Figure 1.

The STRFs of neurons in the inferior colliculus vary with stimulus intensity. a, The sound pressure distribution of the rain stimulus. The pressure signal was normalized to have a maximum value of 1 and a minimum value of −1. b, The spectrogram of a 400 ms segment of the stimulus. The spectrogram was computed with frequency bins spaced at one-tenth octave, time bins spaced at 2 ms, and a 4 ms hamming window. The colors denote the intensity of the stimulus in dB SPL. c, The frequency spectrum of the stimulus in dB SPL, averaged across a 40 s segment. d, The modulation spectrum of the stimulus. The modulation spectrum was obtained by computing the power spectrum of the amplitude modulations within each frequency bin of the spectrogram (after normalizing the amplitude modulations to have a maximum value of 1) and averaging the results across all frequency bins. e, The responses of a typical IC neuron to 50 repeats of a 500 ms segment of the rain stimulus at three intensities: 52, 72, and 92 dB SPL. Inset, The segments surrounding threshold crossings in the extracellular recording of this response that were classified as either spikes (red) or noise (black). f, The RLF relating mean firing rate to the intensity of the rain stimulus for the neuron for which responses are shown in e. The intensities corresponding to the responses in e are marked with colored boxes. g, h, The responses and rate-level function for a second neuron, displayed as in e and f. i, A schematic diagram of a linear–nonlinear (LN) model describing the mapping from stimulus to response in the inferior colliculus. The auditory stimulus in spectrogram form (s) is passed through the STRF (summation across frequency, convolution across time) to produce the intermediate signal y, which reflects the stimulus-related modulations in the membrane potential of the neuron. The intermediate signal is passed through the static NL to yield a time-varying firing rate (r). For more details, see Materials and Methods. j, The FRA relating mean firing rate to the intensity of a pure tone stimulus for the neuron for which responses are shown in e. k, The STRFs measured from responses to a 40 s segment of the rain stimulus (the training stimulus) at 52, 72, and 92 dB SPL for the neuron for which responses are shown in b. Red and blue areas indicate excitatory and inhibitory regions according to the color bar. The units of the STRF are arbitrary (but proportional to hertz per decibel). STRFs were smoothed with a 3 × 3 pixel Gaussian window for plotting. l, The NLs measured from responses to the rain stimulus at 52, 72, and 92 dB SPL for the neuron for which responses are shown in b. Colors correspond to the intensity of the stimulus. The horizontal axis corresponds to the result of passing the stimulus through the STRF, after normalization to unit variance. m–o, The FRA, STRFs, and NLs for the neuron for which responses are shown in g, displayed as in j–l.