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Odor- and context dependent modulation of mitral cell activity in behaving rats Leslie M. Kay & Gilles Laurent Presented by Alexa Hamilton The Olfactory Bulb Connected to everything else in the brain (according to Kay) Signals traveling to the olfactory bulb do not go through the thalamus Prone to disconnection in traumatic head injuries (sieve bone acts as guillotine) In rats, the olfactory bulb is very large, relatively much larger than in humans. OB contains mitral cells that communicate via action potentials. The Olfactory Bulb Question When assessing the function of neurons, does it make a difference whether the animal is anesthetized? Alternatives Yes No Logic Look for a difference in the firing of the mitral cells between two conditions with different contexts. If mitral cell firing is influenced by behavior and/or context, we conclude that yes, there is a difference in behaving vs. anesthetized animals. Otherwise no. 2 conditions: Odor identification Odor-driven behavior Methods Rats are trained to enter a cage and drink from a tube. Electrodes are implanted in the rats’ olfactory bulbs. Trained rats are put through a set of trials, varying odors and liquid. S: “odorless” trials, 10% sucrose solution P1: (“odor identification task”) two odors ,10% sucrose solution P2: (“odor-driven behavior task”) two odors, each consistently paired with either quinine (aversive) or 30% sucrose solution P3: (“odor identification task”) two odors, 10% sucrose solution P4: To ensure rats could not smell the solution, after all trials, animals were tested by randomly pairing quinine or sucrose solution with different odors. Methods - Behavioral Trial structure (every trial, regardless of P-phase): Light goes on Door opens Rat sniffs Rat chooses to drink the liquid or not. Methods - Physiological Results Results S{ N{ Results Results-summary During odor identification task (P1 & P3), 11% of cells were found to have odorselective differences. During odor-driven behavior (P2), 94% of cells were found to have odor-selective differences. It was also found that in P4, the rats performed at chance level. Interpretations Yes, there is a difference between behaving animals and anesthetized animals: Behaving animals are capable of fast sniffing and drinking. This suggests that the function of the mitral cells (what the spike train reflects/encodes) is dynamically dependent on reward history and behavioral context/task. There is a remarkable flexibility in the ability of mitral cells to change what they encode. This might reflect the fundamental nature of the OB, being hooked up to all kinds of other systems (emotional, sensory, behavioral, etc.), flexibly changing the task it’s engaged in/the computations it performs. “Problems” Overall, an extremely solid paper Performed the necessary controls Really shows that the importance of using behaving animals in order to make conclusions about behavior. Does the psychophysics and physiology in the same organism. Does histology. Actual Problems Overall, a very low number of neurons (52 single units). Typical minimum standards are around a 100. Reporting most of the results in terms of INDIVIDUAL neurons, not populations. There is a remarkable, unexplained variance in the response between and within mitral cells that remain poorly understood and might reflect anything, but this possibility is not discussed. Is it possible that the crucial difference between mitral cell firing in response to aversive vs. non-aversive odor in the P2 condition is due to motor activity (drinking vs. nondrinking)? (The authors deny that, but fail to provide evidence for this).