Spatial dissociation of changes of level and signal-to-noise ratio in auditory cortex for tones in noise.

Functional magnetic resonance imaging has been used to investigate the signal representation in human auditory cortex for a sinusoidal signal in the presence of a noise masker. This paradigm is widely used in auditory research to study auditory processing. Five-note tonal melodies were presented in a masking noise for signal-to-noise ratios (S/N) from -18 dB to+24 dB in 6 dB-steps. For small S/N (-18 dB, -12 dB, -6 dB) the overall level of the sound is nearly constant, but the audibility of the tone varies with S/N. For S/N of 0 dB and above, the tone is always clearly audible, and the perceived change is mainly the increase in overall level. This interaction between S/N, overall level and perception is reflected by a spatial dissociation of the respective activation in auditory cortex. Brain regions mainly sensitive to level changes were found in various parts of the superior temporal lobes, including primary auditory cortex and Planum temporale, while those regions mainly sensitive to S/N changes were located at or close to lateral Heschl's gyrus. The overlap between these two regions is small. The results are interpreted as indicating that the coding of overall level and, thus, loudness is different from the coding of audibility of a periodic signal. The S/N-sensitive region largely overlaps with the pitch-sensitive regions in lateral Heschl's gyrus found in previous studies. The results from the present study further suggest that the audibility of a tone in noise is related to the overall pitch strength.