Auditory response properties and spatial response areas of superior collicular neurons of the FM bat, Eptesicus fuscus

1. Electrophysiological properties of 279 single units in the superior colliculus (SC) of the big brown bat,Eptesicus fuscus, were studied by recording their responses to pure tone pulses and frequency-modulated stimuli. 2. The SC units generally fired only a few impulses to acoustic stimulus. They are not tonotopically organized along the dorsoventral axis of the SC (Fig. 1). 3. The response latency of 276 units was between 3.6 and 20 ms, but most (253 units) were below 12.5 ms (Fig. 2). 4. Tuning curves of 164 units are either narrow, intermediate, or broad, according to the bandwidth of the tuning curves (Fig. 3). Q10 dB values range between 1.21 (best frequency (BF) =33.52 kHz) and 68.9 (BF=69.94 kHz), but the majority are below 20. The minimum thresholds (MTs) of these units were between 15 and 104 dB SPL, but most were above 40 dB SPL. 5. MTs to pure tone pulses and one octave upward and downward sweeping FM stimuli were compared (Fig. 4). SC units generally had their lowest MTs to downward sweeping FM stimuli and their highest MTs to pure tone pulses (Table 1). 6. Intensity-rate functions of 16 SC units were measured with both pure tone pulses and upward and downward sweeping FM stimuli. There was no correlation between the type of intensity-rate function obtained and the type of acoustic stimulus used (Fig. 5). 7. Auditory spatial response areas were measured for 47 units. The size of the spatial response area increases with stimulus intensity (Fig. 6). The auditory space is not orderly represented in the SC of a bat (Fig. 7).