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Speech recognition is robust to background noise. One underlying neural mechanism is that the auditory system segregates speech from the listening background and encodes it reliably. Such robust internal representation has been demonstrated in auditory cortex by neural activity entrained to the temporal envelope of speech. A paradox, however, then arises, as the spectro-temporal fine structure rather than the temporal envelope is known to be the major cue to segregate target speech from background noise. Does the reliable cortical entrainment in fact reflect a robust internal “synthesis” of the attended speech stream rather than direct tracking of the acoustic envelope? Here, we test this hypothesis by degrading the spectro-temporal fine structure while preserving the temporal envelope using vocoders. Magnetoencephalography (MEG) recordings reveal that cortical entrainment to vocoded speech is severely …