These experiments investigate the ways that sounds and sound sources are represented or encoded in the activity patterns among individual auditory nerve cells from the ears, and in cells of the brain that make up the central auditory system. Physiological experiments use microelectrodes that detect the action potentials (nerve impulses) set up in the auditory nerve and brain by simple and complex sounds. Anatomical experiments determine the identity of the cells and brain areas that have been probed, and describe the ways that sound-responsive cells are interconnected. One goal of this work is to understand how sound features are represented in physiological responses within and among cells of the nervous system, and how these representations are used by the brain to make decisions and develop perceptions about sound sources.
Work on cells of the auditory nerve, midbrain, and forebrain have shown in the goldfish that most of the responses are quite similar to those studied in amphibians, reptiles, birds and mammals. For example, every cell studied is frequency-selective, meaning that mechanisms exist to activate the cell with a small range of frequencies, and to "filter out" other frequencies. In brain cells, some of this frequency selectivity has been computed by complex excitatory (additive) and inhibitory (subtractive) interactions among cells. In cells of the auditory nerve, a crude frequency selectivity exists that derives from mechanisms probably different from those causing frequency selectivity in mammals. An important finding has been that the results and functional utility of this "filtering" are probably similar in all vertebrate animals while the mechanisms that begin the frequency-selective process in the ears appear to be different. Thus, it appears that vertebrate auditory systems have developed to solve a common set of problems in sound analysis, but that the mechanisms used to accomplish this may not be identical.
Other studies carried out at Parmly and the Marine Biological Laboratory in Woods Hole, MA focus on the ways that the spatial locations of sound sources may be represented in the directional response properties of auditory nerve cells in toadfish (a marine fish that uses vocalization sounds during courtship), and of brain cells in goldfish (a freshwater fish that does not vocalize). In both species it has been found that most nerve cells are direction-selective as well as frequency-selective, and that arrays of cells responding best to different source directions probably represent the direction of any given sound source. Directional responses are found in the brain cells of many vertebrate animals, and this information is probably used to locate sound sources. However, the origin of these directional responses are different in fish and terrestrial animals. Terrestrial animals compute direction by correlating the responses from the two ears, but fish appear to use the inherent directionality of hair cell receptors to solve the problem of directional hearing.