Directionality of sound pressure transformation at the cat's pinna

Abstract

In anesthetized cats, pinna directionality was examined using the amplitude of the round window-recorded cochlear microphonic as a quantitative indicator of tympanic sound pressure level (SPL) associated with free field tonal stimuli in anechoic space. For high frequencies (above 3.5 kHz), there was a circumscribed optimal area for tympanic SPL in the frontal ipsilateral sound field, confirming that the pinna has an acoustical axis. The directionality of the pinna, determined from the solid angle enclosed by the 5-dB optimal area, increased with tonal frequency. Few low frequencies, no circumscribed optimal areas could be discerned. Excision of the pinna abolished the circumscribed optimal area for tympanic SPL and revealed that the pinna may produce up to 30-dB amplification of tonal stimuli delivered “on-axis.” This selective directional amplification is compatible with the view [J. C. Middlebrooks and J. D. Pettigrew, J. Neurosci. 1, 107–120 (1981)] that apart from a role in sound localization, active pinna movements could be used by the cat for the scrutiny of sound sources of interest at the expense of others.