Responses of superior collicular neurons of the big brown bat, Eptesicus fuscus, to stationary and moving sounds.

Responses of 59 superior collicular neurons (SC) of the big brown bat, Eptesicus fuscus, to stationary and moving sounds were studied under free-field stimulation conditions. Neurons responding to sound stimulation were recorded from all three layers of the SC; however, responses of neurons in the superficial and upper intermediate layers generally show poor signal to noise ratio. Spatial sensitivity of each SC neuron was studied by determining variation of the number of impulses and minimum threshold (MT) of the neuron with a sound broadcast from a stationary loudspeaker at several selected azimuthal angles. More than 95% of SC neurons showed a lowest MT and maximal number of impulses to sound broadcast from a contralateral angle. However, the angle of lowest MT and the angle of maximal number of impulses of a given neuron were not always the same. Responses of 40 SC neurons to moving sounds were studied by determining the number of impulses of each neuron to a sound broadcast from a moving loudspeaker at selected azimuthal angles. Most (n = 27, 67%) neurons were sensitive to a sound with low moving velocity (36 degrees/s). There were 9 (23%) neurons which discharged maximally to a sound moved in a preferred direction but poorly when moved in the opposite direction. In addition, 5 (13%) neurons discharged maximally to a sound moving away from the midline and across 20 degrees lateral of the bat's frontal auditory space. These findings suggest that SC neurons may help a bat acoustically "gaze" at insect during hunting.