Download Odor- and context dependent modulation of mitral cell

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
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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).