Critical Bandwidth in Man and Some Other Species in Relation to the Traveling Wave Envelope

Abstract

The hypothesis was offered some years ago (Greenwood, 1961) that critical bandWIdth in man was an exponential function of distance on the basilar membrane. Integration of the function yielded a frequency-position curve fitting Bekesy's data. Scale relations between frequency-position functions of a few species and between displacement envelopes (Greenwood, 1962) suggest that critical bandwidth may correspond to scale-related distances and may be proportional to the apical segm.ent of the traveling wave envelope. A revision of the critical-bandwidth function (Greenwood, 1971), in which critical bandwidth corresponds to about 1.25 mm in man, is reviewed; functions, based on distances scale related to 1.25 mm, are suggested for cat and squirrel monkey. Analysis of data on basilar-membrane motion in squirrel monkey and guinea pig (Johnstone, 1969/10; Kohlloffel, 1972; Rhode, 1970, 1971) indicates that the apical segments of the displacement envelopes closely agree in length with the respective distances to which critical bandwidth may correspond in these species and would imply an apical segment length close to 1.25 mm in man. Historically, students of hearing have sought to relate the findings of psychoacoustics to the physical and physiological properties of the auditory system. Stevens was a major contributor to this objective. His early physiological work and the influential book, Hearing, written with H. Davis in 1938 are well known. But a contribution of perhaps more far-reaching importance was his constant insistence on the fundamental need to search for basic psychophysical relationships possessing a sufficient generality and precision that we might know more clearly what had to be explained. These psychophysical relationships, essentially expressing what the system can do, would be necessary before an explanation of hearing in terms of the physiological properties and mechanisms of the auditory system could be constructed. One such relationship, that of critical bandwidth to other basic measures of audi-tory function such as the pitch scale and to an approximately constant distance on the basilar membrane, interested Stevens for many years. The present paper takes the latter relationship as its point of departure and is presented with the hope that it would have aroused Stevens' interest -along with his usual skepticism. It is also hoped that the ramifications of this empirical relationship will lead to the establish-ment of more links between psychoacoustics and auditory physiology. Critical Bandwidth in Relation to Position on the Basilar Membrane