The inhibition of cat lateral superior olive unit excitatory responses to binaural tone bursts. II. The sustained discharges

C. Tsuchitani Graduate School of Biomedical Sciences, University of Texas Health Science Center, Houston 77030. 1. Preliminary to extending a point process model of lateral superior olive (LSO) unit activity to describe the units' binaural responses, the statistical properties of their discharges to binaural tone bursts were studied. The hypothesis that stimulation of the contralateral ear results in the simple reduction of the ipsilateral input was also examined. Single-unit activity was recorded extracellularly from the LSO of the anesthetized cat. The sustained discharges to characteristic frequency (CF) tone bursts presented simultaneously to the two ears were examined to determine whether the fine temporal (statistical) properties of these discharges differed from those of the discharges elicited by stimulating the ipsilateral ear alone. 2. The major effect of simultaneously stimulating the contralateral ear was the inhibition (i.e., the reduction in the mean discharge rate) of the sustained discharges to the ipsilateral control stimulus. The temporal pattern of discharges to the ipsilateral stimulus was also affected by stimulation of the contralateral ear. The discharges to binaural stimulation were more irregular in pattern: they often produced bimodal or multimodal interval histograms where unimodal interval histograms had been produced by the discharges to the ipsilateral control stimulus alone. The hazard function, an estimate of the unit recovery function, also often differed in form for the binaural and monaural discharges. 3. The binaural discharges could be distinguished from an ipsilaterally elicited discharge of comparable mean rate: there was a greater incidence of "short" interspike intervals in the binaural discharge. These short interspike intervals occurred most frequently in the discharges to the ipsilateral control stimulus alone and infrequently in the discharges to an ipsilateral stimulus that produced a mean rate similar to that of the binaural discharge. Thus the dead time estimates derived from the binaural discharges were more similar to the estimates derived from the ipsilateral control discharges than to those derived from the comparable-rate ipsilaterally elicited discharges. 4. Although the measures of the recovery properties of LSO unit discharges differed under monaural and binaural stimulus conditions, the serial dependence observed between successive interspike intervals in the binaurally elicited discharges was simila