Spontaneous otoacoustic emissions in lizards, air pressure effects on them and the question of point sources and global standing waves

Shera [1] proposed that pressure effects on the middle ear provide a model for distinguishing between a point-source and a global standing-wave model of SOAE generation. A point source is supposed to be insensitive to changes in the boundary conditions for oscillation, whereas a standing wave would be influenced. Changing middle-ear pressure in humans alters both frequency and amplitude of SOAE, supporting Shera’s assumption that mammalian SOAE originate through global standing waves. Lizards are highly reliable generators of SOAE, but their hearing organ differs from that of mammals in size, structure and micromechanics. Thus they provide a good system in which to continue to examine ideas about the generation of spontaneous emissions. In lizards, both negative and positive pressure changes were produced in the ear canal by adding or withdrawing air. Increases in pressure led to no or only small changes in frequency and amplitude, whereas pressure drops led to a fall or rise in SOAE frequency of up to several percent and to amplitude loss. These changes were observed over much smaller pressure ranges than those necessary in humans. The question is discussed as to whether such data permit a clear distinction of the nature of the emission source.