Microelectrode and neuroimaging studies of central auditory function

Imaging studies in humans are revealing parallels with the functional organisation of the auditory brain discovered in microelectrode studies in animals: the rate of amplitude modulation generating the strongest response declines systematically from the lower brain stem to the cortex; an increase in sound level induces a higher level and a greater extent of activity; spectra are represented tonotopically in multiple cortical areas. There are also differences: evidence of organisation reflecting the sound level of the stimulus is absent in animals, but has been found in humans. Additionally, imaging has revealed functional specialisations which have not (yet) been located in animals: areas that respond more strongly to sounds with stronger pitches and to sounds that move in space. Microelectrode studies suggest that vocalisations are represented by spatially distributed populations of neurones in secondary auditory areas. In humans, likewise, activation progressively more specific to speech is found as the search moves from primary to secondary to accessory areas.