This happens when the present circumstances are powerfully suggestive of an immediate event.

The effect is all the more powerful, moreover, in those cases where the object or event expected is interesting or exciting, since here the mental image gains in vividness through the emotional excitement attending it.

Thus, if I am watching a train off and know from all the signs that it is just about to start, I easily delude myself into the conviction that it has begun to start, when it is really still.[53] An intense degree of expectation may, in such cases, produce something indistinguishable from an actual sensation.

This effect is seen in such common experiences as that the sight of food makes the mouth of a hungry man water; that the appearance of a surgical instrument produces a nascent sensation of pain; and that a threatening movement, giving a vivid anticipation of tickling, begets a feeling which closely approximates to the result of actual tickling.
One or two very striking instances of such imagined sensations are given by Dr.Carpenter.[54] Here is one.

An officer who superintended the exhuming of a coffin rendered necessary through a suspicion of crime, declared that he already experienced the odour of decomposition, though it was afterwards found that the coffin was empty.[55] It is, of course, often difficult to say, in such cases as these, how far elements of actual sensation co-operate in the production of the illusions.