The influence of critical bands on neuromagnetic fields evoked by speech stimuli in humans

The various classes of speech sounds differ in their configuration of acoustic features. Vowels are characterized by specific local maxima of spectral energy distribution (formants). Using whole-head magnetoencephalography, the impact of variation of the first (F1) and second formant (F2) on the evoked N1m component (100 ms latency) was studied based on an oddball design. F1 changes yielded N1m enhancements in parallel to the spectral distance between standard and deviant stimuli. By contrast, F2 shifts gave rise to a non-linear relationship: the N1m effect flattened out above a range of two Barks. This frequency domain accords to critical band characteristics of the peripheral and central auditory system. The differences of early neuronal encoding of both formants relate to the predominant role of F2 for the encoding of stop consonants.