Categorical perception is a phenomenon in which we perceive sounds as belonging to specific categories even when they actually form a continuum. A paper published online in Nature Neuroscience this week shows that activity in a specific region of the brain correlates with this perception, rather than the physical characteristics of the sounds heard.
The posterior superior temporal gyrus in the human brain was previously known to be important for processing speech sounds. Edward Chang and his colleagues were able to get direct access to this region, by recording electrical activity in the area from awake patients undergoing brain surgery to treat epilepsy or brain tumours. The patients heard artificially synthesized speech sounds which varied by equal steps along a continuum. However, they heard these sounds perceptually as belonging to either /ba/, /da/ or /ga/ categories. Electrical activity in the superior temporal gyrus showed the same kind of clustering as their perception suggesting that the brain responses were less influenced by the physical characteristics of the speech sounds, but instead reflected how people heard these sounds. These results therefore provide an insight into how the brain transforms incoming stimuli into more abstract representations.