|yes, it does make a good pokemon character|
Before I start explaining, let it be known that I am not saying the brain proper doesn't contribute to the 'food-smells-better-when-you're-hungry' phenomenon, in fact I would be very surprised if it didn't involve modulation of the ventral tegmental area, nucleus accumbens, and hypothalamus.
Mousley et al. (2006) use a technique called electro-olfactogram (EOG) to record the signals from smell receptors. When the cells are excited by an odor, the size of the response can be recorded. They are using this technique in Axolotls, but it can be used in humans too:
|EOG recording in humans (source)|
So what did they find? The found that this peptide (NPY) could modulate the size of the EOG response in hungry axolotls. They applied the same amount of odor molecule and the same amount of NPY for each recording, so the increase in response is not due to more odor molecules or more NPY being present. They suggest that it might be due to mory NPY receptors on the smell cells themselves, indicating that when hungry, the smell cells change in these animals.
|Mousley et al., 2006 Fig4|
So what does that mean? It means that when the animal is hungry, the brain proper has the ability to change the excitability of the smell receptors by dropping some NPY on them (through the terminal nerve).
This study showed the one specific peptide had an effect, but the principle that the brain can actually change the way the peripheral receptors sense things really struck me. I had always thought that the receptors were pretty much stable, and pretty much always sent the same signal to the brain, but that the way the brain interpreted that signal could be different. It fundamentally changed my view of sensory cells to learn that the smell receptors don't always send the same signals to the brain.
However, now I give these wine tasters more credit. Perhaps the untrustworthy smell cells were actually altered by the brain's expectation. I haven't seen any study testing this idea with EOGs on humans, but I think it would make a great experiment.
Mousley A, Polese G, Marks NJ, & Eisthen HL (2006). Terminal nerve-derived neuropeptide y modulates physiological responses in the olfactory epithelium of hungry axolotls (Ambystoma mexicanum). The Journal of neuroscience : the official journal of the Society for Neuroscience, 26 (29), 7707-17 PMID: 16855098