Consciousness in a Cockroach by Douglas Fox
published online January 10, 2007
To Nicholas Strausfeld, a tiny brain is a beautiful thing. Over his 35-year career, the neurobiologist at the University of Arizona at Tucson has probed the minute brain structures of cockroaches, water bugs, velvet worms, brine shrimp, and dozens of other invertebrates. Using microscopes, tweezers, and hand-built electronics, he and his graduate students tease apart—ever so gently—the cell-by-cell workings of brain structures the size of several grains of salt. From this tedious analysis Strausfeld concludes that insects possess "the most sophisticated brains on this planet."
Strausfeld and his students are not alone in their devotion. Bruno van Swinderen, a researcher at the Neurosciences Institute (NSI) in San Diego, finds hints of higher cognitive functions in insects—clues to what one scientific journal called "the remote roots of consciousness."
"Many people would pooh-pooh the notion of insects having brains that are in any way comparable to those of primates," Strausfeld adds. "But one has to think of the principles underlying how you put a brain together, and those principles are likely to be universal."
The findings are controversial. "The evidence that I've seen so far has not convinced me," says Gilles Laurent, a neuroscientist at Caltech. But some researchers are considering possibilities that would shock most lay observers. "We have literally no idea at what level of brain complexity consciousness stops," says Christof Koch, another Caltech neuroscientist. "Most people say, 'For heaven's sake, a bug isn't conscious.' But how do we know? We're not sure anymore. I don't kill bugs needlessly anymore."
Heinrich Reichert of the University of Basel in Switzerland has become more and more interested in "the relatedness of all brains." Reichert's own studies of the brain's origin lead to a little-known ancestor, a humble creature called Urbilateria, which wriggled and swam nearly a billion years ago. The granddaddy of all bilaterally symmetrical animals, Urbilateria is the forebear of spiders, snails, insects, amphibians, fish, worms, birds, reptiles, mammals, crabs, clams—and yes, humans.
There is, of course, good reason to view insect brains as primitive—at least quantitatively. Humans possess 100,000,000,000 brain cells. A cockroach has nearly 1,000,000 brain cells; a fruit fly, only 250,000. Still, insects exercise impressive information management: They pack neurons into their brains 10 times more densely than mammals do. They also use each brain cell more flexibly than mammals. Several far-flung tendrils of a single neuron can each act independently—boosting computing power without increasing the number of cells. Somehow that circuitry allows a honeybee, with barely a million neurons on board, to meander six miles from its hive, find food, and make a beeline directly home. Few humans could do the same even with a map and a compass.
On the surface, the brains of insects and mammals look nothing alike. Only from studies of cell-by-cell connections does the astounding similarity emerge. One afternoon Christopher Theall, one of Strausfeld's Ph.D. students, shows me his own experimental setup for tapping into a portion of the cockroach brain known as the mushroom body. This mushroom-shaped brain structure is thought to be analogous to the mammalian hippocampus, a brain component involved in forming memories of places.
"What we're trying to do," says Theall, as we enter a cramped laboratory, "is scale down the techniques that have been used in rat and primate brains—scale them down to a brain that's a thousandth the size."
Theall's experimental apparatus rests on a table that floats on vibration-absorbing pressurized air. Even a cart rattling in the hallway outside could undermine the experiment. Because Theall needs to record nerve impulses amounting to just one 1/10,0000 of a volt, the table is enclosed in a cage that blocks electromagnetic interference from the room's lights. Working under a microscope with tweezers, steady hands, and held breath, Theall fashions copper wire only twice the diameter of a red blood cell into electrodes that he will insert into the cockroach's brain.
"They're fragile," he says. "Even a breeze from a door opening can ruin a couple hours of work."
After 20 hours of prep, Theall is ready to do the experiment. Twisting a knob while gazing into the microscope, he sinks the electrode into the roach's brain until it rests in one of the mushroom bodies. During the experiment, Theall will train this cockroach to earn a reward: If the insect points its antenna toward certain landmarks, it will receive thrilling puffs of peanut-butter odor. Theall wants to eavesdrop on neurons to determine how they contribute to learning the location of those landmarks.
The final step of the experiment—dissection of the mushroom body—allows Theall to see the two or three cells he has monitored. Because the cells have absorbed copper released from the electrode, he can tell them apart from the 200,000 other brain cells in the mushroom body. Theall then traces the structure of each cell using pen, paper, and a light box. It is like drawing a gnarled oak tree down to the last twig, and reconstructing a single cell can take two days. Theall, a typical student in Strausfeld's lab, will perform hundreds of experiments like these before his Ph.D. is complete.
Theall and Strausfeld never know which of the tens of thousands of cells they're going to hit when they tap into a roach's mushroom body. By repeating the experiment over and over, however, they are assembling a picture of what types of cells exist, how those cells function during tasks of place memory, and what kinds of connections they form with other cells. Cell by cell, they hope to piece together the structure's circuitry.
Paired structures called mushroom bodies in a cockroach brain play a key role in navigation
During a chat in his office, Strausfeld sketches a mushroom body, pointing out several parallels to the hippocampus, the brain center devoted to memory and place location in mammals. The base consists of thousands of parallel nerve fibers running together like the grain in a piece of wood. Further up from the base, the fibers send out connections in loops that look like jug handles on a freeway; this is the shape that has earned this part of the brain the name "mushroom body." The connections rejoin the fibers higher up, near the top. Strausfeld suspects these looping pathways bring together related pieces of information, like the sights and smells of various landmarks that a roach encounters, one after another, as it travels to and from its home.
"The geometry of the structure," he says, "is so strangely reminiscent of the [human] hippocampus." Strausfeld and others are looking for clues as to whether the similarities result from a deep and ancient kinship or simply from analogous solutions that evolved independently to aid survival.
In his underground laboratory at the Neurosciences Institute, van Swinderen is observing a fly suspended in what amounts to a miniature IMAX theater. The setup is designed to monitor the focus of attention in a fly's brain. An LED screen wraps around the fly, displaying a sequence of flashing objects in front of its eyes, two objects at a time. Right now, it's an X and a square. The X is flickering 12 times per second and the square 15 times per second.
"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.
Two electrodes in a fly's brain monitor signals involved in attention.
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