Sounds like NIR spectroscopy

A car alarm and waves crashing on a beach clearly sound very different. Now, two Korean researchers have used functional NIR spectroscopy to investigate how the brain’s auditory cortex responds to these different sounds, as well as to many others.

Keum-Shik Hong and Hendrik Santosa at Pusan National University played these sounds to 18 subjects fitted with an array of NIR emitters and detectors on either side of their heads, right over the location of the auditory cortex in each hemisphere of their brain. This allowed the researchers to use NIR spectroscopy to record variations in the concentration of oxygenated haemoglobin over the auditory cortex, as a measure of brain activity, in response to the sounds.

The advantage of functional NIR spectroscopy over other techniques for monitoring brain activity, such as functional magnetic resonance imaging (MRI), is that it’s much quieter. This obviously makes it ideal for studying the auditory cortex, where unwanted noise needs to be reduced to a minimum.

Hong and Santosa played four different types of sound to the subjects, all of whom only understood English. These were: English speech; non-English speech, including Russian, German and French; annoying sounds, including a car alarm, a police siren and a baby’s cry; and natural sounds, including ocean waves, a forest and rain.

As reported in Hearing Research, all the sounds caused the oxygenated haemoglobin concentrations in the subjects’ auditory cortexes to increase. But English speech caused the concentrations to increase over a broader region of the cortex than the non-English speech and the increases were also greater, which the researchers attributed to the subjects being able to understand the English speech.

Less intuitively, the annoying sounds caused the concentrations to increase over a broader region of the cortex than the natural sounds, but the natural sounds generated greater increases in the concentrations. The researchers attributed this to the fact that most people prefer the calming sounds of waves on a beach to the shrill tones of a car alarm.

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