Even patients who were born without a sense of smell--a rare condition called congenital hyposmia--show activity in the areas of the brain responsible for smell, according to a study in the Journal of Computer Assisted Tomography (JCAT).

The unique study shows that these patients' brains respond even when they imagine odors they have never smelled, report Drs. Robert I. Henkin and Lucien M. Levy of George Washington University Medical Center.

The researchers used a technique called functional magnetic resonance imaging (fMRI)--which shows the brain areas activated in response to specific tasks-- to study nine patients with congenital hyposmia. Because of their condition, the patients (average age 33 years) had never been able to recognize odors.

Even so, the brains of patients with congenital hyposmia became activated in response to odors. The degree of activation was small--about 15 percent of that in subjects with a normal sense of smell. Activation was also lower than in patients who had lost their sense of smell as a result of disease (acquired hyposmia), even though the two groups were similar in their inability to recognize odors.

The regions of the brain activated by odors in patients with congenital hyposmia were similar to those in the other groups. However, patterns of activation varied, and responses did not "lateralize" normally--in normal subjects, pleasant odors tended to activate areas in the left side of the brain and unpleasant odors in the left side.

Brain activation occurred even when the patients were asked to imagine odors they had never smelled, such as ripe bananas or peppermint. Again, the degree of activation was much less than in normal subjects or patients with acquired hyposmia.

Congenital hyposmia, which has many possible causes, affects perhaps two-tenths of one percent (0.2%) of the population. The new fMRI study is the first to show activation of brain areas associated with smell in patients who have never been able to recognize odors. In this way, smell appears unique among the senses--for example, patients with congenital deafness show no brain activation in response to sound.

The sense of smell appears to be "hard-wired" into the central nervous system, Drs. Henkin and Levy conclude. However, from there it may be a case of "use it or lose it"--the brain areas responsible for smell remain underdeveloped if they're never called upon to function. The fact that brain activation occurs even when patients with congenital hyposmia "remember" odors they have never smelled suggests that smell may be evolutionarily more "primitive" than the other senses, the researchers suggest.

---Lippincott Williams & Wilkins

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