In her quest to examine the contents of consciousness, a scientist at The Johns Hopkins University has produced new evidence of the evolutionary path from the monkey brain to the human.
It was not where anyone expected.
Until recently, many scientists argued that storage depots for short-term memory (called "working memory") in humans were probably located in the same place as in monkeys; memory of spatial relationships was thought to be stored in the top half of the frontal cortex and memory of objects was stored in the bottom half.
But using a relatively new brain-mapping technology called functional magnetic resonance imaging (fMRI*), Courtney discovered that the human brain has a dedicated place for spatial working memory located further back in the brain anatomy.
"Until now, it's been rather controversial," Courtney said, "because people have based their theories about human brain physiology on what they have seen in non-human primates. And yet when scientists went looking for a particular place for spatial working memory in humans, they couldn't find it. But now, with the help of new technologies such as fMRI, we can see where the human brain handles some of its short-term storage."
The difference has interesting implications for human brain evolution, Courtney said.
"The comparison indicates that there has been a great expansion of the frontal cortex in the evolution from monkeys to humans, pushing this older area, reserved for spatial memory, out of the way," she said. "Of course, one of the big questions is what replaced it. From the imaging work and from studies of people with damage to that part of the brain, it looks as if these ‘new' areas are involved in functions that are, if not uniquely human, then greatly more developed in humans--for example, abstract reasoning, planning for the future, and manipulation of information."
In the past, researchers who wanted detailed studies of human brain activity struggled because they lacked effective investigative tools. In experiments with people, two of the best technologies brought only limited success in mapping the processes of consciousness. An EEG (electroencephalogram), for example, could not pinpoint where brain signals were coming from, and PET (positron emissions tomography) required radioactive tracers, which limited the exposure times of subjects. PET was also restricted because it could only provide an image of brain activity integrated over a minute or two.
Consequently, researchers have relied on monkeys who have undergone invasive surgery and the implantation of electrodes into the brain for monitoring biological activity. For that reason, the primate brain has served as the model for understanding some of the finer aspects of human brain performance.
Functional MRI, however, can be used repeatedly on human subjects without causing harm or relying on invasive procedures. Plus, the resolution is so good that scientists like Courtney can observe changes in the brain every few seconds, allowing them to chart neural activity nearly in real time.
For Courtney, who is discretely examining what she calls "the contents of consciousness," functional imaging has advanced her work considerably. Besides revealing clues about human brain evolution, her latest work with functional imaging also revealed that the process of retrieving short-term memories of faces shares the same physical mechanism in the brain as that for retrieving long-term memories of faces--forcing yet another wedge into the mysteries of consciousness.
"It's great uncharted territory," she said. "But we're beginning to make a lot of progress."
Her work was funded by the National Institute of Mental Health and reported in the Philosophical Transactions of the Royal Society of London.
(* Functional magnetic resonance imaging (fMRI) is based on the fact that neural activity causes an increase in blood flow, which in turn increases blood oxygenation. The technology maps the brain by charting the relative increase in oxygenated hemoglobin while researchers conduct various experiments on their subjects that may, for example, task memory or test senses.)
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