God Soul Mind Brain

vision is accomplished by a divide-and-conquer method. The overall task of vision is broken down into components including: 1. determining the shape and color of objects; 2. tracking the motion of objects; 3. pinpointing the spatial location of objects; and 4. using vision to guide action, such as in hand-eye coordination.

    Not all of the visual cortex is divided in this way. The initial stages in the hierarchy—V1, V2, and V3—process all aspects of vision. They seem to be performing basic preprocessing useful for any visual task. It is the higher stages of visual processing that are divided into separate categories of information.

    The stream at the bottom of the brain in Diagram 6-3 contains the cryptically named brain areas V4, TEO, and TE. These areas mainly process the shape and color of objects.

    The second stream from the bottom contains the brain areas MT, MST, and STP. These areas emphasize the motion of objects.

    The third stream contains the brain areas V3A, DP, and 7A. These areas seem to emphasize the spatial locations of objects.

    The stream at the top of the brain mainly coordinates vision and action. For example, it helps to guide eye movements when you are scanning a visual scene, and it helps to guide arm movements when you are reaching toward an object. It contains the brain areas V6, V6A, and IP.

    The crazy names for these areas emerged for historical reasons as a result of the work of many different labs with different naming preferences. In fact, most of these areas have more than one name. It has been said that neuroscientists would rather borrow each other’s toothbrushes than each other’s acronyms.

    The idea of separate processing streams, dedicated to different tasks in vision, is actually a terrible simplification and a convenient fiction. It helps neuroscientists keep track of the visual system, but the real brain is much more complex. For example, area MT is in the motion processing stream, and yet its neurons also carry some signals related to color. Area 7A is in the spatial-location stream, and yet its neurons also carry signals related to the motion of objects. The action stream carries a mixture of every kind of signal, including information about object shape, motion, and location. The mixing and swapping of information among the four streams is considerable.

    When the idea of separate visual streams was first proposed, scientists were faced with a puzzle. If the motion of objects is computed separately from the shape of objects, then how does the brain bind the right shape to the right motion? How does it know that the person is running to the left and the baseball is flying to the right, instead of vice versa? The “binding problem” occupied many minds. But it is now understood that motion and shape are not computed separately. The visual streams overlap and exchange so much information that the binding problem no longer seems like such a big deal.

    There is no sharp cutoff at the end of the visual system. Once information is processed in the highest levels of the visual cortex, of course it does not stop there. It is sent on, distributed to a range of brain structures related to memory, to decision making, to emotion, to muscle control. For example, area IP sends signals into the motor system. It contacts the brain structures that control eye and limb movement. Some of the structures beyond the visual system will come up again in later chapters, but for now we have enough background to proceed.

Chapter 7

    The machinery for the perception of mind

    One approach in searching for the social circuitry in the brain is to start at the eye and work inward through the visual system. Much of what we perceive about other people, much of the raw data we use to reconstruct another person’s mind, comes through vision. We see someone’s face and read the facial expression. We see the eyes and deduce what that person is looking at and thinking about. We see the gestures and movement of limbs, the body language that reveals so much about the person’s inner state. All of these computations depend on vision. Therefore we should expect the visual system to flow into the circuitry for social perception. We might also expect the circuitry for social perception to have a back and forth, an inflow and outflow with the circuitry for language, since the conscious mind is obviously capable of understanding language and expressing itself verbally. What structures in the brain lie