Good question about the vascular constrictions, but it does. MSG was an assured migraine for me, I only got the visual type, a scintillating scotoma, and very seldom a headache. From what I've been told and read, vasoconstriction causes the visual migraines, and dilatation causes the headaches. Apparently mine would constrict but not over dilate. Also, I lost my vision twice in one eye, just briefly, had all kinds of tests, but they all came out negative. I was angry at the time, apparently I underwent strong vasoconstriction in my eye and out it went. Also had a Balisar Migraine once, wouldn't recommend one to anyone, it's also attributed to vasoconstriction. I received many medical tests after that as well, CT, CT with Contrast, and an MRI with contrast.
We should always have a doctors exam after anything weird like brief loss of vision or the profound effects of a Basilar Migraine.
Here's a interesting discussion of the smooth muscules that are part of Arteries and Veins that makes these types of constrictions possible.