INFORMATION TECHNOLOGY SYMPOSIUM
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One common procedure to establish the presence of blindsight is to
put the blind proband in front of a video screen that is divided into four
quadrants-upper left, upper right, lower left, lower right-and flash
a series of randomly placed dots of light on the screen. The experimenter
asks the proband to indicate by pressing one of four buttons in which
quadrant a particular dot has appeared. The subject protests. He says to
the experimenter: "Didn't I tell you that I'm blind? I can't see anything."
The experimenter answers: "Never mind, do me a favor and just push
one of the buttons whenever I tell you that a dot has been flashed.
If
you
really didn't see it, just make a guess."
If
that blind proband happens to be blindsighted, he will be able to
report location of the flashed dots to a degree of accuracy that is much
higher than he could have achieved by random guessing. In other words,
the subject did see the dots without having been consciously aware of
them.
It
should be noted, though, that if the subject is asked to report a
dot's position verbally, the correctness of his answers is no greater than
would have been achieved by random guessing.
Blindsight is a particularly favorable phenomenon for the study of
subconscious presence because the neurobiological analysis of normal
vision has progressed farthest among sensory modalities. There are at
least two reasons for this preeminence of vision in research on sensory
perception. First, experiments on the processing of visual input are eas–
ier to carry out than those addressed to other sensory modalities
because visual stimuli can be controlled more precisely and varied more
widely than auditory or olfactory stimuli. And, secondly, our human
language is much better adapted for talking about the things we see than
about the things we hear or smell. So it is not a coincidence that brain
scientists who study visual perception can report the results of their
research much more clearly and in more meaningful detail than those
who address auditory or olfactory perception.
The human visual pathway begins at the array of light receptor cells
in the retina located at the back of the eye, whence the light-induced
electrical activity is passed on to the retinal ganglion cells for the first
stage of processing of the visual information. The retinal ganglion cells
then pass on that information via the optic nerve to a relay station
located in the midbrain. From there it is conducted via the nerve tract of
the optic radiation to an area of the cerebral cortex at the back of the
head designated as primary visual area, or area Vl. The visual informa–
tion is then passed on from the primary visual area to other higher cen–
ters of the cerebral cortex for further processing, and eventually winds
up in the parietal lobe of the cortex. There it is integrated with the input
