1. Single-unit recordings were carried out in primary auditory cortex (AI) of barbiturate-anesthetized cats. Observations were based on a sample of 131 high-best-frequency (>5 kHz), azimuth-sensitive neurons. These were identified by their responses to a set of noise bursts, presented in the free field, that varied in azimuth and sound-pressure level(SPL). Each azimuth-sensitive neuron responded well to some levels at certain azimuths, but did not respond well to any level at other azimuths. 2. Unilateral ear plugging was used to infer each neuron's response to monaural stimulation. Ear plugs, produced by injecting a plastic ear mold compound into the external ear, attenuated sound reaching the tympanic membrane by 25-70 dB. The azimuth tuning of a large proportion of the sample (62/131), referred to as binaural directional (BD), was completely dependent upon binaural stimulation because with one ear plugged, these cells were insensitive to azimuth (either responded well at all azimuths or failed to respond at any azimuth) or in a few cases exhibited striking changes in location of azimuth function peaks. This report describes patterns of monaural responses and binaural interactions exhibited by BD neurons and relates them to each cell's azimuth and level tuning. The response of BD cells to ear plugging is consistent with the hypothesis that they derive azimuth tuning from interaural level differences present in noise bursts. Another component of the sample consisted of monaural directional (27/131) cells that derived azimuth tuning in part or entirely from monaural spectral cues. Cells in the remaining portion of the sample (42/131) responded too unreliably to permit specific conclusions. 3. Binaural interactions were inferred by statistical comparison of a cell's responses to monaural (unilateral plug) and binaural (no plug) stimulation. A larger binaural response than either monaural response was taken as evidence for binaural facilitation. A smaller binaural than monaural response was taken as evidence for binaural inhibition. Binaural facilitation was exhibited by 65% (40/62) of the BD sample ( facilitatory cells). Many of these exhibited mixed interactions, i.e., binaural facilitation occurred in response to some azimuth-level combinations, and binaural inhibition to others. Binaural inhibition in the absence of binaural facilitation occurred in 35% (22/62) of the BD sample, a majority of which were EI cells, so called because they received excitatory (E) input from one ear (excitatory ear) and inhibitory (I) input from the other(inhibitory ear). One cell that exhibited binaural inhibition received excitatory input from each ear. 4. EI cells responded vigorously throughout much or all of one frontal quadrant (preferred side) and responded poorly throughout the other. Plugging of the ear on the nonpreferred side caused an increase in the cell's responsiveness, showing that the ear on the preferred side provided excitatory input and that the other ear provided inhibitory input. For a majority(86%, 18/21), the preferred side was located contralateral to the recording site in AI. 5. The binaural facilitation group was composed of midline-preferring (23%, 9/40) and lateral field (LF) cells (77%, 31/40) that responded selectively to midline and lateral locations, respectively. The LF sample included slightly more ipsilateral-than contralateral-preferring cells (55% vs. 45% and 17/31 vs. 14/31).
