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The Corona Pollentia is the part of the brain that adapts to and controls the supernatural powers that parahumans develop and display.
The Corona Pollentia is the part of the brain that adapts to and controls the powers that come from individual EntityShards. With the appearance of this formation, the subject becomes able to trigger.
Corona Pollentia is a startlingly common feature even in unpowered people.
The Gemma is the part of the Corona Pollentia that controls the active use of a parahumans' abilities and powers, the same way there are parts of the brain that allow regular humans to coordinate and move their hands. Artificial Gemmas were known to give powers that were maladapted to stressful engagements.
The size, shape, and location of the Corona and the Gemma changes from parahuman to parahuman. Usually, it starts as 1-1.6 inch-sized body settled between the frontal and the parietal lobes.
During the trigger event, Corona gets a spurt of development. It is a partial reason for people blacking out. Corona expands slightly, veins swell. Although it is still hard to distinguish a small but active Corona from bigger but inactive.
Young triggers have more pronounced Corona Pollentia with a noticeable effect called 'dimpling', as if it is pressed down into the brain, and, sometimes, expanded webbing, enveloping the brain.
It is possible to get an impression of power expression from how Corona Pollentia is integrated with the brain. E.g. strong cerebellum connection indicates Thinker-type perception powers, and Masters would get a stronger emphasis on the frontal lobe.
It is because of the size and location of the Corona Pollentia and Gemma that prevents people from removing them surgically as, most of the time, trial and error does not work.
Cauldron vials trigger powers regardless of whether or not the subject has a Corona Pollentia. In those that already have one, there is a greater chance of deviant powers. In those that do not have a Corona Pollentia, one develops as they trigger.
The size and shape of the Corona Pollentia is also stated to depend on the powers of the parahuman with it and how their powers work.
This is how Cauldron is able to remove powers - Contessa can (via her power) damage the Corona Pollentia in such a way that the target loses access to their powers.
↑If a shard can give any power classification, depending on the trigger, Wildbow:For the record, some shards can, but not all. Some shards are almost always going to just trigger Master, for example, and you just won't trigger unless you're in a state of mind that lends itself into a frequency match for the scan-manton snap (as Bonesaw describes in 11.h). - Alternative powers.Wildbow, Reddit.com, 2017-06-17
↑ 2.02.1“Corona Pollentia, [...] They take different forms. There aren’t hard and fast rules. But I’ve heard that if you trigger young, it takes more of a hold, with more… it’s called dimpling. Like a hand is actually pressing it down into the brain. That right there looks like dimpling. [...] You get more cloudiness in some kinds of scans where it expands out into the webbing around the brain, but we’re not getting that. It’s been way too long since I studied this,” I said. “The corona starts as a single marker, like a quarter-sized knot in wood, or a ball the size of a golf ball, pushed between the two lobes. Then when we trigger, it surges into life. It’s part of the reason we black out. It expands slightly, veins swell. But most of the time an unactivated corona is hard to tell apart from an activated one, and a surprising number of people have unactivated ones. Sometimes you look at them and the larger veins or structures suggest what the power is linked to.”
“What does this tell us?” Chris asked.
“She triggered young. It’s deeper set. People who had a corona for a long time don’t get dimpling, I’m pretty sure. Looking at the veins, where it seems to have reached out… [...]
Cerebellum is senses, seeing, hearing, coordination of movement.”
“That makes me think of Mama Mathers,” Sveta said.
“If thinkers had an emphasis, I think it leans more toward frontal lobe. This could be a perception power, I guess.” - Excerpt from Torch 7.6
↑The deviations, the ones who didn’t take to the formula, tended to fall into certain categories. There were those who had some minor physical or mental changes; they were little different from the most extreme deviations that appeared in typical trigger cases. Such deviations occurred a mere eighth of a percent of the time. They weren’t what he was thinking of.
The formula wasn’t exact. Though they learned more every day, there were still unknowns regarding powers. Whatever connection the agents formed with individuals before or during a trigger event, it didn’t manifest as strongly through the formula. When the subject was stressed, their body engaged by that distress, the connection grew weaker. - Excerpt from Interlude 21.x