Selective increase in representations of sounds repeated at an ethological rate.

Exposure to sounds during early development causes enlarged cortical representations of those sounds, leading to the commonly held view that the size of stimulus representations increases with stimulus exposure. However, representing stimuli based solely on their prevalence may be inefficient, because many frequent environmental sounds are behaviorally irrelevant. Here, we show that cortical plasticity depends not only on exposure time but also on the temporal modulation rate of the stimulus set. We examined cortical plasticity induced by early exposure to 7 kHz tone pips repeated at a slow (2 Hz), fast (15 Hz), or ethological (6 Hz) rate. Certain rat calls are modulated near 6 Hz. We found that spectral representation of 7 kHz increased only in the ethological-rate-reared animals, whereas improved entrainment of cortical neurons was seen in animals reared in the slow- and fast-rate condition. This temporal rate dependence of spectral plasticity may serve as a filtering mechanism to selectively enlarge representations of species-specific vocalizations. Furthermore, our results indicate that spectral and temporal plasticity can be separately engaged depending on the statistical properties of the input stimuli.