1. Monaural excitatory responses of 181 single units in the central nucleus of the inferior colliculus of 15 anesthetized gerbils (Meriones unguiculatus) were examined quantitatively. Pure-tone stimuli were presented monaurally through sealed, calibrated sound-delivery systems. 2. Most units were excited only by contralateral stimulation (EO); 23% were bilaterally excitable (EE). The threshold frequency tuning curves for contralateral stimulation of EE units were significantly broader than those produced by ipsilateral stimulation of EE units and those produced by contralateral stimulation of EO units. The frequency at which threshold was lowest (best frequency, or BF) was very similar for ipsilateral and contralateral stimulation of individual EE units; however, ipsilateral BFs were slightly but significantly lower than contralateral BFs. 3. For EE units, ipsilateral BF thresholds (mean: 29.2 dB SPL) were significantly higher than contralateral BF thresholds (mean: 14.9 dB SPL). 4. Monotonic and nonmonotonic relationships between discharge rate and stimulus intensity at BF were observed in responses evoked both by contralateral and ipsilateral stimulation. Interestingly, for individual EE units it was not uncommon for the rate/intensity function for one monaural condition to be monotonic although the relationship for stimulation of the other ear was markedly nonmonotonic. There was no qualitative difference between rate/intensity functions evoked by contralateral stimulation in EO and EE units. 5. Ipsilateral discharge rates were characteristically much lower than contralateral rates for a given stimulus intensity. For 50 BF tones of 100 ms duration, the median peak numbers of discharges for contralateral stimulation of EO and EE units were 361 and 339, respectively; the median for ipsilateral stimulation of EE units was 102. 6. The dynamic range of each rate/intensity function was calculated by measuring the intensity range associated with an increase in spike count from 10 to 90% of the peak rate. No differences were detected between the distributions of dynamic range for contralateral stimulation in EO or EE units, or between contralateral and ipsilateral dynamic ranges within individual EE units. For all response types the distributions of dynamic range were approximately normal, with means near 20 dB. 7. The minimum mean latency to the first spike at BF was generally longer for ipsilateral than for contralateral responses. The median ipsilateral latency was 17.4 ms, whereas median latencies for contralateral responses of EO and EE units were 14.0 and 13.9 ms, respectively. 8. The temporal pattern of discharges varied considerably among units and often for a given unit as a function of stimulus frequency or intensity. The proportion of transient or onset responses was much greater for ipsilateral stimulation (49%) than for contralateral stimulation (6.5% for EO; 4.7% for EE contralateral). 9. These results reveal some very different consequences of contralateral and ipsilateral monaural stimulation on the discharge properties of single units in the inferior colliculus. The implications of the apparent imbalance of excitatory monaural inputs to current conventions of binaural classification and to the coding of sounds occurring in the free field are discussed.
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