Read Phantoms in the Brain: Probing the Mysteries of the Human Mind for Free Online Page A

this in Chapter 7.)
    When Goldstein arrived at his diagnosis, it must have seemed like science fiction. But not long after that office visit, the woman died suddenly, probably from a second stroke (no, not from strangling herself). An autopsy confirmed Goldstein's suspicions: Prior to her Strangelovean behavior, she had suffered a massive stroke in her corpus callosum, so that the left side of her brain could not "talk to" nor exert its usual control over the right side. Goldstein had unmasked the dual nature of brain function, showing that the two hemispheres are indeed specialized for different tasks.
    Consider next the simple act of smiling, something we all do every day in social situations. You see a good friend and you grin. But what happens when that friend aims a camera at your face and asks you to smile on command? Instead of a natural expression, you produce a hideous grimace. Paradoxically, an act that you perform effortlessly dozens of times each day becomes extraordinarily difficult to perform when someone simply asks you to do it. You might think it's because of embarrassment. But that can't be the answer because if you walk over to any mirror and try smiling, I assure you that the same grimace will appear.
    The reason these two kinds of smiles differ is that different brain regions handle them, and only one of them contains a specialized "smile circuit." A spontaneous smile is produced by the basal ganglia, clusters of cells found between the brain's higher cortex (where thinking and planning take place) and the evolutionarily older thalamus. When you encounter a friendly face, the visual message from that face eventually reaches the brain's emotional center or limbic system and is subsequently relayed to the basal ganglia, which orchestrate the sequences of facial muscle activity needed for producing a natural smile. When this circuit is activated, your smile is genuine. The entire cascade of events, once set in motion, happens in a fraction of a second without the thinking parts of your cortex ever being involved.
    But what happens when someone asks you to smile while taking your photograph? The verbal instruction from the photographer is received and understood by the higher thinking centers in the brain, including the auditory cortex and language centers. From there it is relayed to the motor cortex in the front of the brain, which specializes in producing voluntary skilled movements, like playing a piano or combing your hair.
    Despite its apparent simplicity, smiling involves the careful orchestration of dozens of tiny muscles in the appropriate sequence. As far as the motor cortex (which is not specialized for generating natural smiles) is concerned, this is as complex a feat as playing Rachmaninoff though it never had lessons, and therefore it fails utterly. Your smile is forced, tight, unnatural.
    Evidence for two different "smile circuits" comes from brain−damaged patients. When a person suffers a stroke in the right motor cortex—the specialized brain region that helps orchestrate complex movements on the left side of the body—problems crop up on the left. Asked to smile, the patient produces that forced, unnatural grin, but now it's even more hideous; it's a half smile on the right side of the face alone. But when this same patient sees a beloved friend or relative walk through the door, her face erupts into a broad, natural smile using both sides of the mouth and face. The reason is that her basal ganglia have not been damaged by the stroke, so the special circuit for making symmetrical smiles is intact.8
    Very rarely, one encounters a patient who has apparently had a small stroke, which neither he nor anyone else notices until he tries to smile. All of a sudden, his loved ones are astonished to see that only one half of his face is grinning. And yet when the neurologist instructs him to smile, he produces a symmetrical, albeit unnatural grin—the exact converse of the previous patient. This