Tom Jonard's Blindsight Page
Conscious
visual perception occurs in the the back of the brain in an area called
the occipital cortex. Each side of the brain has such an
area
-- the left occipital cortex being where the right side of the visual
field
is perceived and the right occipital cortex performing the same
function
for the left side of the visual field. This much is simply
deduced
from accidental damage to these areas. Such damage will result in
the victim losing the ability see part or all of the corresponding
visual
field depending on the extent of the damage.
Notice
that the left eye is not not just connected to the right occipital
cortex
and visa versa. The connections of the optic nerves do not lead
directly
to the occipital cortex. Indeed they are arranged so that each
eye
feeds half of the information it receives each side of the brain.
Therefore the effect of loss of the visual cortex on one side of the
brain
is different from loss of an eye or its optic nerve. Whereas
damage
to the eye effects the whole visual field damage to the visual cortex
on
one side effects only half of the visual field as seen in both eyes.
If
the entire occipital cortex is not completely destroyed the victim will
report a loss of visual perception in part of the visual field.
The
effect is similar to the blind spot in the visual field created where
the
optic nerve exits the eye and can be quite large. With the
victim's
cooperation an examination can map the size and position of the
resulting
blind spot. However victims of stroke in the occipital cortex
often
lose visual perception in the entire left or right visual field.
Destruction of both sides of the visual cortex will result in complete
blindness.
Now
the odd thing about some victims of occipital cortical damage due to
stroke
is that while they experience no conscious visual perception in the
blinded
area they nevertheless exhibit a remarkable ability to guess about its
content. Experiments show that they do much better than average
in
guessing if a light has been flashed in the blinded area or whether a
shape
displayed there was a circle or square or the direction of a movement
there.
This is the phenomenon of blindsight. Blindsight victims
may
be blind but they somehow receive information through their eyes!
The
obvious reaction on first hearing of blindsight is that it is too good
to be true -- surely experimenters or subjects (or both) who report
blindsight
are either faking it or have been fooled by hysterical blindness.
But with careful analysis both possibilities can be ruled out.
For
instance both fakers and patients diagnosed as hysterically blind tend
to do very badly in perceiving their environment -- stereotypically
"dumping
into the furniture" much more even than do really blind subjects.
Additionally blindsight victims are also observed to have damage in the
occipital cortex consistent with the blindness they describe.
Blind
sight victims are not aware of their extra-conscious visual perception
unless it is discovered and pointed out to them. They do not just
volunteer that they have this ability or these perceptions. Once
discovered blindsight can be trained so that the victim's ability to
use
it in experimental situations improves. But it does not replace
conscious
visual perception with a kind of pseudo-perception. The blind
sight
victim is never aware of whatever visual processing is occurring when
they
guess what they cannot see. They are still guessing.
Blindsight
should not be surprising considering the many connections in the brain
into which the optic nerve feeds. Information from the eyes is
relayed
many times before reaching the seat of conscious visual perception
(wherever
that may be). There is plenty of opportunity and ample evidence
for
visual processing along the way. What is surprising is that many
of the functions that we equate with visual perception such as sensing
the presence or absence of light, detecting shapes, collor and
detecting
motion apparently occur without the conscious visual perception that we
equate with seeing.
Conscious
visual perception is such a powerful experience that we believe it to
be
defining and all encompassing of any and all visual perception.
We
do not notice that some of the visual processing that occurs in our
brains
occurs outside of this experience. Here
are some more examples of deficits that support this. We assign
all this processing
and the perceptions that result to conscious visual perception as a
matter
of course. But take away conscious perception and the additional
visual processing done elsewhere in the brain is still left.
The
fact is that apparently we can in some sense see without conscious
visual
perception. Though blindsight is clearly not the real thing what
is it and what does it say about the real thing? Blindsight
allows
us a glimpse of unconscious processes that might be called preconscious
visual perception. It may be that this preconscious perception is
a functional part of conscious visual perception. On the other
hand
it may just be a vestigial capability that is no longer a part of
conscious
visual perception. In either case it raises the possibility of
visual
perception that operates entirely at an unconscious level.
We
are not unfamiliar with other unconscious processes that control
behavior.
Temperature regulation of the body and pulling an appendage away from a
source of pain are but two examples of the way we behave involuntarily
and unconsciously. Various parts of the nervous system perhaps
even
the brain are involved in such actions -- but not consciousness.
Action without consciousness seems mechanistic -- a robot could do the
same. We do not like to think of ourselves as robots and
consciousness
is one thing that we can point to distinguish ourselves from them.
In The
Origin of Consciousness in the Breakdown of the Bicameral Mind
(Houghton
Mifflin, 1976) Julian Jaynes proposes that prior to about 2000, BCE
humans
were robots directed by voices in their heads -- their gods. The
voices originated in one hemisphere of the brain and directed the other
to action. These humans did not think as we do and they were not
conscious. The nature of the mind according to Jaynes abruptly
changed
about 4,000 years ago. According to Jaynes some mental
dysfunctions
we see today are throw-backs to the era of the bicameral mind -- which
was not so long ago.
Whether
we believe Jaynes' theory of the development of the human mind it does
show that we need to take seriously all sides of the issue of how the
mind
works including those that don't correspond to our self image.
The
relationship of the mechanisms of the body and brain to the mind must
be
honestly considered given that we don't really know the truth just
yet.
What these relationships are and how they got that way will each help
us
understand the other and lead to a better knowledge of who we are.
Blindsight
suggests that the mental processing that makes up visual perception is
not a single capacity located in a single area of the brain. This
is an idea that may well apply to more than just visual
perception.
The mind as a whole might be distributed throughout the brain and its
units
function in a coordinated but semiautonomous way. Loss of one
processing
area might result in a fragmented mind. In such a fragmented mind
a whole faculty like conscious vision might be lost but not its
components.
In that case the mind would be unable to construct the whole perception
but might still have access to its components. This seems to be
precisely
what we see in blindsight.
This
raises an interesting question: what happens if the faculty that
is lost in this way is consciousness itself? Is the result a
human
robot? Or a human with Julian Jaynes bicameral mind? Human
robots or automata have figured significantly in the history of the
Philosophy
of Mind. We might wonder if any of our fellows are in fact
robots.
We may also wonder why we all aren't. What is consciousness
for?
Could it be the faculty that holds all other mental processes
together?
Just as blindsight is not real sight in the absence of consciousness
perhaps
no perception is real perception without consciousness. Perhaps
without
conscious no mind can construct a perception of its environment
sufficient
to survive. Human robots might be impossible.
If
the latter were the case it might have radical implications for animal
consciousness and what many suppose is the uniqueness of the human
animal.
Many animals exhibit many behaviors that suggest mental capabilities
little
short of our own. Yet the possibility of animal consciousness
remains
an open question. If consciousness is necessary to forge a
coherent
mind out of preconscious perception then maybe animals must also be
conscious
to survive as they do. In this case the human is not the only
conscious
animal. So much for being "little less than gods".
