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

don’t see a black hole in your visual field. The brain mentally fills it in with its best guess of what should be there.
    The photoreceptors transmit electrical signals to a thin sheet of neurons that immediately begins to transform the pristine image that has fallen on the retina. Even before these signals leave the eye, they have already been changed in a way that is no longer an exact representation of the world. These neurons, which are called
retinal ganglion cells
, serve two primary purposes: to collect the visual information from the photoreceptors and transmit it to the brain; and to perform “gain” control. The retinal ganglion cells sense the intensity of light that hits the retina and adjust their output to stay within a constant range. These cells turn up the gain for night vision and turn it down in daylight. Theretinal ganglion cells also place a limit on how fast visual information can reach the brain. By measuring how frequently the ganglion cells fire and by estimating how many ganglion cells cover the retina, we can estimate the bandwidth of the human eye. One estimate put the bandwidth at about 1 MB per second, which is about the speed of a cable modem. 5
    After traveling down the optic nerve, the electrical impulses make a single synapse in a lime-sized structure called the thalamus and are then transmitted to the cortex, where the real transformations take place and where the mental image is constructed. Interestingly enough, the cortical visual system is laid out in a well-defined topography. The initial site of visual processing is in the part of the brain called the
occipital cortex
, which lies at the very back of the head. This rather large swath of territory is devoted exclusively to the initial processing of visual signals and is called, not surprisingly,
area V1
. If you electrically stimulate the neurons in V1, the person will “see” visual phosphenes. For the same reason, if you get hit in the back of the head, the stars you see result from the electrical discharge of these neurons. There is an almost exact representation of the retina here, with each location in the retina being mapped onto a grid in V1. The neurons here process the incoming information and extract basic features from the image such as the location of edges, their orientation relative to horizontal, and the image disparity between the two eyes, which is one element of depth perception.
    The Anatomy of Perception
     
    Until V1, the visual system behaves like a video camera. Although the neurons perform low-level processing of the visual signals, this type of processing does not yet constitute perception. In fact, we are not even aware of what our brains are doing at this point. After V1, however, the information flows from the back of the head in a generally forward direction toward the frontal lobes. The information takes two paths: thehigh road and the low road (see figure 1-1). The high road, which is a route of information flowing over the top of the brain, extracts information about where objects are located in space relative to the body. The low road, which is a pathway flowing through the temporal lobes above the ears, processes the visual information in a way that categorizes what a person sees. These two routes, the “where” and the “what” pathways, coordinate with each other so that the end result is a seamless perception of what the eyes transmit. For example, although you move your head and eyes constantly, your brain does not lose track of the objects surrounding you. This is a complicated process, and the only way in which it can be done efficiently is through a process called
predictive coding
. So that it is not overwhelmed with information processing, the brain makes predictions about what it is seeing and changes these predictions only when it makes an error. As we delve deeper into how this occurs, we shall see the points at which it becomes difficult to see things in ways different than you