"You can use these flickers," van Swinderen tells me, "to extract what the fly is attending to. At the moment," he says, "it's paying attention to the X."
Van Swinderen has inserted an electrode into the fly's brain to monitor its neural activity. The jagged brain waves percolating through the electrode scroll across a computer screen. Buried deep in the jumble of jagged peaks are two tiny signals: one wave rising and falling 12 times per second and another rising and falling 15 times per second. Those two waves are emanating from thousands of brain cells responding to the two flickering objects. The greater the number of cells firing in unison to a given object, the higher the corresponding wave. By noting which wave is higher, van Swinderen can tell which target the fly is directing more attention to.
Van Swinderen prefers to call it "salience" rather than "attention," because he does not want to imply that flies are conscious. But whatever that perceptual focus is called, finding it in a fly has huge implications for understanding the roots of consciousness in humans. Every second we are flooded by sensory information from our eyes, ears, nose, and every inch of our skin. The roving spotlight of attention — our mind's eye — determines which tiny fraction of this influx we actually admit into our consciousness and, just possibly, file away as memory.
Van Swinderen often records brain waves simultaneously from three locations in a large insect brain region called the medial protocerebrum. At first glance, the jumbled waves from those areas might seem as varied as the sounds of Mozart, the Sex Pistols, and Tuvan throat singing. But as long as the fly is alert and attending to something, there exists within that jumble a chorus of neurons chiming the same wave pattern in concert across all three areas. That wave pattern represents the thing the fly is attending to, and as its attention shifts from one thing to another, the wave pattern also changes. Van Swinderen can detect the chorus because he has carefully designed his experiment with its little IMAX theater to determine what the fly will attend to. It is a beautiful illustration of attention: every neuron singing the same song — the song of the square.
"Attention," says van Swinderen, "is a whole-brain phenomenon. A thing is not purely visual, not purely olfactory. It's a binding together of different parts that for us signify one thing. Why couldn't the fly's mechanism [of attention] be directed to a succession of its memories?" he asks. "That, to me, is just a short hop, skip, and a jump away from what might be consciousness." The difference between the memories of a fly and a human might be a matter of degree. The human can store a lot more memories and can therefore maintain a more sophisticated personal narrative of his past and present. But van Swinderen believes "it could be exactly the same mechanism in a fly and a human." Although there is still no evidence to decide either way, the result could be consciousness.
"Probably what consciousness requires," says Koch of Caltech, "is a sufficiently complicated system with massive feedback. Insects have that. If you look at the mushroom bodies, they're massively parallel and have feedback."
Chemical clues confirm that at least some fundamental brain processes are the same in humans and insects. Van Swinderen and Rozi Andretic, a neuroscientist at NSI, have found that mutant flies producing too little of the neurotransmitter dopamine have impaired salience responses. Feeding the mutant flies methamphetamine — a chemical related to drugs used to treat attention-deficit/hyperactivity disorder — relieves the dopamine shortage and normalizes the flies' attention. But give meth to a normal fly and it cannot attend as well. "Similar mechanisms are present in vertebrates and flies," Andretic told me. "You need optimal concentrations of dopamine, and if you have too little or too much, you will be impaired." In both humans and flies, dopamine-releasing cells may help coordinate distant brain regions involved in the phenomenon of attention.
When you consider that neurons themselves are strikingly similar across the animal kingdom, it all begins to make sense. "You have the same basic building blocks for vertebrates and invertebrates," says Strausfeld, "and there are certain ways you can put these building blocks together [into brains]." So when it came to building a brain center like the hippocampus that can recognize places, there might have been only one way to wire those quirky neurons together to do the job — and evolution arrived at that same solution multiple times independently, just as the genetic instructions for wings evolved multiple times in distinct lineages.
The more startling possibility is that the brain might have evolved only once in the history of life. Distant cousins — cockroaches and humans — could have inherited the basic blueprint from a common ancestor, Urbilateria, the last common forebear of all bilaterally symmetrical animals. No fossils of this creature are known to survive, but by estimating how long it took for DNA sequences to diverge between flies and mice, geneticists estimate that Urbilateria lived 600 million to 1 billion years ago.
By analogy to marine invertebrates today, some scientists believe that Urbilateria burrowed on the sea bottom as an adult and swam as a larva, with simple eyes like those in many living marine larvae.
Clues to Urbilateria's brain plan come from studying the embryonic development of creatures alive today. In both mouse and fruit fly embryos, Detlev Arendt, an evolutionary biologist at the European Molecular Biology Laboratory in Heidelberg, Germany, has found that cells involved in forming the brain and nerve cord divide into three columns of cells. At least some of the genes that govern the column formation are the same in flies and mice. "This pattern is so specific," says Arendt, "that clearly the last common ancestor must have had these three columns."
Heinrich Reichert of the University of Basel has turned up another striking similarity. During embryonic growth, a gene that is similar in both flies and mice (and, by implication, humans) triggers the brain to divide into front, middle, and rear segments. Mice lacking the gene develop severe brain abnormalities. But swapping the fly version of the gene into these mutant mice corrects most of those abnormalities. "It reveals," says Reichert, "a deep relatedness in the brains of flies and fish and mice and men that certainly was not expected by just looking at the superficial anatomy."
Of course, these genes act early, while the embryo is primitive. So Urbilateria could have possessed them and still have had almost no brain. The ultimate answer will come from identifying and comparing dozens more insect and mammal genes. Of particular interest are those involved in forming complex structures that play similar roles, like mushroom bodies and the hippocampus.
"The million-dollar question," Strausfeld says, "would be whether the genes involved in the development of these structures are shared between mouse and fly. That would, of course, be thrilling."