RESPONSES OF INFERIOR COLLICULAR NEURONS OF THE FM BAT, EPTESICUS-FUSCUS, TO PULSE TRAINS WITH VARIED PULSE AMPLITUDES

Under free field stimulation conditions, we studied the responses of i nferior collicular neurons of the FM bat, Eptesicus fuscus, to pulse t rains with varied pulse amplitudes. Each pulse train consisted of 7 pu lses of 4 ms delivered at 24 ms interpulse-intervals (i.e. 42 pulses/s ). For a control pulse train, ah pulse amplitudes were equal to a neur on's best amplitude which, when delivered in single pulses, elicited m aximal number of impulses from the neuron. The amplitudes of individua l pulses of the remaining pulse trains were linearly increased or decr eased at a slope of 0, 14, 28, 42, 56 and 69 dB/s. All 56 inferior col licular neurons discharged to pulse trains were of two main types. Typ e I (N43, 77%) neurons discharged to each pulse within a train while t ype II(N11, 20%) neurons discharged to the first pulse of a train stim ulus only. Discharge patterns of the remaining (N2, 3%) neurons change d between type I and type II when stimulated with different pulse trai ns. The number of impulses discharged by a neuron varied with differen t pulse trains. In addition, the number of impulses discharged to each pulse by type I neurons also varied among individual pulses within th e train. Only 14 neurons (25%) discharged maximally to the control pul se train. Responses of the remaining neurons to other pulse trains wer e either 30%-120% larger than (N17, 30%) or within 30% (N25, 45%) of t he control pulse train response. Furthermore, half of 56 neurons selec tively discharged to a most preferred pulse train with a response magn itude which was at least 50% larger than the response to the least pre ferred pulse train. Possible mechanisms underlying the different disch arge patterns are discussed in terms of a neuron's recovery cycle, min imum threshold and inhibitory period relative to the temporal characte ristics (pulse repetition rate and amplitude) of the pulse trains.