Monaural Processing of Frequency Sound Pressure Level and Sounds with Complex Spectra or Timing

The inferior colliculus receives information about the spectrum of sound through all of its inputs, but most studies of frequency organization and spectral responses have emphasized monaural stimulation. There is only a single tonotopic map of frequency that extends across both the central nucleus and the dorsal cortex of the inferior colliculus. The characteristic frequency of single neurons in the central nucleus changes in the dorsolateral to ventromedial direction in steps of 180-200 mm, which correspond to the layers of the fibrodendritic laminae and the inputs from the auditory cortex. Characteristic frequency is defined as the frequency of sound to which the neuron responds with the lowest threshold, and it often corresponds to the frequency that drives the neuron at the highest rate at higher sound levels. There is evidence that the sharpness of tuning in the central nucleus is not uniform, and neurons with different sharpness of tuning could be mapped within the laminae.

The colliculus also participates in the processing of complex temporal signals such as speech signals, which include amplitude and frequency modulation. Neurons in the inferior colliculus have best modulation frequencies for amplitude modulation that may be mapped along the laminae of the colliculus. In some animals like the bat, there are neurons that are sensitive to the duration of a stimulus and sensitive to particular frequency modulations that are used in echo location. The role of the inferior colliculus in speech processing in the primate remains unclear. However, speech sounds are undoubtedly processed by the colliculus, and it possible that certain inputs to the colliculus are better excited by these complex signals than others.

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