Another compelling example is the rat/man illustration (b) (image here?). When you perceive the rat, you are, effectively, agnosic for the man, and vice versa. For normal people, it is a bistable figure; but for John, perception of neither rat nor man would ever occur, despite his normal visual acuity.
You can also get a feel for agnosia by thinking of what happens when you listen to a foreign tongue. You hear all the sounds, syllables, intonations and rhythms of the speech, but none of it makes any sense to you (c) (image?). You simply cannot create a meaningful perception from these sensations.
Problems in the Pathways
To understand GY’s and John’s predicaments, we will need to take a brief tour through the anatomy of the visual pathways. Those more than 30 visual-processing areas have staggeringly complex connections among them. Fortunately, despite this complexity, we can discern a simple overall pattern.
Messages from the retina of the eye get transmitted along the optic nerve before diverging into two parallel anatomical pathways, which we shall call “old” and “new” pathways to indicate their evolutionary sequence (d). The old pathway, also called the where pathway, goes to a structure called the superior colliculus, which forms a bump on the roof of the brain stem, the stalk that emerges from below the brain and continues as the spinal cord. The colliculus helps to determine the location of an object. When a novel or salient event occurs in your environment (for example, when there is an object looming over your left shoulder), you reflexively orient and swivel your eyeballs toward it without knowing what it is. That is, you orient to it or locate it before you proceed to identify it.
The other pathway, the newer one, as we shall see, is required for identifying an item, even though it is incapable of locating it or orienting to it. The new pathway projects to the visual cortex (V1 for short) in the back of the brain, where the features of the object are analyzed (for color, orientation of edges, movement, and so on). Information from V1 splits again into two pathways farther along the visual-processing course: the how pathway projecting into the parietal lobes (“How” do I use or interact with this object?) and the what pathway (“What” exactly is this object? What does it mean for me?) into the temporal lobes (d). The 30 visual areas we spoke of are shared between these pathways. Bear in mind that we have described a grossly oversimplified caricature: many fibers go back and forth between the areas; they are heavily interconnected and not entirely autonomous. But in science it is not a bad idea to start with a simple picture.
Now let us return to GY, who has blindsight. GY has complete damage to V1. No information reaches either the what or how pathway, rendering him blind in the sense that he cannot consciously see objects. But because his where pathway (going through the superior colliculus and bypassing the damaged V1 en route to higher cortical centers) is intact, he can guide his hand unerringly toward the light spot that he cannot consciously see. It is as if there is an unconscious zombie trapped in him that can point accurately even though the conscious person is oblivious. The paradox of blindsight is resolved.
A curious philosophical implication of all this is that only the new pathway is “conscious”; the old pathway can go about its business without consciousness creeping into it. Both pathways are composed of neural circuits, but only one of them (as far as we can tell) is conscious. Scientists have no idea why, although being linked to tasks such as language and meaning might be important. Activity in the what pathway eventually evokes a verbal label or name (“mother”) and nuances of emotions however pronounced (“terror”) or subtle (“warmth”).