Behavioral and physiological studies have indicated the existence of a temporal window of auditory integration (TWI), within which similar sounds are perceptually grouped. The current study exploits the combined temporal and spatial resolution of fast optical imaging (the event-related optical signal, EROS) to show that brain activity elicited by sounds within and outside the TWI differs in location and latency. In a previous event-related brain potential (ERP) study [Sable, Gratton, and Fabiani (2003) European Journal of Neuroscience, 17, 2492–2496], we found that the mismatch negativity (MMN; a brain response to acoustic irregularities) elicited by deviations in stimulus onset asynchronies (SOAs) had a unique shape when the deviant SOA was within the TWI. In the present study, we extended these ERP results using EROS. Participants heard trains of five tones. The first four tones had SOAs of 96, 192, 288 or 384 ms. The SOA of the fourth and fifth tones was either the same (standard) or one of the other three (deviant) SOAs. With a deviant SOA of 96 ms, the cortical response was approximately 2 cm anterior to responses to longer SOA deviants, and was followed by a later response that was absent in the other conditions. Similarly to the electrical MMN, the optical mismatch response amplitudes were proportional to the magnitude of interval deviance. These results, in combination with our previous findings, indicate that the temporal integration of sounds is reflected in cortical mismatch responses that differ from the typical response to interval deviance.