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Nicholas J. Giordano
Purdue University
Department of Physics and Astronomy

We are currently working on several projects involving the physics and biophysics of the nervous system.

  1. Origins and effects of intrinsic noise. The signals that carry information between neurons usually propagate as action potentials (APs). These are propagating electrical pulses which are made possible by the properties of voltage gated ion channels (typically K and Na channels) generally found in neurons and axons. There are several sources of noise intrinsic to neurons and to APs. One is stochastic channel noise associated with the stochastic opening and and closing of individual ion channels. Another is the thermal noise (i.e., Johnson noise) associated with the conductance an open ion channel. We have performed a detailed study of the effects of these two noise sources in several types of neurons. Ours is the first study of the effect of thermal noise at voltages near and above the threshold for AP generation. We have found that thermal noise is often smaller than stochastic channel noise -- by about a factor of 30 in the squid giant axon. However, in other types of neurons, such as the neurons found in the mammalian auditory system, thermal noise can actually be larger than channel noise.
    The conclusion of this work is that any quantitative modeling of noise and fluctuations in the nervous system must include both channel noise AND thermal noise. A preprint describing this work is available here.
  2. One of the important tasks of the auditory system is sound localization; i.e., the direction that a sound comes from. Many years ago, Lord Rayleigh showed that for a low frequency sound, the azimuthal direction is determined by comparing the phases of the sound signals at the two ears. For example, if a sound comes from the left side of the head, the signal at the left ear will be advanced in phase relative to the signal at the right ear. One must then consider how the auditory system processes these two signals to extract and analyze this phase difference. A model for this auditory comparison process was proposed more than 50 years ago by Jeffress, and continues to be a topic of great interest. We have done some modeling work aimed at understanding in detail how the auditory system carries out this computation.

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