Previous studies indicated that a brain region called the planum temporale (located above and behind the auditory cortex) is responsible for localizing sounds in space. At least, that is, when keeping an ear out for them.
New research now shows that the planum temporale activates automatically when there is noise, even if a person is not anticipating it. For instance, it will jump into action if a phone rings in the bedroom when you are watching TV in the living room.
"Space is a parameter that unifies the different senses; it allows us to merge information from, say, vision and audition when the spatial location of the source matches," says Leon Deouell, a cognitive neuroscientist at The Hebrew University of Jerusalem and co-author of the report published in Neuron. "It was important in my view to show that the planum temporale does its job even 'behind the scenes' when you don't intend to listen or localize."
This ability allows a person to shift attention toward the source of a new sound and react appropriately—such as returning a greeting from a neighbor passing by or running from a bear growling in the distance.
The research team, also including scientists from the University of California, Berkeley, and the Weizmann Institute of Science in Rehovot, Israel, says it took great pain to assure that they produced the highest quality sound transmission for their 13 subjects. To avoid the noise of the functional magnetic resonance imaging (fMRI) scanner, the scientists played the sounds between scans to allow for an undisturbed tone. (Because an fMRI measures blood flow to a part of the brain in response to electrical activity, there is a slight delay that the researchers were able to exploit by scanning immediately after paying a sound.) Through headphones modified to work in the scanner, the researchers also used a combination of environmental sounds, such as water or frogs. "Natural sounds, which contain many sound frequencies, stimulate the auditory cortex to a greater extent than pure tones," Deouell notes.
Sounds were tailored to individual subjects. The researchers used the tones not as they were played for a recording or a synthesizer. Prior to getting into the scanner, the participants were played each sound and a recording within each subject's ear was made; that sound was used during the fMRI trials. "The effect is quite amazing—when we test our subjects outside the scanner with their sounds they frequently turn around to look for the source," Deouell says. "They find it hard to believe that the sound comes from the headphones and not from out there in space."
Deouell and his team performed a series of experiments, each involving a different distraction and orientation of sounds. In the first setup, subjects watched a silent movie while sounds were played to them through headphones and their brains were imaged with an fMRI scan. In another, they were asked to perform a button-pushing task to keep their attention occupied.
In each case, whenever the location of a sound was modified, the subjects showed increased activity in their planum temporales; if the sound moved to more locations, heightened activity was elicited from the brain region.
Deouell plans to further study the planum temporale to see how close sounds can get to one another before the region interprets them as nondistinct and whether its activity can be overridden by another part of the brain, perhaps during a task that does not allow for any distraction.