visual stimuli are processed in the brain.
B DEGELDER, M TAMIETTO, G VANBOXTEL, R GOEBEL, A SAHRAIE, J VANDENSTOCK, B STIENEN, L WEISKRANTZ, A PEGNA (2008). Intact navigation skills after bilateral loss of striate cortex Current Biology, 18 (24) DOI: 10.1016/j.cub.2008.11.002