Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
If the visual cortex is damaged by stroke or other injury, patients lose the ability to see things in part of the visual field. The abnormal blind area in the visual field is called a hemianopia (hem-i-an-NO-pia). Some patients with hemianopias involving as much as half the visual field can nevertheless reach out and touch objects in the "blind" area. This is called blindsight.
Blindsight is relatively rare. It is not found in every patient with hemianopias. However, blindsight intrigues investigators because it seems to suggest that visual information can find its way into the brain through some unconscious route.
What is blindsight? What is an explanation for blindsight?
The leading theory of blindsight suggests that visual information reaches the brain through the second visual system in the brain, which runs through the superior colliculi of the brain. The second visual system is a localizing system, specialized for guiding eye movements.
The surface of the retina is mapped onto the superior colliculi much the way it is mapped onto the visual cortex at the back of the brain. In other words, both the visual cortex and the colliculi have a map of the retina: a retinotopic map. When movement occurs somewhere in the visual field, the superior colliculi generate an automatic eye movement toward that location. If this circuit is still working in a person who has a damaged visual cortex, the eye movement might be used as a cue to the location of the object.
What is evidence that cells in the superior colliculi might respond to specific patterns in the periphery?
The second (collicular) visual circuit may "know" more about an object than merely its location. The superior colliculi are richly supplied with incoming fibers from other areas of the brain. Cells in the colliculi habituate (cease firing) when the same object is presented again and again in the periphery of the field of vision. Yet, after habituation, those same cells respond to a new, unfamiliar object. So pattern recognition of some type takes place in the collicular pathway. This might result in ESP-like experiences. For example, people might sense they are being watched if the circuitry in the superior colliculus detects a pair of watching eyes somewhere in the periphery of the visual field.
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