Breaking the wave: effects of attention and learning on concurrent sound perception.

The auditory surrounding is often complex with many sound sources active simultaneously. Yet listeners are proficient in breaking apart the composite acoustic wave reaching the ears. This achievement is thought to be the result of bottom-up as well as top-down processes that reflect listeners' experience and knowledge of the auditory environment. Here, specific findings concerning the role of bottom-up and top-down (schema-driven) processes on concurrent sound perception are reviewed, with particular emphasis on studies that have used scalp recording of event-related brain potentials. Findings from several studies indicate that frequency periodicity, upon which concurrent sound perception partly depends, is quickly and automatically registered in primary auditory cortex. Moreover, success in identifying concurrent vowels is accompanied by enhanced neural activity, as revealed by functional magnetic resonance imaging, in thalamus, primary auditory cortex and planum temporale. Lastly, listeners' ability to segregate concurrent vowels improves with training and these neuroplastic changes occur rapidly, demonstrating the flexibility of human speech segregation mechanisms. Together, these studies suggest that the primary auditory cortex and the planum temporale play an important role in concurrent sound perception, and reveal a link between thalamo-cortical activation and the successful separation and identification of speech sounds presented simultaneously.