Tonotopical organization and pure tone response characteristics of single units in the auditory cortex of the Greater Horseshoe Bat

Abstract

Tonotopical organization and frequency representation in the auditory cortex of Greater Horseshoe Bats was studied using multi-unit recordings. The auditory responsive cortical area can be divided into a primary and a secondary region on the basis of response characteristics forming a core/belt structure. In the primary area units with best frequencies in the range of echolocation signals are strongly overrepresented (Figs. 6–8). There are two separate large areas concerned with the processing of the two components of the echolocation signals. In one area frequencies between the individual resting frequency and about 2 kHz above are represented, which normally occur in the constant frequency (CF) part of the echoes (CF-area), in a second one best frequencies between resting frequency and about 8 kHz below are found (FM-area). In the CF-area tonotopical organization differs from the usual mammalian scheme of dorso-ventral isofrequency slabs. Here isofrequency contours are arranged in a semicircular pattern. The representation of the cochlear partition (cochleotopic organization) was calculated. In the inferior colliculus and auditory cortex there is a disproportionate representation of the basilar membrane. This finding is in contradiction to the current opinion that frequency representation in the auditory system of Horseshoe Bats is only determined by the mechanical tuning properties of the basilar membrane. Response characteristics for single units were studied using pure tone stimuli. Most units showed transient responses. In 25% of units response characteristics depended on the combination of frequency and sound pressure level used. Frequency selectivity of units with best frequencies in the range of echolocation sounds is very high. Q-10dB values of up to 400 were found in a small frequency band just above resting frequency.