Start at the Retina

In a sense, light landing on the retinathe sensory surface at the back of the eyeis already inside the brain. The whole central nervous system (the brain and spinal column [Hack #7]) is contained within a number of membranes, the outermost of which is called the dura mater. The white of your eye, the surface that protects the eye itself, is a continuation of this membrane, meaning the eye is inside the same sac. It's as if two parts of your brain had decided to bulge out of your head and become your eyes, but without becoming separate organs.

The retina is a surface of cells at the back of your eye, containing a layer of photoreceptors, cells that detect light and convert it to electrical signals. For most of the eye, signals are aggregateda hundred photoreceptors will pass their signal onto a single cell further along in the chain. In the center of the eye, a place called the fovea, there is no such signal compression. (The population density of photoreceptors changes considerably across the retina [Hack #14] .) The resolution at the fovea is as high as it can be, with cells packed in, and the uncompressed signal dispatched, along with all the other information from other cells, down the optic nerve. The optic nerve is a bundle of projections from the neurons that sit behind the photoreceptors in the retina, carrying electrical information toward the brain, the path of information out of the eye. The size of the optic nerve is such that it creates a hole in our field of vision, as photoreceptors can't sit over the spot where it quits the eyeball (that's what's referred to as the blind spot [Hack #16] ).

Behind the Eyes

Just behind the eyes, in the middle, the optic nerves from each eye meet, split, and recombine in a new fashion, at the optic chiasm. Both the right halves of the two retinas are dispatched to the left of the brain and vice versa (from here on, the two hemispheres of the brain are mirror images of each other). It seems a little odd to divide processing directly down the center of the visual field, rather than by eye, but this allows a single side of the brain to compare the same scene as observed by both eyes, which it needs to get access to depth information.

The route plan now is a dash from the optic chiasm right to the back of the brain, to reach the visual cortex, which is where the real work starts happening. Along the way, there's a single pit stop at a small region buried deep within the brain called the lateral geniculate nucleus, or LGN (there's one of these in each hemisphere, of course).

Already, this is where it gets a little messy. Not every signal that passes through the optic chiasm goes to the visual cortex. Some go to the superior colliculus, which is like an emergency visual system. Sitting in the midbrain, it helps with decisions on head and eye orienting. The midbrain is an evolutionary, ancient part of the brain, involved with more basic responses than the cortex and forebrain, which are both better developed in humans. (See [Hack #7] for a quick tour.) So it looks as if this region is all low-level functioning. But also, confusingly, the superior colliculus influences high-level functions, as when it suddenly pushes urgent visual signals into conscious awareness [Hack #37] .

Actually, the LGN isn't a simple relay station. It deals almost entirely with optical information, all 1.5 million cells of it. But it also takes input from areas of the brain that deal with what you're paying attention to, as well as from the cortex in general, and mixes that in too. Before visual features have as been extracted from the raw visual information, sophisticated input from elsewhere is being addedwe're not really sure of what's happening here.

There's another division of the visual signal here, too. The LGN has processing pathways for two separate signals: coarse, low-resolution data (lacking in color) goes into the magnocellular pathway. High-resolution information goes along the parvocellular pathway. Although there are many subsequent crossovers, this division remains throughout the visual system.

Date: 2015-12-11; view: 751