In the olfactory bulb, respiration gates windows of activation of mitral/tufted cells at a frequency (theta) similar to the respiration rate. These cells are thought to encode odor concentration through spike timing on a faster (gamma) timescale nested within the theta local field potential. This proposal is comprised of two Aims that seek to support the hypothesis that mitral cell theta phase precession encodes odor concentration.
The first Aim employs an optetrode implanted in the mitral cell layer of the olfactory bulb of mice in an odor concentration-driven go no-go task to test for respiration and theta phase coupling with gamma power in the olfactory bulb. Using the same preparation with variable optogenetic stimulus strength (accomplished with theta- and respiration-phase-specific photostimulation of olfactory sensory neurons), the second Aim will test the hypothesis that olfactory sensory neuron theta-activation phase changes perceived stimulus strength. Thus, these experiments seek to disambiguate the previously conflated low frequency oscillations of theta and respiration to determine the rhythm by which stimulus strength is encoded.
Odor concentration is a fundamental characteristic of smell, yet how the brain processes it remains a mystery. Understanding this mechanism may lead to clinical tools for the early diagnosis of neurodegenerative diseases in which a loss of smell precedes cognitive decline.