Neurons in the medial superior olive (MSO) encode interaural time differences (ITDs) in the range of Ás. The underlying mechanism is thought to be a coincidence detection of binaural excitatory inputs. Key features for this assumption are (1) the observed stability of peaks in the ITD functions of individual neurons that are independent of the frequency of test tones, and (2) the predictability of best ITDs on the basis of responses to monaural stimulation.
However, there are also prominent binaural inhibitory projections to the MSO. There role in ITD coding is still unknown. However, recent recordings from MSO neurons in a small echolocating bat, Tadarida brasiliensis mexicana, indicate an important role for inhibition in shaping ITD functions in response to sinusoidally amplitude modulated (SAM) high frequency tones. The recorded ITD functions show stable peaks in face of changes of the modulation frequency and the best ITDs could be predicted by the responses to monaural stimulation (Grothe an Park; J. Neurosci 16: 6608, 1998).
The ITD functions measured using SAM stimuli can be explained by simply modeling the time course of the two excitatory and two inhibitory inputs with the inhibitory input slightly delayed compared to the excitation form the same side. A crucial factor in this model turns out to be an decrease of the duty cycle of the cyclic inputs when the modulation frequency is raised.
In contrast to high frequency neurons low-frequency neurons phase lock to pure tones Therefore, MSO neurons can code ITDs of ongoing low frequency tones. However, changes in duty cycle of the inhibitory inputs due to changes in the test tone frequency would not occur when low frequency pure tones are used. Thus, the model that can explain ITD coding of SAM envelopes does not work for low frequency pure tones or has to be extended. A possible extension would be a prominent asymmetry in the frequency tuning of the inhibitory inputs to a given MSO neuron. Such an asymmetry could cause different durations of IPSPs for different frequencies similar to the different duty cycles in response to SAM frequency.
Connect With Us: