W. Wayt Gibbs
Side Splitting
Scientific American
January 2001

Jokes, ice water and magnetism can change your view of the world--literally

In John D. Pettigrew's lab, there is less to human experience than meets the eyes. Over the past several years, dozens of test subjects have stared through goggles and pressed keys while the neuroscientist squirted ice water into the volunteers' ear canals, fired strong magnetic pulses into their heads or told jokes that made them giggle. These unusual experiments, which were reported in part last March in Current Biology and presented more fully in November at a neuroscience conference in New Orleans, confirmed that people often cannot see what is plainly before their eyes. More important, the studies suggest that many optical illusions may work not by deceiving our visual system, as long suspected, but rather by making visible a natural contention between the two hemispheres of the human brain. If Pettigrew's theory is correct, then the reason an optical illusion such as the Necker cube outline, which seems to turn inside out periodically, works is that, in some deep biological sense, you are of two minds on the question of what to see.

Reversible figures, such as the Necker cube and drawings of a white vase between black faces, have been curiosities for centuries. And it was in 1838 that Charles Wheatstone first reported an even more peculiar phenomenon called binocular rivalry. When people look through a stereoscope that presents irreconcilable patterns, such as horizontal stripes before one eye and vertical bars before the other, most don't perceive a blend of the two. Instead they report seeing the left pattern, then the right, alternating every few seconds. "Every couple of seconds something goes ‘click' in the brain," Pettigrew says. "But where is the switch?" The answer is still unknown.

For many years, scientists believed that neurons connected to each eye were fighting for dominance. But this theory never explained why reversible illusions work even when one eye is closed. And in monkey studies during the late 1990s, only higher-cognitive areas--parts of the brain that process patterns and not raw sensory data--consistently fired in sync with changes in the animals' perception. That discovery buttressed a new theory: that the brain constructs conflicting representations of the scene and that the representations compete somehow for attention and consciousness.

Pettigrew, a neurobiologist at the University of Queensland in Brisbane, Australia, came up with a different theory: it is not just clusters of neurons that compete in binocular rivalry, but the left and right hemispheres of the cerebral cortex. To test this ambitious hypothesis, Pettigrew, Steven M. Miller and their colleagues measured how long volunteers dwelled on each possible perception of either a Necker cube or a bars-and-stripes stereoscopic display. Their plan was to fiddle with one hemisphere to see how that affected what the subjects saw.

There are several ways to do this. Ice-cold water dribbled against one eardrum causes vertigo and makes the eyes sway woozily. After the vertigo passes, however, the half of the brain opposite the chilled ear practically hums with activity. Conversely, zapping the parietal lobe on one side of the brain with a highly focused, one-tesla magnetic field temporarily interrupts much of the neural activity in just that hemisphere.

And then there is laughter. No one knows very precisely what a good guffaw does to the brain. But long bouts can cause weakness, lack of coordination, difficulty breathing, and even embarrassing wetness. Those afflicted with cataplexy, a form of narcolepsy, sometimes suffer partial or complete paralysis for several minutes after a good laugh. These seizurelike effects suggested to Pettigrew that mirth might involve neural circuits that connect the two hemispheres.

The results were "astounding," wrote Frank Sengpiel of the Cardiff School of Biosciences in Wales in a recent review. Although every test subject showed a different bias--some seeing bars for longer periods than stripes, others vice versa--most showed a statistically significant change in that bias after ice water stimulated their left hemisphere. Control subjects, who got earfuls of tepid water, showed no such change. Magnetic pulses beamed at the left hemisphere similarly allowed five of seven people tested to interrupt their perceptive cycles, effectively controlling whether they saw bars or stripes.

And among all the 20 volunteers tested, a good belly laugh either obliterated the binocular rivalry phenomenon altogether--so that subjects saw a crosshatch of both bars and stripes--or significantly reduced whatever natural bias the individuals showed toward one of the two forms, for up to half an hour.

The result seems to support, though hardly prove, Pettigrew's theory that when the brain is faced with conflicting or ambiguous scenes, the left hemisphere constructs one interpretation, the right hemisphere forms another, and an "interhemispheric switch" waffles between the two. Laughter, he speculates, either short-circuits the switch or toggles it so fast that we see both interpretations at once. "It rebalances the brain," Pettigrew says, "and literally creates a new state of mind."

Pettigrew, who has bipolar disorder, found that his own brain took 10 times longer than normal to switch between bars and stripes, an anomaly borne out by studies on his bipolar patients. A clinical trial is gearing up in Australia to test whether this may offer the first simple physical diagnostic for manic depression. Meanwhile Keith D. White of the University of Florida has discovered that many schizophrenics have distinctly abnormal binocular rivalry. "It is much too early to say whether this might serve as a diagnostic test," White cautions. "But I wonder whether this isn't the only perceptual difference that we can measure in schizophrenia."





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