An investigation of the activity of individual human nerve cells during the act of memory indicates that the brain's nerve cells are even more specialized than many people think - no pun intended.
Although nerve cells that change activity during the use of memory are widely distributed in the brain, individual neurons generally respond to specific aspects of memory.
"For the first time, we've been able to show differences within regions of the temporal lobe in the way individual neurons respond to memory. Everything we've done to this point was to show that there are individual neurons that change activity --but we hadn't been able to sort them out in any meaningful way. Now we can," says Dr. George Ojemann, professor of neurological surgery at the University of Washington.
The findings appear in the January 2002 issue of Nature Neuroscience.
Ojemann is an internationally renowned neurosurgeon who has developed surgical techniques for treating epilepsy, brain tumors and Parkinsonism, and ways to explore the detailed organization of the human brain for language, memory, thought and learning. He has co-authored two books for lay readers on the higher functions of the brain: Inside the Brain and Conversations with Neil's Brain.
This research involves patients with epilepsy who were awake during surgery and agreed to respond to requests to recall words, names of pictures and sounds. The recordings were from relatively healthy brain tissue that must be removed in order to reach problematic parts of the brain responsible for epileptic seizures. In a typical procedure, surgeons insert four microelectrodes and record the electrical activity as neurons communicate with other cells. After the microelectrodes are in place, patients are asked questions that measure stages of memory.
The microelectrodes, sharpened tungsten wire about the thickness of a human hair, identify electric impulses from neurons. There are only a few programs worldwide that have investigated neuronal activity changes with human cognition. Given the size and complexity of neurons and their interconnections, it is difficult to measure the activity of any given neuron for a given time. The electrodes pick up discharges of a pool of neurons that are then separated into activity of individual neurons based on the shape of their individual discharges.
The latest study was able to identify the behavior of 105 neurons at 57 sites in 26 patients; before, Ojemann says, his team's largest sample was about 25 neurons.
The findings reinforce the message that neurons are very specialized. For example, researchers identified 16 of the 105 neurons that significantly changed activity with different stages of memory - encoding, storage and retrieval - and found that in 13 of those, changes were observed in only one modality (auditory, six; text, four; objects, three).
"We just don't find neurons that are generic memory neurons. What we find are neurons that show statistically significant relationships to memory for a particular thing," Ojemann says.
There are three regional differences in brain activity that have not been noted before:
- There is a cluster of neurons that changes activity from encoding, to storage, to retrieval, in the basal temporal area, below the temporal lobe.
- Neurons that may help people recall something quickly after they have seen it earlier in the day - what scientists call 'implicit memory' -- seem very active in the superior temporal gyrus of the temporal lobe.
- There are neurons in the language-dominant hemisphere that respond to more than one modality - memory of both visual and auditory material.
At this point, the research is helping to illuminate the vast mysteries of the human brain. Someday, scientists may be able to use this knowledge to assist ailing brains. For example, it may be possible to externally activate neurons related to memory encoding in order to enhance memory.
TELEVISION PROGRAM: Dr. Ojemann's work will be featured on the Discovery Health cable channel during a documentary on brain surgery.
The program airs 6 p.m. P.T., 9 p.m. E.T., on Sunday, Jan. 6.
These studies are supported by a grant from the National Institute of Neurological Disorders and Stroke, and are a collaborative project with Professor David Corina of the UW Department of Psychology.