The formation of auditory images from complex sound sources is an important function of the human auditory system. Several physical characteristics of complex sounds have been described to be important for the formation of auditory images. In this project, we describe experiments directed at gaining an understanding of the role of harmonicity and intensity in auditory image formation. Intensity can refer to the overall sound level of a stimulus or it may refer to the intensity profile of a stimulus. Harmonicity is closely related to the perception of pitch which is an important perception of many complex sounds, including speech and music. We will specifically focus on the auditory processing of stimulus features related to harmonicity and intensity of different types of noise signals, particularly rippled noises and bandlimited noises. Rippled noises produce the perception of pitch in humans and can be broadband or bandlimited signals. The proposed experiments involve human psychophysics and computer simulations, animal behavior, and single neuron recording. In human psychophysical experiments, we will continue to define the sensory and perceptual capabilities of the auditory system for processing spectrally rippled stimuli, including iterated-rippled noises and log-rippled stimuli. These studies will also include simulations using the Pulse- Ribbon Model of Roy Patterson. In animal experiments, we will study rippled noise processing in the chinchilla using positive reinforcement, psychophysical procedures which will allow us to define the sensory capabilities of this animal for processing rippled noise stimuli. As part of this effort, we will also study the ability of the chinchilla to detect intensity increments of bandlimited noise. Finally, we will study the neural representations of the temporal and spectral characteristics of rippled noises and bandlimited noises in the discharge patterns of neurons in the chinchilla cochlear nucleus. These studies will allow us to compare the results from the chinchilla psychophysical experiments to neuronal responses in the chinchilla cochlear nucleus and to human psychophysical data. These experiments will provide important insights into the neural processing of harmonicity and intensity of complex sounds in humans.
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