Cross-polarized reflected light measurement of fast optical responses associated with neural activation
Xin-Cheng Yao 1, Amanda Foust 2, David M Rector 2, Benjamin Barrowes 1 and John S George 1*
1 Los Alamos National Laboratory
2 Washington State University
* To whom correspondence should be addressed. E-mail: jsg{at}lanl.gov.
Submitted on September 7, 2004
Revised on December 12, 2004
Accepted on 10 March 2005
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Abstract |
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We developed a novel optical probe for cross-polarized reflected light measurements and investigated optical signals associated with electrophysiological activation in isolated lobster nerves. The cross-polarized baseline light intensity (structural signal) and the amplitude of the transient response to stimulation (functional signal) measured in reflected light mode were dependent on the orientation of the nerve axis relative to the polarization plane of the incident light. The maximum structural signal and functional response amplitude were observed at 45o, and the ratio of functional to structural signals was approximately constant across orientations. Functional responses could be measured in single trials in both transmitted and reflected light geometries and responses had similar waveforms. Both structural and functional signals were an order of magnitude smaller in reflected than in transmitted light measurements, but functional responses had similar signal-to-noise ratios. We propose a simple theoretical model based on geometrical optics that is consistent with key experimental results. In the model, the cross-polarized structural signal results from light reflection from cylindrical fibers and the transient functional response arises from axonal swelling associated with neural activation. Polarization-sensitive reflected light measurements could greatly enhance in vivo imaging of neural activation since cross-polarized responses are much larger than scattering signals now employed for dynamic functional neuroimaging.
Key Words:
Axon, Electrical stimulation, Nerve, Optical imaging
