1. The monaural and binaural response properties and the horizontal sound location sensitivity of 78 binaurally excited neurons from 26 bats were examined with a combined closed-field and free-field stimulation paradigm. The aim was to determine how the response properties of these neurons shape their selectivity for auditory space. All neurons were recorded within a single, greatly enlarged isofrequency (60 kHz) region of the mustache bat's central nucleus of the inferior colliculus (ICC). In this and two companion papers (Wenstrup et al. 1988a,b) that focused on binaurally inhibited neurons in this isofrequency region, our goal was to examine the neural mechanisms for binaural processing within a single frequency channel of the primary auditory system. 2. Binaurally excited neurons were defined as either excited by monaural input from both ears or, if excited by monaural stimulation of only one ear, facilitated by binaural stimulation. Some neurons also exhibited binaural inhibition. These neurons were divided into functional classes based on their responses to monaural and binaural stimulation presented under dichotic, closed-field conditions. The following notation was used: response to contralateral stimulation (E, excitatory; O, no effect), response to ipsilateral stimulation (E, excitatory; O, no effect)/response to binaural stimulation (F, facilitatory; I, inhibitory; O, no effect). Seven functional classes were observed: EE/O, EE/F, EE/I, EE/FI, EO/F, EO/FI, and OO/F. 3. Among EE neurons, thresholds for contralateral monaural stimulation were typically lower than for ipsilateral stimulation, and response magnitudes for contralateral stimulation were typically greater. Among EO/F and EO/FI neurons, only one, an OE/FI neuron, was excited by ipsilateral monaural stimulation. These results suggest that contralateral input provides the dominant excitatory influence. EE/FI and EO/FI neurons, which exhibited both binaural facilitation and inhibition, were typically inhibited at interaural intensity differences (IIDs) favoring the ipsilateral ear, suggesting that ipsilateral input provides the dominant inhibitory influence. 4. Neurons were tested over the range of naturally occurring IIDs (±30 dB) at intensities of -20 to 30 dB relative to threshold. The IID functions of these neurons assumed three configurations: flat, with facilitation occurring at all IIDs; stepped, with facilitation occurring over part of the IID range, and peaked, with facilitation occurring over a limited band of IIDs. The majority of cells (90%) exhibited peaked IID functions, and most of these (73%), regardless of functional class, were maximally facilitated at an IID of 0 dB. 5. Neurons differed considerably in the strength of their binaural interactions. Binaural facilitation could contribute from 0 to 95% of the maximum response. Binaural facilitation was typically weaker in EE/F and EE/FI neurons than in EO/F and EO/FI neurons. In some EE/FI and EO/FI neurons, binaural inhibition occurred at IIDs flanking those that facilitated the cell, thereby sharpening their IID selectivity. 6. Under free-field conditions, neurons displayed considerable diversity in their spatial selectivity. In general, the stronger the binaural interactions of a neuron and the sharper its IID selectivity, the more restricted its sound location selectivity. Three types of receptive fields were observed. Omnidirectional fields expanded with intensity to encompass the entire horizontal plane. These were typical of, but not limited to, EE/O, EE/F, and weakly facilitated EO/F neurons. Hemispheric fields were restricted to the contralateral hemifield and were often observed in EE/FI and EO/FI neurons. Only one circumscribed field was observed, and it was restricted to the center of the frontal sound field. 7. The near-threshold spatial sensitivity of most neurons was the same as the directional selectivity of the external ears. A shift in threshold sensitivity was observed only in neurons that responded almost exclusively to binaural stimulation. A neuron's binaural response properties had the most obvious effect on its best area, where it responded with >75% maximum response. Because most neurons were maximally facilitated at an IID of 0 dB, best areas were typically shifted toward the bat's midsagittal plane.
