A friend of mine had an experience on LSD recently where he started 'looping.' He was repeating the same series of meaningless actions over and over again, with no real idea why or how to stop.
Similarly, I've had experiences with shrooms where I fall into thought-loops, just going over the same distressing idea again and again.

Both of these sound a lot like symptoms usually associated with OCD, a disorder that we have known selective serotonin re-uptake inhibitors are reasonably effective at treating.
It makes me wonder if agonism of certain serotonin receptors can trigger thought processes similar to OCD.

If this is true, then what does this tell us about serotonin's role in creating behavior and extracting 'meaning' from actions and the environment?

Blessings
~ND