3.31 - Invertebrate Auditory Pathways

Abstract

Invertebrate auditory pathways are used for processing of species-specific signals and predator sounds. Although many invertebrates have sensory structures that can respond to sound, hearing is well developed and widely used only in insects, which form the subject of this chapter. Primary auditory afferents respond to sound pressure-induced mechanical oscillations of a tympanic membrane. The afferents forward their spike activity to an auditory neuropil in the central nervous system. Here, the first steps of signal processing are performed by local and ascending auditory neurons. Information on intensity, frequency, or direction is refined and sent to the brain via ascending axons. In the brain, a more complex processing takes place; based on the specific pattern of sound signals, different behavioral responses may be initiated. Although hearing has evolved in different groups of insects, the functional organization of the auditory pathway appears to be similar across the taxa. A brief overview presents the different hearing organs that have evolved in insects. Then the spatial organization of afferents in the auditory organs and the arrangement of their central projection patterns are compared with their functional properties. This demonstrates a highly structured organization of the auditory pathways already at the first stages of neural processing. The central auditory pathways of different groups of insects are described with particular emphasis on functional properties such as frequency analysis, temporal processing, pattern recognition, and the processing of directional acoustic information. Neural mechanisms are analyzed and the contribution of single neurons to behavior is highlighted. Finally we describe the impact of motor activity on hearing and what mechanisms may be employed to maintain auditory sensitivity during sound production.