Neural correlates of cubic difference tones in the medial geniculate body of the cat

Single unit responses to the cubic difference tone CDT (2f₁ ? f₂ = CF) and the difference tone DT (f₂ ? f₁ = CF) were studied in the medial geniculate body (MGB) of the cat. Out of 66 units tested with CDT stimuli and having characteristic frequencies (CF) below 10 kHz, 77% gave a response to the two-tone combination stimulus. The component tones when presented alone evoked no responses, or in some cases a response pattern that was different from the one observed for the combination tone. The CDT response pattern was always similar to that seen for a pure tone at the CF. The threshold of response for the CDT was 10–70 dB higher than for a pure tone stimulus at the CF.The few units which were phase-locked could be synchronised with the CF, CDT, or DT, depending on the particular stimulus conditions. The index of synchrony was in many cases found to be higher for CDT responses than for a pure tone at CF.