In Our Messy, Reptilian Brains

April 9, 2007 issue - Let others rhapsodize about the elegant design and astounding complexity of the human brain—the most complicated, most sophisticated entity in the known universe, as they say. David Linden, a professor of neuroscience at Johns Hopkins University, doesn't see it that way. To him, the brain is a "cobbled-together mess." Impressive in function, sure. But in its design the brain is "quirky, inefficient and bizarre ... a weird agglomeration of ad hoc solutions that have accumulated throughout millions of years of evolutionary history," he argues in his new book, "The Accidental Mind," from Harvard University Press. More than another salvo in the battle over whether biological structures are the products of supernatural design or biological evolution (though Linden has no doubt it's the latter), research on our brain's primitive foundation is cracking such puzzles as why we cannot tickle ourselves, why we are driven to spin narratives even in our dreams and why reptilian traits persist in our gray matter.

Just as the mouse brain is a lizard brain "with some extra stuff thrown on top," Linden writes, the human brain is essentially a mouse brain with extra toppings. That's how we wound up with two vision systems. In amphibians, signals from the eye are processed in a region called the midbrain, which, for instance, guides a frog's tongue to insects in midair and enables us to duck as an errant fastball bears down on us. Our kludgy brain retains this primitive visual structure even though most signals from the eye are processed in the visual cortex, a newer addition. If the latter is damaged, patients typically say they cannot see a thing. Yet if asked to reach for an object, many of them can grab it on the first try. And if asked to judge the emotional expression on a face, they get it right more often than chance would predict—especially if that expression is anger.

They're not lying about being unable to see. In such "blindsight," people who have lost what most of us think of as vision are seeing with the amphibian visual system. But because the midbrain is not connected to higher cognitive regions, they have no conscious awareness of an object's location or a face's expression. Consciously, the world looks inky black. But unconsciously, signals from the midbrain are merrily zipping along to the amygdala (which assesses emotion) and the motor cortex (which makes the arm reach out).

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