Evoked potential correlates of echolocation in the mustached bat, Pteronotus p. parnellii

The biosonar signals of the greater mustached bats are characterized by a long constant frequency component that is preceded and terminated by frequency modulated components. It has generally been concluded that the terminal FM (TFM) is important for target ranging while the initial FM (IFM), or beginning of the signal, is relatively insignificant. With the aid of chronically implanted electrodes, acoustically evoked brainstem potentials were recorded from bats during simulated flight on a pendulum and when targets were placed at fixed distances from the bat's head. Distinct pulse- and echo-evoked potentials were recorded in relation to the onset of both the IFM and TFM, or the onset of the CF when no IFM was present. Echo-evoked potentials were often as high in amplitude as pulse-evoked potentials and the timing of the IFM- and TFM-pulse and echo-evoked potentials seemed to accurately reflect target distance. Data indicate that the IFM, or signal onset, must be a significant part of the echo even though it is usually faint, overlaps the intense outgoing CF component, and returns to the ear when the middle ear muscles are contracting.