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".

Home.Return to Tom Jonard's Consciousness and Mind page

Created November 5, 2002, 
© 2002, Thomas A. Jonard