Periodically throughout recorded history, puzzling instances of psychiatric and neurologic symptoms have presented en mass<span>: outbursts of thrashing and screaming, or je rky spasms and abrupt vocal tics affecting a group of individuals at once and often attributed to causes like possession, witchcraft, and malingering. Such occurrences of so-called "mass hysteria" continue to confound the medical community, but growing experience has improved the understanding and approach to these seemingly contagious psychogenic events. </span>
This "italicized" part of the sentence is an adjectival dependent clause. It acts as an adjective to the word motorist and cannot stand alone without putting it with an independent sentence, such as "The motorist lost his way."
The prepositional phrase is "on a high shelf" since ON is a preposition.
