Animals and ICE: meaning, origin, and diversity

Abstract

ICE stands for internally coupled ears. More than half of the terrestrial vertebrates, such as frogs, lizards, and birds, as well as many insects are equipped with ICE, that utilize an air-filled cavity connecting the two eardrums. Its effect is pronounced and two-fold. On the basis of a solid experimental and mathematical foundation it is known that there is a low-frequency regime where the internal time difference (iTD) as perceived by the animal may well be 2-5 times higher than the external ITD, the interaural time difference, and that there is a frequency plateau over which the fraction iTD/ITD is constant. There is also a high-frequency regime where the internal level (amplitude) difference iLD as perceived by the animal is much higher than the interaural level difference ILD measured externally between the two ears. The fundamental tympanic frequency segregates the two regimes. The present special issue provides an overview of many aspects of ICE, be they acoustic, anatomical, auditory, mathematical, or neurobiological. A focus is on the hotly debated topic of what aspects of ICE animals actually exploit neuronally to localize a sound source.

A circuit for detection of interaural time differences in the nucleus laminaris of turtles

“…Two strategies have emerged to improve localization when lowfrequency hearing and small heads limit animals' ability to encode sound source location. Other reptiles, such as lizards and archosaurs, have coupled ears, which increase the range of ITDs available at low frequencies (Calford and Piddington, 1988;Christensen-Dalsgaard and Manley, 2008;Larsen et al, 2016;van Hemmen et al, 2016). Small mammals, such as the gerbil and guinea-pig, show a trend that places the sensitive slope of the function where greatest ITD discrimination is required (McAlpine et al, 2001; for reviews see Grothe et al, 2010;Grothe and Pecka, 2014).…”

“…The rapid Golgi technique was used on five young turtles (carapace length 2.5 cm) (Valverde, 1970). Brains were removed and placed in Golgi fixative, an aqueous solution containing 2.33% potassium dichromate and 0.19% osmium tetroxide.…”

Geometric perturbation theory and Acoustic Boundary Condition Dynamics

“…We start by describing a typical situation, viz., that of internally coupled ears (ICE), which is present in more than half of the terrestrial vertebrates. Though ICE has been observed and described since long, it has attracted considerable attention only recently [37]. It is used here as a typical example from acoustics to illustrate the general idea of geometric perturbation theory.…”

“…Whereas mammals have independent ears, which do not influence each other, most of the terrestrial vertebrates have ICE at their disposal, which means an internal, air-filled, cavity that connects the two eardrums. ICE allows in particular small animals with small head and, hence, small ITD to greatly increase its effective interaural distance; typically, for low frequencies by a multiplicative factor of 2-4 [37]. What the animal then actually perceives is the so-called the internal time difference (iTD) as the superposition of the external auditory stimulus p ext operating at the two eardrums located at, say, x = 0 and x = L, and the internal pressure p; see Fig.…”

Frequency tuning and directional sensitivity of tympanal vibrations in the field cricketGryllus bimaculatus

“…To achieve their remarkable directional sensitivity crickets, like many other small animals with good directional hearing, have evolved specialized internal air-filled pathways between the left and right tympanal membranes [24][25][26]. Cricket ears are located on the proximal part of their prothoracic tibias.…”