  Neurons in the main center of convergence in the auditory midbrain, the
central nucleus of the inferior colliculus (ICC) have been shown to display
either linear significant receptive fields, or both, linear and nonlinear
significant receptive fields. In this study, we used reverse correlation to
probe linear and nonlinear response properties of single neurons in the cat
ICC. The receptive fields display areas of stimulus parameters leading to
enhanced or inhibited spiking activity, and thus allow investigating the
interplay to process complex sounds. Spiking responses were obtained from
neural recordings of anesthetized cats in response to dynamic moving ripple
(DMR) stimuli. The DMR sound contains amplitude and frequency modulations and
allows systematically mapping neural preferences. Correlations of the stimulus
envelope that preferably excite neurons can be mapped with the spike-triggered
covariance. The spike-triggered average and -covariance were computed for the
envelope of the DMR, separately for each frequency carrier (spanning a range of
0-5.5 octaves). This enables studying processing of the sound envelope, and to
investigate whether nonlinearities are more pronounced at the neurons'
preferred frequencies rather than at other frequencies. We find that more than
half of the neurons (n=120) display significant nonlinear response properties
at least at one frequency carrier. Nonlinearities are dominant at the neuron's
best frequency. The nonlinear preferences can have either the same or opposite
temporal receptive field pattern (e.g. on-off) as the linear preferences. No
relationship to other neural properties such as feature-selectivity,
phase-locking, or the like has been found. Thus, these nonlinearities do not
seem to be linked to a specific type of neuron but to be inherent to ICC
neurons indicating a diverse range of filtering characteristics.
