Neuronal populations in the human brain extracting invariant relationships from acoustic variance.

The ability to extract invariant relationships from physically varying stimulation is critical for example to categorical perception of complex auditory information such as speech and music. Human subjects were presented with tone pairs randomly varying over a wide frequency range, there being no physically constant tone pair at all. Instead, the invariant feature was either the direction of the tone pairs (ascending: the second tone was higher in frequency than the first tone) or the frequency ratio (musical interval) of the two tones. The subjects ignored the tone pairs, and instead attended a silent video. Occasional deviant pairs (either descending in direction or having a different frequency ratio) elicited the mismatch negativity (MMN) of the event-related potential, demonstrating the existence of neuronal populations which automatically (independently of attention) extract invariant relationships from acoustical variance.