On Neurophysiological Mechanisms of the Echolocating Apparatus in Bats (Frequency Parameters)

Abstract

The total electric reactions and action potentials of single units of Cochlear nuclei, inferior colliculi, medial geniculate bodies and auditory cortex were studied. Maximum sensibility to ultrasonic sounds on all levels of the auditory analyzer in Myotis oxygnathus (fam. Vespertilionidae) is revealed at frequencies comprising their echolocating cries. In Rhinolophys ferrum equinum (fam. Rhinolophidae) in the range of 80 kc-the chief frequency of their echolocating cries-thresholds of electric reactions are registered higher by 15–30 db than at the closelying 70–76 and 81–86 kc frequencies.

The response areas of single neurons in both species of bats are similar to those registered in other mammals at corresponding levels. At the same time certain peculiarities in the reactions of single neurons, the characteristic frequencies of which relate to the 70–90 kc range, are revealed in Rhinolophus ferrum equinum. Thus, on the Cochlear nucleus level, three peculiar types may be marked out: neurons with a response area of either up to 78 kc, or from 80 to 90 kc; neurons reacting in the 40–90 kc frequency band, but not sensitive to stimulation with a 78–80 kc frequency; and, finally, a group of neurons having a response area including the 78–80 kc region, but in which the character of responses changed, depending on the stimulus frequency, from tonic responses to phasic responses; to switching the stimulus ‘ON’ or switching it ‘ON’ and ‘OFF’.

The neuron tuning sharpness is found to be at the maximum in Rhinolophus ferrum equinum in the 70–90 kc range. The total number of finely tuned neurons in this species increases from the Cochlear nucleus to the auditory cortex, while as in Myotis oxygnatus this process is not expressed.