Neural Mechanisms of Hearing in Fishes

Abstract

Our understanding of the sense of hearing in fishes has been greatly enhanced in recent years. This chapter selectively reviews important findings of neural mechanisms of hearing in fishes including both hearing specialists and generalists. It first introduces basic knowledge of physical properties of underwater sounds and structure of fish’s auditory organs, and then describes sound detection pathways in different fish species. Major auditory nuclei and their connections that are involved in central auditory processing are also included. Research on fish hearing has previously focused on neural representation of simple sounds, with the majority of studies done on a few species, particularly the goldfish (Carassius auratus). This review summarizes transformations of temporal response patterns and changes of frequency selectivity at several levels of the goldfish’s auditory pathway. Recent studies of auditory research in fishes have aimed at the understanding of neural coding of biologically relevant sounds such as those produced by fishes themselves. Males of the mormyrid Pollimyrus adspersus and midshipman Porichthys notatus, two of many vocalizing fishes, produce species-specific sounds for acoustic communication during courtship behavior. Although we do not fully understand how the communication sounds are processed in the brains of these species, some experimental evidence has begun to uncover mechanisms of neural coding of acoustic signals that resemble their courtship sounds. At the end of the chapter, hypotheses and experimental data regarding how fish localize a sound source are reviewed. Sensory hair cells in fish’s hearing organs are spatially oriented in three dimensional space, and these hair cells are most sensitive to detecting a sound traveling along the directions of their morphological polarizations. These have led us to believe that each sensory hair cell in the fish’s ear is a directional sensor. Experimental results have revealed that response directionality of auditory afférents correlates significantly with morphological polarity of the hair cells innervated by them, indicating that the response directionality of hair cells is faithfully relayed to the brain via auditory afferents. However, it is still not clear how peripherally encoded directional information, the axis at which a sound propagates, is further processed in the brain to extract the actual direction of the sound source.