If it truly is a settled question that MSG causes headaches in humans, when the MSG is ingested (not injected) in amounts humans normally eat, then it should be easy to find studies in reputable journals to this effect. Do you have a link to such studies?
The injection/ingestion distinction is important, because when one ingests something, the entire quantity is not instantly dumped in to the bloodstream as it is when injected. The food MSG is ingested with might also have an effect on its rate of absorption and its effects. It might also undergo some changes in the digestive tract before it gets in to the blood stream.
Because this particular researcher is looking for very specific effects, that she's trying to measure accurately. She's shaved an area of the rat's skulls down to a thin window through which she's able to observe blood flow changes.
> Do you have a link to such studies?
This paper has a bunch of these types of references; for that alone it's useful. See section 2.4.
By the way, injection would be a good way to double-blind an MSG experiment with humans properly: you can't taste an injection. Nobody can tell whether they are being injected saline or MSG.
And nobody is claiming that the effects of MSG are not dose-dependent; that there is no consumption threshold below which sufferers feel no ill effects. Those who believe that any amount of MSG, no matter how miniscule, can cause them discomfort are deranged. If it were so, they would get these effects from all sorts of common foods that naturally contain glutamate.
The injection/ingestion distinction is important, because when one ingests something, the entire quantity is not instantly dumped in to the bloodstream as it is when injected. The food MSG is ingested with might also have an effect on its rate of absorption and its effects. It might also undergo some changes in the digestive tract before it gets in to the blood stream.