Fig. 1. Graphical illustration of the model developed to explain the characteristics of mammalian auditory nerve rate–intensity functions (Sachs et al., 1989; Yates, 1990a). Three examples of auditory nerve responses at the same characteristic frequency are illustrated. A, Relationship between sound pressure level and basilar-membrane motion, pointing out the two characteristic response segments: a linear growth at low levels, followed, above a certain breakpoint, by a compressive growth. This mechanical input is common to all three model fibers. A value of 0.2, commonly encountered in the guinea pig (Yates et al., 1990), has been chosen for the exponent of the slope of the compressive segment (equal to parameter A4 in Equation 1) in this example. B, Relationship between basilar-membrane motion and neural discharge rate. The three curves represent examples that differ in their sensitivity and spontaneous discharge rate in a way typically found in mammals, i.e., the most sensitive fiber has the highest spontaneous rate. They are labeled according to the shape of rate–intensity function that will result. C, Neural rate–intensity functions (solid curves), i.e., the relationship between sound pressure level and discharge rate. The three curves result from the combination of the inputs shown inA and B. The dashed linesillustrate different parameters used in the model, according to Equations 1 and 2. The parameters A1 (maximal rate) and A3 (breakpoint) are common to all three model fibers in this example; A0 (spontaneous rate) and A2 (half-maximum point) are only indicated for the flat–saturating curve to avoid confusion.