I am a scientist by nature, and I don't believe at all in God or any form of spirituality or the Holy Spirit (of course).
However, it does confuse me as to how some people genuinely do seem to be overcome by 'spirit' in certain more extreme churches.
http://www.youtube.com/watch?v=BuOBdGeArZw Watch this clip, and at about 3:35 the man goes into a strange state, and states like this do happen.
As well as this, my dad once began crying for no reason in a Church or Mosque or something at the altar.
Are there any scientifically/theoretical psychological explanations as to why this could be possible without the need for any sort of spirituality?
One of the common answers is that it's driven by social demands and depends on the cultural expectation and the suggestibility of the individual.
I remember an example Spanos used to like to bring up regarding demonic possession. Catholics and protestants had different symptoms for demonic possession and people who exhibited it as a phenomenon each had the symptoms they were supposed to have. The long history of hypnosis, hypno-therapy and a variety of social and suggestibility related phenomenon suggests that's one potential cause for what you witness.
More recent theorizing talks about social emotional constructs such as social joy. The act of being with a group of people can make someone happy when all participating together and there is a strong feeling of shared belief. I think there's something to that and the general idea of spirituality that either comes from social contexts or strong isolation (isolation impacts an individual partly because of their reliance on the social). There is clearly some emotion that we can get that we attribute causally to God or a spirit that we do not fully understand. Of course, we've always attributed to God or a spirit that which we do not fully understand. Until we can better understand that and how it's important to the well being of humans, atheism will perpetually have an uphill battle.
I remember seeing something that mentioned that for people who actually believe that something spiritual is happening to them their actual brain chemistry is changing.
Heightened emotions emit serotonin, dopamine, etc, and that could have an effect on people with a combination of other chemicals. Any time that our emotions are heightened we experience physiological and sometimes psychological changes. To explain why people see strange things or angels and such, those are just hallucinations. These people are not mentally ill, but the way you behave and perceive the world depends on your brain chemistry and brain chemistry is malleable given certain circumstances, so over time, someone could go from a Christian to a pagan and feel and see totally different things.
For an example I want to mention near death experiences. Near death experiences are typically explained by some scientists as auditory, visual, and sensory hallucinations, some scientists think that the brain is still active to a degree after clinical death. Of course this is still being researched and debated but it would explain near death experiences and why they are all similar. Near death experiences are influenced by culture. Culture is a learned trait and for most people this culture is the same, especially when isolated. That could also be the reason why some people experience the same things as they do rituals, prey, etc. It is shared belief and it creates memories, among other things, as our brains are wired to fit these beliefs.
a 2009 study by Inzlicht et al. found that "religious conviction is marked by reduced reactivity in the
anterior cingulate cortex (ACC), a cortical system that is
involved in the experience of anxiety and is important for
self-regulation [...] These correlations remained strong even after we controlled for personality and cognitive ability." And the paper goes as far as having a "xanax of the people" heading for one its sections, and obvious pun on the better known phrase of Marx. The authors also point out that a similar anxiolytic effect was observed in studies of other strong beliefs, but not necessarily religious, e.g. ideological/political, citing Amodio et al. 2007 and Kay et al. 2008. (And starting to digress here, but the Amodio study has elicited strong reactions in the press).