The present work is concerned with the processing of spectral information by stochastic coding, at the output of the cochlea. We study the collective behaviour of a large number of cells based on a measure of the correlation of sequences of spikes between neighbour units. Our first results show that we can obtain significant discrimination of spectral components of vowels without any kind of cabled lateral inhibition or second filter device. This collective behaviour is made possible by the synchronization of inputs of adjacent units. This could provide an interesting and rather new way of making profit of both geographical coding (tonotopy) and temporal coding (synchronization) particularly useful for formant detection.