No. 98: Mar-Apr 1995
Blindsight is an eerie phenomenon. Humans with cortical blindness; that is, they have lost their primary visual cortex through brain damage or disease; can still detect objects and yet be unaware of them. Doesn't sound right, does it? The situation is this: A person, apparently totally blind, can somehow discern the location, form, and size of objects, but they will swear that they "see" nothing at all. In fact, they are blind by all tests. They have blindsight.
One explanation of blindsight maintains that the visual cortex has not been totally destroyed, and that functional remnants remain. In scientific terms, blindsight represents "suboptimal functioning of the primary visual cortex."
But now, A. Cowey and P. Stoerig report that they have totally removed the primary visual cortex from monkeys' brains. (Something one would not try with humans!) Tests with the visual cortex-less monkeys demonstrated that they possessed blindsight. Therefore, blindsight does not seem to be "suboptimal functioning" of a damaged brain -- at least in monkeys.
Blindsight thus remains a mysterious biological function. How do blindsighted humans detect objects of which they are not visually aware? Somehow information about the visual world appears in the brain.
(Cowey, Alan, and Stoerig, Petra; "Blindsight in Monkeys," Nature, 373: 247, 1995. Also: Kaas, Jon H.; "Vision without Awareness," Nature, 373:195, 1995)
Blindsight in humans is cataloged under code BHT3 in Biological Anomalies: Humans I. Ordering information here.