Advances allow neuroscientists to 'see' nerves at work
This article first appeared in the St. Louis Beacon, April 5, 2011 - The $30 million Human Connectome project, a consortium of nine institutions, is aimed at creating a wiring diagram for the brain in the next five years. The hope is that the research could someday lead to therapies for neurological and psychological disorders. And thanks to cutting-edge technology, the brains at nine institutions nationwide are making substantial progress.
The main imaging tool for the Connectome Project will be magnetic resonance imaging (MRI). Many of us have undergone MRI scans that produce a three-dimensional image of soft tissue anatomy. This type of MRI will be just one of the scans done on the Connectome subjects. The MR scanner is versatile. Changing the settings allows its computer to generate images based on properties of various molecules.
At Washington University, "regular" MRI scans will create a 3-D image of each subject's basic brain. The same scanner set differently will make 3-D maps of the bundles of nerve axons (white matter) that connect the gray matter areas of the brain. This method, called DTI, generates images like the one in the photo as it follows the movement of water molecules moving down the length of those long axon processes.
DTI is a relatively new technique, and radiologist Josh Shimony points out that the technology is advancing so quickly that data from even five years ago is out of date.
With still different settings the same MRI scanner will create images of functional brain connections.
How can a scientist "see" functional brain connections? The principal at work is that neurons use energy to fire electrical impulses. Blood flow increases to the active area to restore oxygen and glucose (fuel), in a response about 20 seconds long. The MRI scanner can be set to distinguish between oxygenated and de-oxygenated blood, and successive scans can detect how blood oxygen levels at any point change with time.
The assumption in functional MRI imaging is that regions of the brain showing exactly simultaneous fluctuations in blood oxygen levels are connected to each other through one or more synapses.
Research by Steven Nelson in the laboratory of Steven Petersen at Washington University gives an example of how maps showing functional connections are made. Nelson was interested an area of the brain linked to attention and memory retrieval.
To construct the map of relationships, Nelson and Petersen investigated functional MRI connections in both resting brains and in brains engaged in a specific task. Resting was defined as having the subjects focus their eyes on a fixed point, They selected 15 small areas in the left parietal cortex to be mapped in detail. Some of the areas showed strong functional connections to each other as well as to other areas across the brain.
The functional connections were confirmed by having subjects do a mental task. The areas functionally connected at rest also responded "in sync" when the subjects were thinking.
Functional Imaging In Real Time
Functional MRI responses occur after the neuron fires and the time scale is seconds to minutes. The time scale for a very different type of imaging called MEG (Magnetoencephalography) is milliseconds. MEG detects the magnetic waves that are generated every time an electrical signal varies.
Richard Bucholz of Saint Louis University uses MEG to "see" nerves firing when a subject does a task such as moving a finger. The subject sits in a device that looks like a hair dryer, but is really a helmet of about 300 magnetic wave detectors connected to a computer. That computer places the source of the magnetic wave in three-dimensional space. By superimposing the 3-D magnetic image on an anatomical MRI brain scan, he can see where in the brain the finger movement comes from. It may even be possible to detect the sequence in which connected neurons fire.
Jo Seltzer is a freelance writer with more than 30 years on the research faculty at the Washington University School of Medicine and seven years teaching technical writing at WU's engineering school.