Chapter 9 - Circuit Oscillations in Odor Perception and Memory
Introduction
The mammalian olfactory system produces an alphabet of rhythmic oscillations of the local field potential (LFP) and electrocorticogram/electroencephalogram (EEG) that track behavioral and cognitive processes associated with its many structures and modes. Cortical rhythms produce a spectrum of frequencies known as a 1/f power spectrum. In this type of spectrum, each successively higher frequency produces less power (amplitude) than the adjacent lower frequencies. This means that the signal itself will appear the same no matter what timescale is used, which is known as self-similarity. The olfactory bulb (OB) power spectrum contains significant and persistent deviations from this power falloff, showing elevated frequency band peaks where signals are more periodic than the irregular 1/f background activity (Fig. 1A). Some of these oscillatory bands have been very well studied, others have only recently been described, but all are still objects of active investigation.
Soon after the first frequency descriptions of the EEG by Beck, Neminsky, Cybulski, Berger, and Jasper (1891–1938; reviewed in Ahmed and Cash, 2013) came Adrian’s papers describing oscillations in the mammalian OB (Adrian, 1942, Adrian, 1950). These fast oscillations were evoked in anesthetized hedgehogs, cats, and rabbits to many different odors. Around the same time were reported similar fast oscillations of the LFP in the human OB (Dodge et al., 1956). By the end of the 1950s, this effect had also been reported in waking cats and was then followed by many papers that have stood as foundational works on olfactory oscillations (Freeman, 1974a, Freeman, 1974b, Freeman, 1975, Rall and Shepherd, 1968).
Section snippets
The LFP in the Mammalian Olfactory System
The LFP represents the sum over a spatial average of many local neurons. Depending on the cortical area and size and impedance of the recording electrode, the number can vary from on the order of 100 to 10,000 neurons. The types of neuronal activity that contribute to this signal vary and must be studied separately, because each cortical area contains different neuronal circuits with different properties. In the mammalian OB, the main source of the LFP comes from granule cells, because of their
Oscillatory Bands in the Olfactory System
Three major bands have been identified as having functional significance for olfactory processing in rats, mice, cats, and rabbits (frequency bands are for rats and mice): the theta or respiratory band (2–12 Hz), the gamma band (40–100 Hz), and the beta band (18–30 Hz). These bands are associated with specific types of behavioral events and have known relationships to each other. Theta oscillations track and represent respiratory behavior in the OB. Gamma oscillations are associated with local
Summary and Conclusion
Neural population rhythms in mammalian olfactory systems occur in discrete frequency bands that are driven by many circuits within the OB and other brain regions. Theta and beta rhythms represent processes at different ends of the cognitive spectrum, with theta primarily associated with sensory drive or respiration and beta associated with learning and experience. However, theta rhythms can be linked with learning and performance in some circumstances, and beta rhythms are linked with
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