The Versatile Mind: Seeing without Visual Cortex

On Dec. 22, 2008, Benedict Carey published an article online in The New York Times with the title "Blind, Yet Seeing: The Brain’s Subconscious Visual Sense" in which he describes a recent demonstration of blindsight. Researchers in Switzerland reported a remarkable performance of a patient who had recovered from a massive stroke that completely destroyed the visual areas of cerebral cortex on both hemispheres (de Gelder and others, 2008). Despite this impediment the patient successfully managed to negotiate an obstacle course set out for him in a hallway. Already 60 years ago, the eminent American neuroscientist Karl Lashley observed in a series of meticulous cortical ablation experiments that decerebrated rats were able to wend their way through their environment with surprising competence.

Nerve cells in a midbrain structure known as superior colliculus may play a crucial role in this process. When the cortical hemispheres of our brain are parted at the midline, the observer recognizes four small mounds rising from the surface of the underlying midbrain. To the early anatomists, the structures resembled little hills, called colliculi in Latin. One pair rises somewhat higher than the other, and thus was named the colliculi superior. The superior colliculi are composed of layers of nerve cells and nerve cell fibers. Three layers were shown to contain maps of our visual, acoustic and tactile space from the top to the bottom, respectively. The space maps normally overlap with great accuracy. As a consequence, the three senses permit us to localize an object in the same location with great precision. The congruence of spatial representation in three senses evolves during a critical period in brain development. Thirty years ago, Mazakasu Konishi and colleagues demonstrated a remarkable degree of plasticity in these maps in a series of elegant studies on barn owls in which the midbrain space representations realigned in compensation for manipulations of sensory input (Goldberg, 2008). Therefore, the recent observations in Switzerland do not come entirely as a surprise. A major focus of neurological research will remain on which additional functions are preserved after strokes that destroy visual cortex only partially.

Addenda

• You may wish to read my post dated Dec. 18, 2007, on recovery of function after stroke (02/25/2009).
• In her report on Reuters today, Maggie Fox describes a research study that provides evidence for improved recovery through visual exercise, perhaps aided by blind sight (04/01/09).
• If you are considering stem cell therapy, you may find the information on the International Society for Stem Cell Research site helpful (07/26/10).

References

• de Gelder B, Tamietto M, van Boxtel G, Goebel R, Sahraie A, van den Stock J, Stienen BM, Weiskrantz L, Pegna A (2008) Intact navigation skills after bilateral loss of striate cortex. Curr Biol 18:R1128-R1129.
• Goldberg J (2008) Locating a Mouse by Its Sound: A Brain Map of Auditory Space. HMMI Report.