PROCESSING OF INTERAURAL INTENSITY DIFFERENCES IN THE LSO - ROLE OF INTERAURAL THRESHOLD DIFFERENCES

Cells in the lateral superior olive (LSO) are known to be sensitive to interaural intensity differences (IIDs) in that they are excited by I IDs that favor the ipsilateral ear and inhibited by IIDs that favor th e contralateral ear. For each LSO neuron there is a particular IID tha t causes a complete inhibition of discharges, and the IID of complete inhibition varies from neuron to neuron. This variability in IID sensi tivity among LSO neurons is a key factor that allows for the coding of a variety of IIDs among the population of cells. A fundamental questi on concerning the coding of IIDs is: how does each cell in the LSO der ive its particular IID sensitivity? Although there have been a large n umber of neurophysiological studies on the LSO, this question has rece ived little attention. Indeed, the only reports that have directly add ressed this question are those of Reed and Blum, who modeled the binau ral properties of LSO neurons and proposed that the IID at which disch arges are completely suppressed should correspond to the difference in threshold between the excitatory, ipsilateral and inhibitory, contral ateral inputs that innervate each LSO cell. The main purpose of this s tudy was to test the threshold difference hypothesis proposed by Reed and Blum by recording responses to monaural stimulation and to IIDs fr om single cells in the LSO of the mustache bat. Our results show that although the IID sensitivities of some LSO cells correspond to the dif ference in threshold between the excitatory and inhibitory ears, in th e majority of cells the difference in thresholds did not correspond to the cell's IID sensitivity. The results lead us to propose two models to account for IID sensitivities. One model is similar to that propos ed by Reed and Blum and emphasizes differences in the thresholds of th e excitatory and inhibitory inputs. This model accounts for the minori ty of cells in which the IID of complete inhibition corresponded to th e difference in threshold of the inputs from the two ears. The other m odel, which accounts for the cells in which the IID of complete inhibi tion did not correspond to the difference in the thresholds of the inp uts from the two ears (the majority of cells), places emphasis on diff erences in latencies of the excitatory and inhibitory inputs. The mode ls incorporate features that are concordant with the known properties of the neurons that project to the LSO and together can account for th e diversity of IID sensitivities among the population of LSO neurons.