Wednesday, July 8, 2009
The human ability to know roughly where in space a sound is coming from is so integral to our experience of the world that it’s basically invisible. A friend calls out our name, and we turn to the left, or turn to the right, and there they are.
Yet that ability, which seems so simple, not only depends on neurons in our brain that are exceptionally fast and precise, it’s an ability that we develop only after we begin hearing. We’re not born with the ability either to hear well enough to understand language or to locate a sound in space.
“Upon the opening of the auditory canal, which enables hearing onset in animals, the system is immature,” says Nace Golding, associate professor of neurobiology. “In humans this occurs in utero. The auditory system then begins a period of refinement. The electrical properties of the neurons themselves undergo very drastic and rapid changes.”
Using a technique called “patch clamping,” Golding and his lab are able to observe, in real time, what happens to a particular group of auditory neurons over the course of the first few weeks of post-natal development.
They take incredibly thin slices of rodent brain tissue and keep the slices alive in an artificial cerebrospinal solution. A glass pipette with a diameter at its tip of about one micron—approximately 1/100th of the width of a human hair—is then attached directly to the cell membrane. Suction is applied, creating a tight seal, which ultimately provides access to the inside of the cell. The pipettes then act essentially as lenses through which the electrical activity of the ion channels in the cells is directly observable.
At first, says Golding, the neurons don’t look too different from other kinds of neurons. Very quickly, however, they begin changing.
“In general, there are developmental changes found everywhere in the brain,” says Golding. “So the fact that there were changes wasn’t unexpected. What was unexpected was the magnitude of the changes.”
Over the course of approximately 5 days, for instance, one particular ion channel, which is involved in the speed of neural signaling, increases in density almost ten-fold. At the same time, the electrical properties of these channels change in a way that increases their impact on neural precision.
By the time such maturation is complete, the auditory neurons are so specialized for their task that the human auditory system, for instance, is able to locate a sound in space by calculating from the incredibly small (submillisecond) time differential between when a sound reaches the left and the right ears.
“Everything about these neurons is built for speed,” Golding says.
One of his lab’s most striking findings, he says, is how well-correlated these dramatic developmental changes are to the onset of hearing. The correlation, says Golding, suggests something rather profound about our relationship to the external world. We’re made by it.
“Intrinsic developmental processes can get things approximately wired correctly, get the inputs into the right place,” he says, “but for really fine tuning it requires other processes. One of those processes is the nature of the activity itself. The sound experience seems to actually shape the connections that are formed, as well as the electrical properties of the cells themselves.”