Spectral and Temporal Modulation Tradeoff in the Inferior Colliculus

1 Biomedical Engineering, 2 Psychology, 3 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut Submitted 9 September 2009; accepted in final form 11 December 2009 ABSTRACT The cochlea encodes sounds through frequency-selective channels that exhibit low-pass modulation sensitivity. Unlike the cochlea, neurons in the auditory midbrain are tuned for spectral and temporal modulations found in natural sounds, yet the role of this transformation is not known. We report a distinct tradeoff in modulation sensitivity and tuning that is topographically ordered within the central nucleus of the inferior colliculus (CNIC). Spectrotemporal receptive fields (STRFs) were obtained with 16-channel electrodes inserted orthogonal to the isofrequency lamina. Surprisingly, temporal and spectral characteristics exhibited an opposing relationship along the tonotopic axis. For low best frequencies (BFs), units were selective for fast temporal and broad spectral modulations. A systematic progression was observed toward slower temporal and finer spectral modulation sensitivity at high BF. This tradeoff was strongly reflected in the arrangement of excitation and inhibition and, consequently, in the modulation tuning characteristics. Comparisons with auditory nerve fibers show that these trends oppose the pattern imposed by the peripheral filters. These results suggest that spectrotemporal preferences are reordered within the tonotopic axis of the CNIC. This topographic organization has profound implications for the coding of spectrotemporal features in natural sounds and could underlie a number of perceptual phenomena. Address for reprint requests and other correspondence: M. A. Escabí, University of Connecticut Electrical, and Computer Engineering, 371 Fairfield Rd., Unit 2157, Storrs, CT 06269 x 1157 (E-mail: escabi{at}engr.uconn.edu ).