This functional magnetic resonance imaging study investigates the neural underpinnings of spectro-temporal integration during speech perception. Participants performed an auditory discrimination task on a set of sine-wave analogues that could be perceived as either nonspeech or speech. Behavioural results revealed a difference in the processing mode; spectro-temporal integration occurred during speech perception, but not when stimuli were perceived as nonspeech. In terms of neuroimaging, we observed an activation increase in the left posterior primary and secondary auditory cortex, namely Heschl's gyrus and planum temporale encroaching onto the superior temporal sulcus, reflecting a shift from auditory to speech perception. This finding demonstrates that the left posterior superior temporal lobe is essential for spectro-temporal processing during speech perception.