cortex only that difference information—that is, the bit that wasn’t predicted away. This unpredicted information adjusts the internal model so there will be less of a mismatch in the future. In this way, the brain refines its model of the world by paying attention to its mistakes. MacKay pointed out that this model is consistent with the anatomical fact that there are ten times as many fibers projecting from the primary visual cortex back to the visual thalamus as there are going the other direction—just what you’d expect if detailed expectations were sent from the cortex to the thalamus and the forward-moving information represented only a small signal carrying the difference.
What all this tells us is that perception reflects the active comparison of sensoryinputs with internal predictions. And this gives us a way to understand a bigger concept: awareness of yoursurroundings occurs only when sensory inputs
violate
expectations. When the world is successfully predicted away, awareness is not needed because the brain is doing its job well. For example, when you first learn how to ride a bicycle, a great deal of conscious concentration is required; after some time, when your sensory-motor predictions have been perfected, riding becomes unconscious. I don’t mean you’re unaware that you’re riding a bicycle, but you
are
unaware of how you’re holding the handlebars, applying pressure to the pedals, and balancing your torso. From extensive experience, your brain knows exactly what to expect as you make your movements. So you’re conscious neither of the movements nor of the sensations unless something changes—like a strong wind or a flat tire. When these new situations cause your normal expectations to be violated, consciousness comes online and your internal model adjusts.
This predictability that you develop between your own actions and the resulting sensations is the reason you cannot tickle yourself. Other people can tickle you because their tickling maneuvers are not predictable to you. And if you’d really like to, there are ways to take predictability away from your own actions so that you can tickle yourself. Imagine controlling the position of a feather with a time-delay joystick: when you move the stick, at least one second passes before the feather moves accordingly. This takes away the predictability and grants you the ability to self-tickle. Interestingly, schizophrenics can tickle themselves because of a problem with their timing that does not allow their motor actions and resulting sensations to be correctly sequenced. 47
Recognizing the brain as a loopy system with its own internal dynamics allows us to understand otherwise bizarre disorders. TakeAnton’s syndrome, a disorder in which a stroke renders a person blind—and the patient
denies
her blindness. 48 A group of doctors will stand around the bedside and say, “Mrs. Johnson, how many of us are around your bed?” and she’ll confidently answer, “Four,” even though in fact there are seven of them. A doctor will say, “Mrs. Johnson, how many fingers am I holding up?” She’ll say,“Three,” while in fact he is holding up none. When he asks, “What color is my shirt?” she’ll tell him it is white when it is blue. Those with Anton’s syndrome are not
pretending
they are not blind; they truly believe they are not blind. Their verbal reports, while inaccurate, are not lies. Instead, they are experiencing what they take to be vision, but it is all internally generated. Often a patient with Anton’s syndrome will not seek medical attention for a little while after the stroke, because she has no idea she is blind. It is only after bumping into enough furniture and walls that she begins to feel that something is amiss. While the patient’s answers seem bizarre, they can be understood as her internal model: the external data is not getting to the right places because of the stroke, and so the patient’s reality is simply that
What all this tells us is that perception reflects the active comparison of sensoryinputs with internal predictions. And this gives us a way to understand a bigger concept: awareness of yoursurroundings occurs only when sensory inputs
violate
expectations. When the world is successfully predicted away, awareness is not needed because the brain is doing its job well. For example, when you first learn how to ride a bicycle, a great deal of conscious concentration is required; after some time, when your sensory-motor predictions have been perfected, riding becomes unconscious. I don’t mean you’re unaware that you’re riding a bicycle, but you
are
unaware of how you’re holding the handlebars, applying pressure to the pedals, and balancing your torso. From extensive experience, your brain knows exactly what to expect as you make your movements. So you’re conscious neither of the movements nor of the sensations unless something changes—like a strong wind or a flat tire. When these new situations cause your normal expectations to be violated, consciousness comes online and your internal model adjusts.
This predictability that you develop between your own actions and the resulting sensations is the reason you cannot tickle yourself. Other people can tickle you because their tickling maneuvers are not predictable to you. And if you’d really like to, there are ways to take predictability away from your own actions so that you can tickle yourself. Imagine controlling the position of a feather with a time-delay joystick: when you move the stick, at least one second passes before the feather moves accordingly. This takes away the predictability and grants you the ability to self-tickle. Interestingly, schizophrenics can tickle themselves because of a problem with their timing that does not allow their motor actions and resulting sensations to be correctly sequenced. 47
Recognizing the brain as a loopy system with its own internal dynamics allows us to understand otherwise bizarre disorders. TakeAnton’s syndrome, a disorder in which a stroke renders a person blind—and the patient
denies
her blindness. 48 A group of doctors will stand around the bedside and say, “Mrs. Johnson, how many of us are around your bed?” and she’ll confidently answer, “Four,” even though in fact there are seven of them. A doctor will say, “Mrs. Johnson, how many fingers am I holding up?” She’ll say,“Three,” while in fact he is holding up none. When he asks, “What color is my shirt?” she’ll tell him it is white when it is blue. Those with Anton’s syndrome are not
pretending
they are not blind; they truly believe they are not blind. Their verbal reports, while inaccurate, are not lies. Instead, they are experiencing what they take to be vision, but it is all internally generated. Often a patient with Anton’s syndrome will not seek medical attention for a little while after the stroke, because she has no idea she is blind. It is only after bumping into enough furniture and walls that she begins to feel that something is amiss. While the patient’s answers seem bizarre, they can be understood as her internal model: the external data is not getting to the right places because of the stroke, and so the patient’s reality is simply that
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