Analysis of information-bearing elements in complex sounds by auditory neurons of bats

Abstract

Communication sounds of higher animals, including humans, often consist of three information-bearing elements, constant-frequency (or pure tone), frequency-modulated, and noise components. For communication and echolocation, bats and dolphins emit sounds including these three elements. Echoes returning from different objects overlap each other and show complex envelopes and structures. Analysis of complex sound is very essential for these animals. In order to explore the neural mechanisms of the analysis of complex sounds, two problems were particularly studied: (1) how single neurons respond to each of the information-bearing elements composing complex sounds, and (2) how single neurons respond to the complex sounds consisting of these elements.

In both the inferior colliculus and the auditory cortex of bats, neurons exist which resond either to only one of the three elements, to two of the three, or to all of them. Most of the characteristics of these various types of neurons, which are not found in the cochlear nucleus, can be explained by interaction between excitation and inhibition. In the neurons which are specialized to analyze the information-bearing elements, the structure of complex sound is an important factor for excitation.

Key Words:

• Frequency-modulated (FM) sounds
• Responses of single neurons
• Pure tone-specialized neuron
• FM-specialized neuron
• Noise-specialized neuron
• Excitatory and inhibitory areas
• Neural network model