After enough damage is done to the primary
visual cortex, parts of it may cease functioning, leaving the sufferer either
wholly or partially blind. When damage is light the symptoms can manifest as a
blind spot, which if small enough can be ignored just as everybody ignores the
optic nerve's retinal junction. With heavier damage comes larger areas of blindness, and with profound damage comes complete loss of
sight. This form of blindness is just as damning as that caused by genetic disease or damage to the eyes, and the sufferer must learn the same coping mechanisms and receive the same aid as in other forms. However, when
vision is tested in certain ways it can have 80 or 90 percent of the
acuity of normal vision -- this phenomenon is called blindsight.
Common to all of the remaining visual abilities is that they are completely subconscious. When asked what they see or what is in front of them, the blind person will respond that he or she can't see anything, or that there's nothing there to be seen. It is when they are asked to somehow use their visual input for something, to answer a specific question or reach out and grasp an object, that the extent of their remaining vision can be seen. For instance, a blindsighted person can trace parallel lines on a piece of paper when asked to imagine a line and trace it with their hand. They can identify colors and simple shapes presented on a monitor, when told there's something there and asked to guess what it is. They can even put their hand into the appropriate shape to pick up, say, an eggshell or a kilogram weight, if asked to take what is in front of their hand.
Thus, the primary confusion surrounding blindsight is the disparity between what the mind is conscious of and what it is actually processing, between what is thought about and what is known. Even in primate experiments where the primary visual cortex -- and thus one-hundred percent of the thalamus-mediated optic nervous input -- has been removed, the primates exhibit the same spectrum of abilities, while exhibiting all the symptoms of perfect blindness. The very existence of blindsight is proof that a person's mind can conduct very complex active processing of information, completely without the person's awareness of it.
A now-classic discovery in neuroscience is that visual processing seems to travel in two functionally and physiologically separate paths through the brain. One path, called the What stream, is responsible for object recognition and recall, for identifying what an object is. Physiologically, this is the more direct neural pathway, going from the retina, through the lateral geniculate nucleus in the thalamus, and terminating in area V1 of the primary visual cortex. The other path, called the Where stream, is used for spatial orientation and relationships between visible objects and the viewer; it calculates where an object is, and where a moving object is going. This path is phylogenetically older (i.e. it evolved first) than the other, and projects input from the post-thalamic optic nerves through the superior colliculus, and out to motor and pre-motor areas of the brainstem, and also to the parietal cortex, which helps control movement. Notably, this visual information has no terminus in the temporal lobe, which is the brain area most associated with consciousness.
Hopefully, after reading that paragraph the most widely accepted explanation of blindsight is beginning to become clear. It claims that when the primary visual cortex is disrupted, no visual information makes it to the temporal lobe, so there can be no consciousness of it. However, since the visual pathway to the motor system (via the Where stream) is left intact, motor processes and some vague mental access are unaffected. This would explain perfectly the abilities of grasping and tracing with blindsight, though it leaves questions regarding the identification of colors and shapes.