Simultaneous EEG Recordings from Olfactory and Limbic Brain Structures: Limbic Markers during Olfactory Perception
We approach the question of how the brain may expect a known olfactory stimulus. EEG data were recorded from rats simultaneously from the olfactory bulb (OB), prepiriform cortex (PPC), entorhinal cortex (EC), and dentate gyms (DG) of the hippocampus. The animals were trained in a classical paradigm to identify two odors.
The problem of nonstationarity in analyzing the EEG time series was compounded by the brief duration and only loosely time-locked perceptual event. We used several statistical and dynamical methods to show the relationship of activity among the four structures during odor identification. Tools used were correlation distributions and coherence in different behavioral time segments and a new technique to measure the system’s transit in and out of putative chaotic attracted states.
Results show three significant features of the system, which occur reliably during odor identification and unreliably or insignificantly in control periods. We show gamma burst (40–160 Hz) coherence among all four studied structures during odor identification. We have previously identified this burst period as a perceptually defined event, so this ties the hippocampus strongly to olfactory perception. The second feature is the apparent centrifugal transmission of a proposed biasing signal in the beta band (15–35 Hz) from the entorhinal cortex to the olfactory bulb just before the onset of the gamma burst in the olfactory bulb. We have coined the term “preafference” to identify this as an expectation signal, after a generalization of the model of reafference presented by von Holst and Mittelstaedt in 1950. The third feature is a gamma burst apparently initiated in the prepyriform cortex, which is then passed back to the olfactory bulb just after the primary bulb burst. We label this the reafferent or “handshaking” signal, which could serve to adjust the dynamics of the system as it diverges from what was expected before the stimulus arrival.
KeywordsOlfactory Bulb Dentate Gyrus Entorhinal Cortex Theta Rhythm Gamma Band
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