Corticofugal Modulation of Duration-tuned Neurons in the Midbrain Auditory Nucleus in Bats

Animal sounds, as well as human speech sounds, are characterized by multiple parameters such as frequency, intensity, duration, etc. The central auditory system produces neurons tuned to particular durations and frequencies of sounds emitted by a species. In bats, "duration-tuned" neurons are mostly sensitive to short durations and high frequencies of sounds used for echolocation. They are scattered in the frequency maps of the inferior colliculus and auditory cortex. We found that electric stimulation of cortical duration-tuned neurons modulates collicular duration-tuned neurons in both duration and frequency tuning only when collicular and cortical neurons paired for studies are within +/-4 ms in best duration and within +/-6 kHz in best frequency. There are four types of modulations: sharpening or broadening of duration tuning, and lengthening or shortening of best duration. Sharpening is observed in "matched" collicular neurons whose best durations are the same as those of stimulated cortical neurons, and it is accompanied by augmentation of the auditory responses at their best durations. The other three types of modulations are observed in "unmatched" collicular neurons whose best durations are different from those of stimulated cortical neurons. Lengthening or shortening of best duration is linearly related to the amount of the difference in best duration between collicular and cortical neurons. Corticofugal modulation is specific and systematic according to relationships in both duration and frequency between stimulated cortical and recorded collicular neurons.