[Olfaction and neurological diseases: a review of the literature]. / Olfaction et pathologies neurologiques: revue de la littérature.

Olfactory disorders are often misjudged and rarely rated in the clinical setting. They are nevertheless described in a wide range of neurological disorders, and their evaluation can be useful for diagnosis. Usually irreversible olfactory dysfunction is a well-known complication after head trauma. Severe changes in olfactory tests are observed in Parkinson's disease. Dysfunction is present at disease onset and evidenced with all behavioral tests. Regarding other parkinsonian syndromes, olfactory performances are severely impaired in Lewy body disease, less pronounced in multiple system atrophy and usually preserved in corticobasal degeneration. Olfactory deficits are an early feature in Alzheimer's disease and worsen with disease progression. Rarely reported by patients, they must be searched for with olfactory tests. Though epilepsy is mainly known for its olfactory hallucinatory disorders, alterations of olfactory abilities are also described, especially in mesial temporal epilepsy. Disorders of olfactory perception are finally reported in patients with multiple sclerosis and migraine. After a reminder of anatomical data on the olfactory system, and the different methods of testing used to rate olfactory performances, the current review focuses on the type of olfactory dysfunction and damaged brain areas of the olfactory system encountered in the main neurological diseases.

Emotional responses to pleasant and unpleasant olfactory, visual, and auditory stimuli: a positron emission tomography study.

Neural correlates of responses to emotionally valenced olfactory, visual, and auditory stimuli were examined using positron emission tomography. Twelve volunteers were scanned using the water bolus method. For each sensory modality, regional cerebral blood flow (rCBF) during presentation of both pleasant and unpleasant stimuli was compared with that measured during presentation of neutral stimuli. During the emotionally valenced conditions, subjects performed forced-choice pleasant and unpleasant judgments. During the neutral conditions, subjects were asked to select at random one of a two key-press buttons. All stimulations were synchronized with inspiration, using an airflow olfactometer, to present the same number of stimuli for each sensory modality. A no-stimulation control condition was also performed in which no stimulus was presented. For all three sensory modalities, emotionally valenced stimuli led to increased rCBF in the orbitofrontal cortex, the temporal pole, and the superior frontal gyrus, in the left hemisphere. Emotionally valenced olfactory and visual but not auditory stimuli produced additional rCBF increases in the hypothalamus and the subcallosal gyrus. Only emotionally valenced olfactory stimuli induced bilateral rCBF increases in the amygdala. These findings suggest that pleasant and unpleasant emotional judgments recruit the same core network in the left hemisphere, regardless of the sensory modality. This core network is activated in addition to a number of circuits that are specific to individual sensory modalities. Finally, the data suggest a superior potency of emotionally valenced olfactory over visual and auditory stimuli in activating the amygdala.