From Rachel Herz’s quite interesting The Scent of Desire:
In one study that contrasted the trauma of being blinded or becoming anosmic [losing you sense of smell] after an accident, it was found that those who were blinded initially felt much more traumatized by their loss than those who had lost their sense of smell. But follow-up analyses on the emotional health of these patients one year later showed that the anosmics were faring much more poorly than the blind. The emotional health of anosmic patients typically continues to deteriorate with passing time, in some cases requiring hospitalization.
I cite some of Herz’s work in my own book, including her charming experiment “A Naturalistic Study of Autobiographical Memories Evoked By Olfactory and Visual Cues: Testing the Proustian Hypothesis”. As you can probably guess, the French novelist was right: there is something uniquely sentimental about the buttery whiff of a seashell-shaped cookie (or any other odor). This is because smell and taste are the only senses that connect directly to the hippocampus, the center of the brain’s long-term memory. Their mark is primal and indelible. All our other senses (sight, touch and hearing) are first processed by the thalamus and only then sent along to the hippocampus. As a result, these senses are much less efficient at summoning up our past.