The Tonotopic Array

Since the spectral resolution of the auditory filters plays an important role in almost all auditory phenomena, a perceptual method is described with which their bandwidths can be determined. These bandwidths are neither constant in Hertz nor in musical intervals such as semitones. This makes it necessary to calculate a frequency array on which equal distances are neither equal in Hertz nor in semitones, but in terms of the bandwidths of the auditory filters. This array is called the tonotopic array. This tonotopic array will be specified in detail because it plays a crucial role in the following chapters of this book. Distances on this array will be expressed in Cams, derived from Cambridge where this tonotopic scale was first described. One Cam naturally corresponds to the bandwidth of an auditory filter. The spectral properties of a sound will be expressed as excitation on this tonotopic array. The resultant excitation pattern expresses, grossly speaking, how much the various frequency components of a sound contribute to its auditory attributes, such as its loudness, its brightness, and its roughness. The temporal properties of the auditory filters are described by the temporal fine structure of the spike trains which is derived from the interval distributions of these spike trains. This temporal fine structure specifies, below about 5 kHz, not only the frequency of the stimulus but also the subharmonics of this frequency. This plays an important role in, e.g., pitch perception.