To find out exactly what happens in the prefrontal cortex, Rizzuto and his advisor, Richard Andersen, a Caltech neuroscience professor, piggybacked on clinical work done by Adam Mamelak, a neurosurgeon at Huntington Memorial Hospital in Pasadena.
Mamelak was treating three patients with severe epilepsy. Trying to identify the brain areas where the seizures occurred, the neurosurgeon implanted electrodes into the patients' ventrolateral prefrontal cortex.
"So for a couple of weeks these patients are lying there, bored, waiting for a seizure," Rizzuto said. "I was able to get their permission to do my study, taking advantage of the electrodes that were already [surgically implanted] there."
The patients had to watch a computer screen for a flashing target, then remember the target location for a short time, then reach to that location on a touch screen.
Monitoring their brain activity, Rizzuto was able to show conclusively that the ventrolateral prefrontal cortex is involved in how we process spatial information.
"These findings were not surprising to our group, because we understand that object and spatial processing do not necessarily require different processing domains in the brain," Rizzuto said.
"There is more research showing that the brain areas dedicated to object and spatial processing actually have a lot of interconnections with each other," he added. "However, some scientists still hold these traditional ideas of separate object and spatial processing domains, and they may be surprised at our results."
Recent studies of monkeys have shown that neurons in the monkeys' ventrolateral prefrontal cortex also carry spatial signals when monkeys are planning movements.
"The monkey brain is a model for the human brain, and studies like these provide critical evidence that there are indeed [structural likenesses] between them, even at the highest levels of brain processing," said Earl Miller, a neuroscience professor at the Massachusetts Institute of Technology in Cambridge.
The Caltech research is important, Miller said, because it identifies the ventral prefrontal cortex as a site where motor, or body movement, planning takes place, meaning it could be used to drive neural prosthetic devices.
"There may be big advantages to using the prefrontal cortex [for neural prostheses] rather than [the] lower-level motor cortex, because the prefrontal cortex seems less hardwired and specific to the details of the movements than [the] primary motor cortex," Miller said. (The motor cortex is a brain region that controls voluntary muscle movement.)
Movement-planning areas are also less susceptible to damage than areas of the brain directly responsible for movement. In the case of a spinal cord injury, for example, communication to and from the primary motor cortex is cut off: For example, severed nerves might prevent a person's brain from sending signals telling their legs to step forward.
However, the brain still performs the computations associated with planning to move. Scientists could, in theory, tap into these planning calculations and decode where a person is thinking of moving.
"We believe that motor-planning areas will be more resistant to pathological reorganization after spinal injury, as it is already known that neurons in primary motor cortex die after such injuries," Rizzuto said. In other words, neurons in motor-planning parts of the brain are probably less likely to be "scrambled" after an injury than they are in areas that are directly involved in carrying out movement.
As the control center of the brain, the prefrontal cortex could possibly be used to control multiple prosthetic devices simultaneously.
"For instance, a patient could navigate his wheelchair and use an LCD interface to type a letter at the same time," Rizzuto said. "This could be accomplished by having the patient learn different neural codes, or contexts, for different devices."
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