Cross Correlation by Neurons of the Medial Superior Olive: a Reexamination

Abstract

Initial analysis of interaural temporal disparities (ITDs), a cue for sound localization, occurs in the superior olivary complex. The medial superior olive (MSO) receives excitatory input from the left and right cochlear nuclei. Its neurons are believed to be coincidence detectors, discharging when input arrives simultaneously from the two sides. Many current psychophysical models assume a strict version of coincidence, in which neurons of the MSO cross correlate their left and right inputs. However, there have been few tests of this assumption. Here we examine data derived from two earlier studies of the MSO and compare the responses to the output of a computational model. We find that the MSO is not an ideal cross correlator. Ideal cross correlation implies a strict relationship between the precision of phaselocking of the inputs and the range of ITDs to which a neuron responds. This relationship does not appear to be met. Instead, the modeling implies that a neuron responds over a wider range of ITDs than expected from the inferred precision of phase-locking of the inputs. The responses are more consistent with a scheme in which the neuron can also be activated by the input from one side alone. Such activation degrades the tuning of neurons in the MSO to ITDs.

Congenital and Prolonged Adult-Onset Deafness Cause Distinct Degradations in Neural ITD Coding with Bilateral Cochlear Implants

“…In other words, an IC neuron that is capable of temporal coding up to relatively high pulse rates is not more likely to show good ITD sensitivity at the low pulse rates (10-80 pps) that we used to characterize ITD coding. This dissociation between ITD sensitivity and temporal coding, which is also observed for pure-tone stimulation in normalhearing animals (Kuwada et al 1984;Kuwada et al 1987), is consistent with the fact that the IC is not the primary site of binaural interaction where ITD sensitivity first arises (Batra and Yin 2004), and suggests that pulse locking in the IC is limited by processing at sites beyond the primary binaural neurons in MSO and LSO.…”

Sensitivity of Inferior Colliculus Neurons to Interaural Time Differences in the Envelope Versus the Fine Structure With Bilateral Cochlear Implants

“…The binaural processor is implemented as multiplication between a delayed input from the contralateral side and an undelayed input from the ispsilateral side. This architecture is comparable to Jeffress-type models of binaural coincidence detection (Jeffress 1948) and mimics the operation of the MSO when operating on spike probabilities (Batra and Yin 2004;Goldberg and Brown 1969;Yin and Chan 1990). The envelope processor implements the interaction between excitation from the output of the binaural coincidence detector with delayed inhibition from the same output.…”

Neural and Behavioral Sensitivity to Interaural Time Differences Using Amplitude Modulated Tones with Mismatched Carrier Frequencies

“…Because the dynamic range of electric hearing is compressed relative to the range of environmentally relevant acoustic signals, the speech processor compresses the incoming signal, reducing the fidelity of intensity coding. Any loss of monaural synchronization would be even more important for binaural processing because the process of coincidence detection produces binaural synchronization that is approximately the square of the monaural synchronies Batra and Yin 2004). Implant listeners tend to have acute sensitivity to the presence of amplitude modulation (Shannon 1992), but there is some evidence that the ability to detect small changes in stimulus rate is relatively poor (Zeng 2002).…”