PERIODICITY CODING IN THE AUDITORY CORTEX: WHAT CAN WE LEARN FROM LEARNING EXPERIMENTS?

Many natural sounds are characterized by periodic envelope or amplitude modulation (AM). The perceptual qualities corresponding to this signal characteristic are rhythm for low and pitch for high modulation frequencies (f_m). We demonstrate that f_m ranges corresponding to these two perceptual qualities seem to be represented by different codes in the primary auditory cortex (AI) of the Mongolian gerbil: Low f_m are represented by a synchrony code whereas high f_m are represented by a rate-place code. For the rate-place code we show that in response to AM neurons integrate over spectral ranges much wider than expected on the basis of their classical frequency receptive fields. Furthermore, we present preliminary data from learning experiments where gerbils were trained to discriminate between AM with different f_m. It seems that the different cortical codes used to represent AM with different perceptual qualities are also reflected in the learning curves: learning performance is better for low f_m ranges, where phase-locking is still present, than for high f_m ranges that are encoded by a rate-place code.