Speech comprehension has been shown to be a strikingly bilateral process, but the differential contributions of the subfields of left and right auditory cortices have remained elusive. The hypothesis that left auditory areas engage predominantly in decoding fast temporal perturbations of a signal whereas the right areas are relatively more driven by changes of the frequency spectrum has not been directly tested in speech or music. This brain-imaging study independently manipulated the speech signal itself along the spectral and the temporal domain using noise-band vocoding. In a parametric design with five temporal and five spectral degradation levels in word comprehension, a functional distinction of the left and right auditory association cortices emerged: increases in the temporal detail of the signal were most effective in driving brain activation of the left anterolateral superior temporal sulcus (STS), whereas the right homolog areas exhibited stronger sensitivity to the variations in spectral detail. In accordance with behavioral measures of speech comprehension acquired in parallel, change of spectral detail exhibited a stronger coupling with the STS BOLD signal. The relative pattern of lateralization (quantified using lateralization quotients) proved reliable in a jack-knifed iterative reanalysis of the group functional magnetic resonance imaging model. This study supplies direct evidence to the often implied functional distinction of the two cerebral hemispheres in speech processing. Applying direct manipulations to the speech signal rather than to low-level surrogates, the results lend plausibility to the notion of complementary roles for the left and right superior temporal sulci in comprehending the speech signal.