[carbocation]

"The study group was additionally treated with a series of 15 daily visits to a cryogenic chamber"

So, the controls should also be treated with a series of 15 daily visits to a chamber that is simply the same temperature as its surroundings.

Having 15 daily visits worth of contact with study personnel, in a chamber that exists for a specific purpose, may have a big impact, regardless of temperature. But you won't know unless you use a proper control.

[heyjudy]

Yeah, superficially-cold by say contacting metal at room-temperature.

[carbocation]

That would add another variable and would prevent you from knowing whether the effect is due to temperature (say, a threshold effect) or the other things. I would run this control at room temperature.