The first time we witness Othello falling into a trance is in Act 4, scene 1. Othello is clearly overwhelmed and distraught by the insinuations Iago makes when he suggests that Desdemona might have slept with Cassio. Iago is not quite forthcoming and plays with Othello's emotions. Othello becomes more and more upset and finally blurts out:
"Lie with her! lie on her! We say lie on her, when
they belie her. Lie with her! that's fulsome.
confess, and be hanged for his labour;--first, to be
hanged, and then to confess.--I tremble at it.
Nature would not invest herself in such shadowing
passion without some instruction. It is not words
that shake me thus. Pish! Noses, ears, and lips.
--Is't possible?--Confess--handkerchief!--O devil!-"
It is clear that Othello has worked himself up to such an extent that he loses it completely and then faints. Iago has effectively manipulated him into believing that Desdemona and Cassio have been having an affair - an idea too shocking for Othello to contemplate.
Othello has previously shown that his emotional distress has a physical effect on him. In Act 3, scene 3, Iago commented that :"Ha! I like not that!" after he and Othello witnessed Cassio departing from Desdemona in a secretive manner. When Othello questions him, he intimates that Desdemona and Cassio might be having an affair. Othello is anguished and develops a headache.
It seems as if Othello suffers from some form of epilepsy which is activated during moments of deep emotional turmoil or stress. It is only when he is truly distraught that Othello falls into a trance.
Iago derives great pleasure from seeing Othello so vulnerable. He enjoys seeing that his deception and manipulation working and states:
My medicine, work! Thus credulous fools are caught; And many worthy and chaste dames even thus, All guiltless, meet reproach."
Othello, like Julius Caesar and Alexander the Great, suffered from epilepsy. His "trance" is not caused by spiritual influences but is a physical disease. However, up until the recent past, people who suffered from epilepsy were often thought to be possessed of some evil spirit. Because of the loss of control he experienced when in an epileptic seizure, Othello tries to hide his disease from others. He appears to others as being very self-controlled. Othello even says,"He that stirs next to carve for his own rage / Holds his soul light; he dies upon his motion" (II.iii.164-165). However, his epilepsy seizures render some of his actions uncontrollable and leave him in a vulnerable state. So, in addition to being a Moor and being considered an outsider, his disease contributes to his lack of self-esteem because he never knows when he will lose control of his behavior.
I think it was the brain condition that he has along side with Iago telling him lies about his wife, as he Iago say, work on my medicine, which was referring to all the lies he told him.