To what extent does the awarness of the presence of others affect brain function and cognitive state? This scope pertains to non-life threatening interactions and the awareness of the presence of a single or multiple individuals - (not direct conversation, not group mentality) just the presence.

To what extent does the presence of others affect brain function, emotion and cognitive state?

What are the differences within the host that cause certain individuals to evoke stronger cognitive changes vs. those that evoke weaker changes? 

What are the changes in state when one is completely sure nobody is around?

Are there any studies that measure this or show what a reasonable standard deviation would be for this phenomenon?

In addition, what are the factors that allow the host to completely block any changes in cognition regardless of the presence of others? (i.e. - higher serotonin, larger frontal cortex, etc?)
I recently came across an article (1) examining the effect of an observer on cognitive task performance. The meta-analysis showed an overall negative effect associated with the presence of others leading to a poorer cognitive performance. 

I am not aware of an article investigating such an observer effect using brain imaging techniques. It didn't stop researchers (2) to speculate on a cognitive/neuropsychological model of social inhibition. They stated that "the presence of others places high demands on the frontal and executive systems, leaving less available processing capacity to dedicate to performing tasks that require executive and frontal processing. The presence of others may, in turn, facilitate performance on tasks that rely less on frontal and executive processing". 

I leave them the final sentence of this post:

"there is clearly potential for more collaboration between social psychologists
and those working in cognitive psychology and neuroscience"

(1) Eastvold et al. (2012). Does a Third Party Observer Affect Neuropsychological Test Performance? It Depends. The Clinical Neuropsychologist, 26(3).

(2) Wagstaff et al. (2008). Some cognitive and neuropsychological aspects of social inhibition and facilitation. European Journal of Cognitive Psychology, 20(4).