Andrea Megela Simmons - US grants

This research will explore the acoustic dimensions of auditory perception in anuran amphibians to determine the limits of the acoustical world of these animals. Experiments will identify the acoustic features which influence the detection and discrimination of biologically relevant sounds from other sounds in the environment. A powerful new technique which measures the performance of the anuran auditory system to its real limits will be employed to test the prevailing hypotheses of vocal communication in anurans and to take advantage of these unique animals to develop our understanding of the comparative aspects of hearing in vertebrates. This new technique, reflex modification, is the first sensitive psychophysical procedure yielding precise, quantitative data on auditory detection in anurans. The results of the proposed experiments will be relevant to hypotheses concerning specializations of the auditory system for detecting sounds of biological interest to animals, and will also contribute to the understanding of the relation of morphological structures in the inner ear to hearing capabilities in vertebrates. This research can provide important contributions to the scientific disciplines of sensory physiology and communication sciences. The development of a sensitive procedure for audiometric measurements in anurans based on reflex modification has implications for the study of sensory function in other animals for which alternative measures based on standard conditioning techniques are inefficient or difficult to achieve. Because reflex modification requires no training and produces effects with relatively few trials, it may be a useful procedure for clinical and diagnostic work with infants and children, and for assessing effects of various drugs on sensory function.

DESCRIPTION (adapted from the Abstract): The objectives of this project are to analyze the representation of acoustic features of complex sounds in the central auditory system, and the relation of this neural processing to vocal communication in the natural environment. The auditory system of anuran amphibians will be used as a model system for understanding how neural processing in the brain underlies and guides natural acoustic behavior. This application outlines a series of experiments using physiological, anatomical, and behavioral techniques designed to test specific hypotheses of the role of temporal acoustic features in organizing and driving responses of auditory brainstem nuclei, and in mediating acoustic communication behaviorally. Physiological experiments will focus on neural coding of temporal features of complex sounds in two auditory nuclei, the torus semicircularis in the midbrain and the dorsolateral nucleus in the medulla. The goal of these experiments is to examine the operation of two competing neural codes for temporal information processing, one based on changes in spike rate of individual neurons and one based on the ability of individual neurons to phase lock or synchronize to the period of complex sounds. Competing hypotheses of how the periodicity or pitch of complex sounds is extracted in the central nervous system using different time domain algorithms will also be tested. The organization and representation of periodicity information in the midbrain and medulla will be analyzed by neuronal tract-tracing and cell filling techniques. These experiments will examine interconnections of functionally-described areas of these nuclei, and how these interconnections might modulate neural response properties. Of particular interest is the examination of different kinds of spatial representation of pitch information, as discrete periodotopic maps or as anatomical gradients of changes in neural codes representing pitch. These experiments will also examine how differences in cell morphology might reflect differences in acoustic information processing. Behavioral experiments using the evoked calling technique will be conducted to identify the roles of temporal properties of complex sounds in guiding vocal communication in the natural environment. These experiments directly impact upon theories of the neural bases for periodicity pitch perception in humans. Pitch is a critical dimension of auditory perception in humans, playing an important role in the identification, localization and interpretation of many kinds of sounds, including speech and music. The physiological bases for pitch perception has long been one of the dominant themes in research on hearing, but many questions about the neural extraction of pitch remain unanswered. Because pitch perception is so fundamental to hearing in humans, understanding its neural bases will be of value in the development of improved auditory prosthetic devices.

The overall objective of this research program is an understanding of the neuroethological bases of auditory communication. To achieve this objective, behavioral experiments on processing of biologically-relevant vocal signals in a "simple" vertebrate auditory system will be conducted and the neurophysiological correlates of these phenomena will be examined. The major theme of the project is a quantitative analysis of temporal cues mediating recognition of species-specific communication sounds. An integrative neuroethological approach will be used to estimate absolute sensitivity and selectivity in noise to different kinds of temporal cues, particularly waveform periodicity and fine-temporal structure. Experiments will be conducted on 3 levels: (1) psychophysical; (2) ethological, studying the natural communication behavior; and (3) physiological. Because vocal communication in a noisy environment is a fundamental problem for many animals, the principles of neural coding derived from this research are relevant for understanding the representation of complex speech sounds in the human's nervous system. This research will also contribute to the understanding of the mechanism of pitch perception, a fundamental but difficult problem in psychoacoustics.

The objectives of this project are to analyze the role of time-domain processing in mediating perception of complex sounds, specifically in relation to periodicity ('pitch') perception. Pitch is a critical dimension of auditory perception in humans; it is important, for example, in the identification, localization, and interpretation of many kinds of sounds, including speech and music. The physiological basis for pitch perception has long been one of the dominant themes in research on hearing, but many questions about the neural extraction of pitch in both the ear and the central auditory pathways remain unanswered. Because pitch perception is so fundamental to hearing, understanding its neural bases will be of value in the development of improved auditory prosthetic devices. Part of the difficulty involved in studying pitch stems from the multiplicity of neural codes available for pitch extraction. In this project, the representation of acoustic parameters influencing pitch perception in humans will be examined in the auditory system of an animal species in which the different neural codes available for periodicity extraction can be separated more readily than in most mammalian auditory systems. Anuran amphibians use pitch-like phenomena for mediating intraspecies communication. The behavioral sensitivity of these animals to acoustic parameters influencing 'pitch' will be examined using both ethologically-based and laboratory-based behavioral techniques. The organization of the anuran inner ear allows separation of place (frequency domain) from temporal (time domain) cues in periodicity extraction. Physiological recordings from the auditory nerve and auditory midbrain of these animals will examine the relative roles of frequency domain and time domain processes in mediating periodicity perception. These data will be used to test hypotheses derived from existing models of 'pitch' extraction, and to develop a computational model of time-domain processing in and anuran auditory system.