Basic response determinants of single neurons to amplitude modulation in the auditory midbrain

Amplitude-modulated (AM) signals represent important components of environm ental sounds. While single-cell responses to AM tones in the central audito ry system were often studied using repetitive modulation, owing to its pres ence in vocalization signals, the AM response has not been fully depicted i n terms of receptive field in the stimulus domain. This study was aimed to characterize the receptive field of AM response with respect to nonrepetiti ve AM stimuli and to understand how complex acoustic signals may be coded i n the brain. A novel AM stimulus was implemented with a random envelope and a systemic change in intensity across trials. From 393 single units record ed in the inferior colliculus (IC) of urethane-anesthetized rats, responses to the AM stimulus were first characterized in terms of dot-raster pattern . Three types of response were identified: type I showing a monotonic respo nse to mainly the steady states of the AM envelope and type Il to rising ph ases of the AM envelope with a clear intensity preference. Type III showed a mixed response of both type I and type II. A small number of units, calle d type TV, responded to both rising and falling phases of the modulation. U sing perispike averaging, the AM receptive field, or "level temporal recept ive field" (LTRF), was displayed in a "stimulus level versus perispike time " plane. The LTRF particularly of the type II response, clearly revealed tr iggering features of the cell. The triggering features are consistent with the representation of the cell's response in a receptive space formed by th e Cartesian axes of the velocity of amplitude modulation, the intensity of the sound, and the range of modulation. We therefore considered these stimu lus parameters as the three basic determinants of the AM response in the au ditory midbrain.