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 o