Binaural hearing enables multiple functions of the hearing system, including sound localization. In the mammalian auditory circuitry, the first neurons responsible for comparing sound from the two ears are found in the Medial Superior Olive (MSO). While there have been many studies on MSO neuron physiology, details concerning how these neurons generate meaningful patterns of activity in response to synaptic input are poorly understood. In this study, we used dynamic clamp to study neurons from acute brain slices from Mongolian gerbils in vitro. Whole cell dynamic clamp provides naturalistic synaptic input that neurons are predicted to receive in vivo. We simulated synaptic input to MSO neurons evoked by acoustic stimuli at both 100 Hz (low frequency) and 1000 Hz (high frequency). We found that with lower frequency stimuli, the cells respond significantly more at 180 degrees out of phase in comparison to 90 degrees out of phase. At the higher frequency, the opposite is true (which is the situation in vivo.) We attempted to modulate this effect by changing the strength of the synapse. Neither increasing nor decreasing the strength of each synapse eliminated the out of phase spiking. We also changed the number of inputs per side, and while the sample size is small, the data show a trend toward no effect on out of phase spiking. These results suggest that inhibition may be required to process low frequencies compared to higher frequencies. We speculate that this could be a result of the interaction between stimulus frequency and the refractory period of the neuron. Future studies will include adding monaural (EEI) and binaural (EEII) inhibition to the dynamic clamp protocols in order to observe the changes to the out of phase stimuli.
Sean Maloney was a behavioral neuroscience major at Lehigh University, graduating with a bachelors of science. The research he conducted took place in Dr. R. Michael Burger's Auditory Neuroscience lab. It consisted of a year long project involving electrophysiological studies of gerbil brain stems, specifically the Medial Superior Olive. The results of this study will be used in future research in Dr. Burger's lab. Sean will be attending the University of Massachusetts Medical School in the Fall.