The detection of novel signals in the auditory scene is an elementary task of any hearing system. In Neoconocephalus katydids, a primary auditory interneuron (TN-1) with broad spectral sensitivity, responded preferentially to rare deviant pulses (7 pulses/s repetition rate) embedded among common standard pulses (140 pulses/s repetition rate). Eliminating inhibitory input did not affect the detection of the deviant pulses. Detection thresholds for deviant pulses increased significantly with increasing amplitude of standard pulses. Responses to deviant pulses occurred when the carrier frequencies of deviant and standard were sufficiently different, both when the deviant had a higher or lower carrier frequency than the standard. Recordings from receptor neurons revealed that TN-1 responses to the deviant pulses did not depend on the population response strength of the receptors, but on the distribution of the receptor cell activity. TN-1 responses to the deviant pulse occurred only when the standard and deviant pulses were transmitted by different groups of receptor cells. TN-1 responses parallel stimulus specific adaptation (SSA) described in mammalian auditory system. The results support the hypothesis that the mechanisms underlying SSA and change-detection are located in the TN-1 dendrite, rather than the receptor cells.