|
At Caltech in the mid-1970s, Masakazu ("Mark") Konishi began studying the auditory system of barn owls in an effort to resolve a seemingly simple question: Why do we have two ears?
While most sounds can be distinguished quite well with one ear alone, the task of pinpointing where sounds are coming from in space requires a complex process called binaural fusion, in which the brain must compare information received from each ear, then translate subtle differences into a unified perception of a single sound—say a dog's bark—coming from a particular location.
Konishi, a zoologist and expert on the nervous system of birds, chose to study this process in owls. The ability to identify where sounds are coming from based on auditory cues alone is common to all hearing creatures, but owls—especially barn owls—excel at the task. These birds exhibit such extraordinary sound localization abilities that they are able to hunt in total darkness.
Together with Eric Knudsen, who is now conducting his own research on owls at Stanford University, Konishi undertook a series of experiments in 1977 to identify networks of neurons in the brains of owls that could distinguish sounds coming from different locations.
He used a technique pioneered by vision researchers, probing the brains of anesthetized owls with fine electrodes. With the electrodes in place, a remote-controlled sound speaker was moved to different locations around the owl's head along an imaginary sphere. As the speaker moved, imitating sounds the owl would hear in the wild, the investigators recorded the firing of neurons in the vicinity of the electrodes.
Over the course of several months, Konishi and Knudsen were able to identify an area in the midbrain of the birds containing cells called space-specific neurons—about 10,000 in all—which would fire only when sounds were presented in a particular location. Astonishingly, the cells were organized in a precise topographic array, similar to maps of cells in the visual cortex of the brain. Aggregates of space-specific neurons, corresponding to the precise vertical and horizontal coordinates of the speaker, fired when a tone was played at that location.
"Regardless of the level of the sound or the content of the sound, these cells always responded to the sources at the same place in space. Each group of cells across the circuit was sensitive to sound coming from a different place in space, so when the sound moved, the pattern of firing shifted across the map of cells," Knudsen recalls.
The discovery of auditory brain cells that could identify the location of sounds in space quickly produced a new mystery. "The lens of the eye projects visual space onto receptors on a 2-dimensional sheet, the retina, and the optic nerve fibers project the same spatial relationships to the brain," says Konishi. "But in the auditory system, only the frequency of sound waves is mapped on the receptor layer, and the auditory nerve fibers project this map of frequency to the brain. How can the brain create a map of auditory space, based only on frequency cues?"
The answer, Konishi believes, may shed light on how the brain and the auditory system process all sounds. |
|
In total darkness a barn owl swoops down on a mouse.
Photo: Masakau Konishi, California Institute of Technology
|
