Olfactory hallucinations coupled with feelings of deja vu occur in "uncinate seizures", a form of temporal lobe epilepsy, and sometimes there is a generalised intensification of smell. The uncus, phylogenetically part of the "smell-brain" (or rhinencephalon), is functionally associated with the whole limbic system (which includes such brain areas as the amygdala, hippocampus, pyriforn cortex and hypothalamus - the cooloured bits in the figure above), which is increasingly recognised to be crucial in determining and regulating the entire emotional 'tone'. Excitation of this, by whatever means, produces heightened emotionalism and an intensification of the senses.
Preliminary work has demonstrated that smell can be used to reduce the occurrence of seizures in epilepsy. One possible explanation is that because olfactory centres (primary olfactory cortex, entorhinal cortex) are next door to regions where seizures begin in temporal lobe epilepsy, activity generated in these areas by the presentation of a smell prevents the spread of the synchronous activity from the epileptic focus.
posted at http://www.cf.ac.uk/biosi/staffinfo/jacob/teaching/sensory/olfact1.html#Memory