Thomas J. Park
The Neurobiology Group
Department of Biological Sciences
University of Illinois at Chicago
Chicago, IL 60607
Interaural intensity differences (IIDs) are important cues that animals use to localize high frequency sounds. Neurons sensitive to IIDs are excited by stimulation of one ear and inhibited by stimulation of the other ear, such that the response magnitude of the cell depends on the relative strengths of the two inputs, which in turn, depends on the sound intensities at the ears. In the auditory midbrain nucleus, the inferior colliculus (IC), many IID sensitive neurons have response functions that decline steeply from maximum to zero spikes as a function of IID. However, there are also many neurons with much more shallow response functions that do not decline to zero spikes. We present evidence from single unit recordings in the Free-tailed bat's IC that this partially inhibited response pattern is a result of the inhibitory input to these cells being very brief (~2 ms). 54 of 137 cells sampled (40%) achieved partial inhibition when tested with 60 ms tones, and the inhibition to these 54 cells occurred primarily during the first few ms of the excitatory response. Consequently, the initial component of the response was highly sensitive to IIDs while the later component was primarily insensitive to IIDs. Each of the 54 "partially inhibited" cells was able to reach complete inhibition with very short stimuli, such as simulated bat echolocation calls that invoked only the initial, IID sensitive component. Local application of inhibitory transmitter antagonists disabled the short inhibitory input, indicating that this response pattern is created within the IC.