A key computational principle for encoding time-varying signals in auditory and somatosensory cortices of monkeys is the opponent model of rate-coding by two distinct populations of neurons.However,the subthreshold mechanisms that give rise to this computation have not been revealed.Because the rate-coding neurons are only observed in awake conditions,it is especially challenging to probe their underlying cellular mechanisms.By using a novel intracellular recording technique that we developed in awake marmosets,we found that the two types of rate-coding neurons in auditory cortex exhibited distinct subthreshold responses.While the positive-monotonic neurons(monotonically increasing firing rate with increasing stimulus repetition frequency)displayed sustained depolarization at high repetition frequency,the negative-monotonic neurons(opposite trend)exhibited instead hyperpolarization at high repetition frequency but sustained depolarization at low repetition frequency.The combination of excitatory and inhibitory subthreshold events allows the cortex to represent time-varying signals through these two opponent neuronal populations.