Iconoclast: A Neuroscientist Reveals How to Think Differently Read Online Free Page B

a picture. The first thing to realize is that vision is not the same as perception. Vision is the process by which photons enter the eye and are transformed into neural signals in the brain. Perception, on the other hand, is the much more complex process by which the brain interprets these signals. The end result is a mental image that reaches consciousness. The eye is not much more than an optical lens and an image detector (the retina). After that, what people’s brains do with the image is a rather individualistic process.
    The Anatomy of Vision
     
    Vision begins in the eye. 4 The human eye is divided into two components: a lens system and a detector system. The outermost part of the eye, the cornea, collects incoming light rays and passes them through the lens. The lens takes the incoming light rays and focuses them onto the retina, which covers the inner surface of the rear of the eyeball. The lens functions in the same way as a camera lens, but unlike a camera lens, which is made from glass or plastic, the human lens is living tissue made up of cells that have elongated into very thin fibers. These fibers attach to muscles that surround the periphery of the lens. When the muscles contract, the lens flattens out, changing the focus point.
    Light entering the eye is projected onto the retina, and it is here that the first transformation from physical image to mental image occurs. The light strikes a specialized type of nerve cell called a
photo-receptor
. These cells contain special pigments that absorb energy from incoming photons and convert this energy into an electrical impulse. There are two types of photoreceptors:
rods
and
cones
, which are named for their shapes under a microscope. The rods have a larger surface area and can detect a few photons at a time, which makes them ideally suited for night vision. The tip of a cone is much smaller and is less sensitive, but the cones are packed close together in the center of the retina. This tight packing makes them ideal for picking up fine details. The cones also contain three different pigments, and the relative concentration of these pigments in a particular cone determines the range of colors it responds to.
    Until this point, the eye functions much like a digital camera. But unlike a camera’s detector, the photoreceptors in the retina are not spaced uniformly on a grid. Because the cones are packed densely near the center of the retina, and the rod spacing is sparse near the periphery, our ability to make out details of objects declines with distance from the center of vision. So even before the photoreceptors transmitelectrical signals to the brain, the image has been fractionated in a way that gives premium bandwidth to things that are in the center of the visual field. By constantly moving your eyes, however, you’re able to construct a mental image of your surroundings. Your brain can keep track of this mental image and fill in the gaps in vision by making guesses that are generally pretty good. There are circumstances, however, in which these guesses fail, and it is under these conditions that the brain makes incorrect assumptions about what it is seeing. It turns out that the ways in which the brain makes these assumptions are the same ways it makes it difficult to think like an iconoclast.
    For example, there is the blind spot. Cats and dogs don’t have blind spots. The phenomenon is unique to humans and other primates. In humans, the photoreceptors are covered by a thin sheet of neurons that make connections (synapses) with the photoreceptors. This sheet of neurons performs basic image processing and then passes the signals on to the brain through the optic nerve. The optic nerve is a cordlike structure that contains the fibers from all the retinal neurons. Because they are collected into one place and pass through a hole in the retina, no photoreceptors can occupy that space, and a blind spot results. But even though you have a hole in the retina, you