Recovery cycles of single-on and double-on neurons in the inferior colliculus of the leaf-nosed bat, Hipposideros armiger.

Our previous study showed that when stimulated with constant frequency-frequency modulation (CF-FM) sounds, neurons in the central nucleus of the inferior colliculus of the CF-FM bat, Hipposideros armiger, either only discharged impulses to the onset of CF-FM sounds (76%, single-on neurons) or to the onset of both CF and FM components of CF-FM sounds (24%, double-on neuron) (Fu et al., 2010). The present paper reports the recovery cycles of these two types of neurons using paired CF, FM and CF-FM sounds as stimuli. Both types of neurons had similar recovery cycle for CF sounds but had the shortest recovery cycle for FM sounds. Whereas single-on neurons had similar recovery cycle for CF and CF-FM sounds, double-on neurons had longer recovery cycle for CF sounds than for CF-FM sounds. In addition, double-on neurons had significantly shorter recovery cycles than single-on neurons for FM and CF-FM sounds. Most neurons did not respond to the second sound when each pair of sounds overlapped. However, when stimulated with paired CF-FM sounds, 3 single-on and 7 double-on neurons discharged to the second sound even when both sounds overlapped. As such, they had "cyclic" recovery cycles that varied between maximum and minimum with inter-pulse intervals. Possible mechanisms underlying the different recovery cycles of these neurons are proposed. Possible biological significance of these neurons in relation to responding to varied pulse repetition rate during hunting is discussed.