Amplitude Spectrum Representation in the Doppler-Shifted-CF Processing Area of the Auditory Cortex of the Mustache Bat

Abstract

The mustache bat, Pteronotus parnellii rubiginosus, emits orientation sounds containing a long constant-frequency (CF) component that is ideal for echo detection and Doppler shift measurement. About 30 percent of the primary auditory cortex of this bat is chiefly devoted to processing the second harmonic of the CF component in Doppler-shifted echoes. In this Doppler-shifted-CF processing area, single neurons recorded in any electrode penetration perpendicular to the cortical surface have nearly identical best frequencies and best amplitudes (or best pressure levels) at which the neurons show maximum excitation. The best frequency and best amplitude vary systematically with the location of the neurons in the cerebral cortex, so that there are tonotopic and "amplitopic" representation axes, which are radial and eccentric, respectively. In other words, the best-frequency and best-amplitude contours are eccentric and radial, respectively. The amplitude spectrum of a signal is thus represented in the coordinates of amplitude and frequency parallel to the cortical surface. This amplitude spectrum representation is disproportionate according to perceptual significance, so that a signal of 61.5 to 62.0 kilohertz and 30 to 50 decibels SPL (sound pressure level) is projected to a larger area than other signals. Just outside this Doppler-shifted-CF processing area, neurons are found which are specialized for responding to a particular information-bearing element or a particular combination of information-bearing elements in orientation sounds and echoes consisting of CF and frequency-modulated components.