The Phenomena of Blindsight
by Karen Wilson

Introduction

Numerous deficits can arise from damage to the visual
system.  Such deficits can affect any number of abilities
such as color processing, motion perception, spatial
attention, and spatial knowledge and thought (Ellis and
Young, 1988).  Damage to the visual cortex can result in
cortical blindness.  Depending upon the lesion, the area
of blindness can be for whole or part of the visual
field.  Traditionally, it has been thought that patients
with cortical blindness were unable to detect stimuli in
the blind areas.  Evidence has shown that patients with
cortical blindness are able to detect stimuli in the
blind field, but they are unaware of the presence of the
stimulus.  This phenomena is known as blindsight.

The phenomena of blindsight is characterized by the
ability to attend to stimuli presented in an area of the
visual field where a patient has lost the ability to see. 
Often patients who demonstrate blindsight are cortically
blind and generally do not have damage to the eyes
themselves.  Cowey and Stoerig (1991) state that
patients' pupils still respond to changes in light,
pattern, and contrast.  Conscious visual experience may
be gone for all or part of the visual field, but patients
are still able to make correct judgments as to the
presence and location of stimuli (Ellis and Young, 1988). 
Patients with blindsight are often hemianopic, meaning
that there is a loss of vision for stimuli falling to
either the left or right visual field (Marcel, 1983;
Ellis and Young, 1988).

Experimental evidence of blindsight

Some have been skeptical of the phenomena of blindsight,
stating that patients' cortex was not completely
destroyed, only damaged or that researchers used loose
standards for detection of stimuli by patients (Cowey and
Stoerig, 1991).  In many cases the skepticism is
unjustified for several reasons:  blindsight has been
demonstrated in patients who have had complete removal of
the visual cortex; stimuli presented in the natural
blindspot are undetectable; and stimuli presented to the
blind area can influence the response to stimuli
presented in intact areas (Cowey and Stoerig, 1991).

Poppel et al (1973) discuss four patients with lesions to
the visual cortex exhibiting blindsight.  None of the
patients had complete loss of vision, and each had
deficits in different areas of the visual field.  All of
the patients reportedly had intact foveal vision. 
Patients were presented with visual stimuli in the blind
areas and asked to guess where the stimulus was using eye
movements to show the stimulus location.  A sound was
elicited by the experimenters to signify to the patients
that the stimulus had been shown.  Throughout the
experiment, the patients never reported being able to see
the stimulus presented.  Despite insisting that they did
not see or sense the stimulus, the subjects performed at
levels greater than chance when asked to identify where
the stimulus had been presented.  The subjects had no
conscious awareness of the stimuli, and believed that
they were merely guessing the location of the stimulus. 
Poppel et al (1973) suggest that because their patients
were able to detect the stimulus, that there is some
visual mechanism within the brain still functioning. 
They propose that the area involved is the midbrain
because it receives a strong input from the retina.

Weiskrantz (1977) discusses the case of a male patient
who had a tumor removed from the right calcarine fissure
and subsequently sustained visual deficits in the left
field of vision in both eyes.  The patient was asked to
reach out and touch a screen that had an image projected
onto it.  The subject responded remarkably well, but
insisted, like subjects in the previous study that he had
been merely guessing as to where the stimulus had been
projected.

Further evidence for blindsight comes from Marcel (1983). 
Marcel (1983) reports that when reaching for objects of
differing size, shape, orientation, location, and
distance, that were presented in the blind field,
patients performed at levels better than chance.  Marcel
(1983) hypothesizes that the problem is not that the
stimulus isn't being analyzed by the patient, but that
the problem lies in patients' awareness of the sensory
information.

Marshall and Halligan (1988) report the case of P.S., a
49-year-old woman who suffered damage to the right
cerebral hemisphere after a subarachnoid hemorrhage.  She
subsequently failed to respond to information presented
to the contralateral (left) side of the lesion.  When
presented with visual information to both the left and
right sides of her visual field, P.S. ignored the stimuli
presented to the left side of the visual field.  P.S.
reportedly had no conscious awareness of any information
presented to the left visual field.  When presented with
two pictures of a house, one of which was burning, she
replied that both pictures were the same.  She neglected
the burning house because it was presented to the left
visual field.  When asked which house she would prefer
live in, she chose the non-burning house, though she also
stated that there was only one picture.  When finally
presented with the house with the flames to the right
visual field, P.S. noticed the flames in all trials. 
Although she thought the pictures were identical, there
was an implied awareness of stimuli in the left visual
field, due to the fact that the patient consistently
preferred the non-burning house.  Marshall and Halligan
(1988) hypothesize that the ignored stimulus is able to
affect cognitive functioning at a pre-conscious level.

Forced-choice test

All of the studies reviewed above employed the use of the
forced-choice test to demonstrate blindsight.  In the
forced-choice test, the subject must prove that they can
detect the stimulus and the researcher is able to
objectively confirm that the stimulus has been detected
(Sekuler and Blake, 1994).  In studies of the visual
system, in order to verify detection of a stimulus, the
researcher will present a stimulus either to the left or
right visual field, and the subject is instructed to
state the position of the stimulus.  This
characteristically has been the method used in research
into blindsight.  According to Sekuler and Blake (1994),
the forced-choice test is useful because it demonstrates
that the sensory system is able to detect more
information than a person is consciously aware.  In cases
of subjects with blindsight this may indicate that the
part of the visual system responsible for detecting
stimuli is still intact, but that the areas responsible
for the conscious awareness of stimuli are the ones that
are damaged.

Neurobiology of blindsight

Patients who have exhibited the phenomenon of blindsight
display damage to the cortex and the pathways from the
eyes to the visual cortex rather than to the eyes
themselves.  Marcel (1983) characterizes blindsight as a
dissociation  between the geniculo-occipital system and
the pulvinar-collicular system, whose projections are to
the parietal lobe.  The area damaged seems to be the
geniculo-occipital pathway (Marcel, 1983).  Ellis and
Young (1988) report that damage to the pathway of the
lateral geniculate nucleus (LGN) and the occipital lobe
can be responsible for a loss of sensitivity to stimuli
falling within the corresponding visual field.  The loss
of vision is often hemianopic and does not affect the
entire visual field.  Like Ellis and Young (1988),
Carlson (1994) discusses evidence that blindsight is due
to the connections that are received from the visual
association cortex from the superior colliculus and the
dorsal LGN.  Carlson (1994) goes on to say that the exact
function of these connections is still unknown.

Conclusion

Research into blindsight has raised questions as to the
function of the primary visual cortex and its pathways. 
Because patients with blindsight are able to detect
stimuli, it is thought that the primary visual cortex may
be more involved with conscious awareness of stimuli
rather than detection of stimuli (Marcel, 1983). 
Blindsight is seen as  a problem of awareness rather than
detection of stimuli.  Blindsight also raises questions
about consciousness and how we are made aware of the
sensory data that is imposed upon us.  

Blindsight is a fascinating phenomena that has given much
insight into how visual processes work and how we are
consciously made aware of stimuli in our environment.  It
is still unclear as to what the exact mechanisms of
blindsight are.  With more research, will come more
understanding of the phenomena of blindsight.

References

Carlson, Neil R., (1994).  Physiology of behavior. 
Boston:  Allyn and Bacon.

Cowey, Alan, and Stoerig, Petra (1991).  The neurobiology
of blindsight.  Trends in Neuroscience, 14, 140-145.

Ellis, Andrew, and Young, Andrew (1988).  Human Cognitive
Neuroscience.  London:  Lawrence Erlbaum Associates.

Marcel, Anthony J. (1983).  Conscious and unconscious
perception:  an approach to the relations between
phenomenal experience and perceptual processes. 
Cognitive Psychology, 15, 238-300.

Marshall, John C., and Halligan, Peter W. (1988). 
Blindsight and insight in visuo-spatial neglect.  Nature,
336, 766-767.

Poppel, Ernst, et al (1973).  Residual visual function
after brain wounds involving the central visual pathways
in man.  Nature, 243, 295-296.

Sekuler, Robert, and Blake, Randolph (1994).  Perception. 
New York:  McGraw Hill.

Weiskrantz, L. (1977).  Trying to bridge some
neuropsychological gaps between monkey and man.  British
Journal of Psychology, 68, 431-445.

Copyright © 1997, Dr. John M. Morgan, All rights reserved - This page last edited March 17, 1997
If you have any feedback for the author, E-mail me