The goal of this study was to determine the temporal response properties of different auditory cortical areas in humans. This is achieved by recording the phase-locked neural activity to white noises modulated sinusoidally in amplitude (AM) at frequencies between 4 and 128 Hz, in the left and right cortices of 20 subjects. Phase-locked neural responses are recorded in four auditory cortical areas with intracerebral electrodes, and modulation transfer functions (MTFs) are computed from these responses. A number of MTFs are bandpass in shape, demonstrating a selective encoding of AM frequencies below 64 Hz in the auditory cortex. This result provides strong physiological support to the idea that the human auditory system decomposes the temporal envelope of sounds (such as speech) into its constituting AM components. Moreover, the results show a predominant response of cortical auditory areas to the lowest AM frequencies (4-16 Hz). This range matches the range of AM frequencies crucial for speech intelligibility, emphasizing therefore the role played by these initial stations of cortical processing in the analysis of speech. Finally, the results show differences in AM sensitivity across cortical areas and hemispheres, and provide a physiological foundation for claims of functional specialization of auditory areas based on previous population measures.