Linear and nonlinear pathways of spectral information transmission in the cochlear nucleus

At the level of the cochlear nucleus (CN), the auditory pathway divides int o several parallel circuits, each of which provides a different representat ion of the acoustic signal, Here, the representation of the power spectrum of an acoustic signal is analyzed for two CN principal cells-chopper neuron s of the ventral CN and type IV neurons of the dorsal CN, The analysis is b ased on a weighting function model that relates the discharge rate of a neu ron to first- and second-order transformations of the power spectrum. In ch opper neurons, the transformation of spectral level into rate is a linear ( i.e., first-order) or nearly linear function, This transformation is a pred ominantly excitatory process involving multiple frequency components, cente red in a narrow frequency range about best frequency, that usually are proc essed independently of each other, In contrast, type IV neurons encode spec tral information linearly only near threshold. At higher stimulus levels, t hese neurons are strongly inhibited by spectral notches, a behavior that ca nnot be explained by level transformations of first- or second-order. Type IV weighting functions reveal complex excitatory. and inhibitory interactio ns that involve frequency components spanning a wider range than that seen in choppers, These findings suggest that chopper and type IV neurons form p arallel pathways of spectral information transmission that are governed by two different mechanisms, Although choppers use a predominantly linear mech anism to transmit tonotopic representations of spectra, type IV neurons use highly nonlinear processes to signal the presence of wide-band spectral fe atures.