Functional Anatomy of Perceptual and Semantic Processing for Odors

Abstract

The functional anatomy of perceptual and semantic processings for odors was studied using positron emission tomography (PET). The first experiment was a pretest in which 71 normal subjects were asked to rate 185 odorants in terms of intensity, familiarity, hedonicity, and comestibility and to name the odorants. This pretest was necessary to select the most appropriate stimuli for the different cognitive tasks of the second experiment. The second one was a PET experiment in which 15 normal subjects were scanned using the water bolus method to measure regional cerebral blood flow (rCBF) during the performance in three conditions. In the first (perceptual) condition, subjects were asked to judge whether an odor was familiar or not. In the second (semantic) condition, subjects had to decide whether an odor corresponded to a comestible item or not. In the third (detection) condition, subjects had to judge whether the perceived stimulus was made of an odor or was just air. It was hypothetized that the three tasks were hierarchically organized from a superficial detection level to a deep semantic level. Odorants were presented with an air-flow olfactometer, which allowed the stimulations to be synchronized with breathing. Subtraction of activation images obtained between familiarity and control judgments revealed that familiarity judgments were mainly associated with the activity of the right orbito-frontal area, the subcallosal gyrus, the left inferior frontal gyrus, the left superior frontal gyrus, and the anterior cingulate (Brodmann's areas 11, 25, 47, 9, and 32, respectively). The comestibility minus familiarity comparison showed that comestibility judgments selectively activated the primary visual areas. In contrast, a decrease in rCBF was observed in these same visual areas for familiarity judgments and in the orbito-frontal area for comestibility judgments. These results suggest that orbito-frontal and visual regions interact in odor processing in a complementary way, depending on the task requirements.

Odour identification in frontotemporal lobar degeneration

“…While patient numbers in our study were small, odour deficits do not appear to be strongly correlated with other types of nonverbal knowledge (for example, face identification: Tables 1 and 2). Our findings await corroboration in larger prospective cohorts, however the present evidence supports current models of the organisation of human olfactory cortex emerging from functional imaging studies [7][8][9][10][11][12][13][14] which have implicated a bilateral network of areas including the antero-mesial temporal lobes. These regions are sites of heavy disease involvement in FTLD [2].…”

“…A preferential role for the right hemisphere in both the perceptual and semantic analysis of odours is supported by functional imaging evidence in normal subjects [9,12], and psychophysical evidence in normal subjects [28] and in patients with focal cerebral resections [15,17]. However, other evidence [7,10,11,13,18] argues against a simple serial transfer of olfactory information within or between hemispheres.…”

“…FTLD is both pathologically and clinically heterogeneous [2][3][4][5][6]. Diseases in the FTLD spectrum involve cortical areas that have been implicated in olfactory processing in functional imaging studies of healthy subjects [7][8][9][10][11][12][13][14] and in human lesion studies [15][16][17][18]. Deficits of olfactory processing have been described in a number of common neurodegenerative conditions, including idiopathic Parkinson's disease and Alzheimer's disease [7,8].…”

Left hippocampal volume loss in Alzheimer's disease is reflected in performance on odor identification: A structural MRI study

“…More recent PET findings have noted some hemispheric specialization based on the familiarity of odors. Royet et al (1999) demonstrated activation in the left inferior frontal lobe while subjects rated odor familiarity. Interestingly, unfamiliar odors have tended to evoke greater activation on the right cerebral hemisphere while familiar odors have tended to evoke bilateral activation (Savic & Berglund, 2000).…”

Brain activation when hearing one's own and others' names

“…In a PET study of the cognitive component of olfactory processing, left cuneus was activated when participants named the odors (Qureshy et al, 2000). The authors suggest that the task involved some imagery to ascertain the name of the odor and indicated that the visual cortex is involved in mental imagery tasks (see also Kosslyn, 1988;Royet et al, 1999). In the current study, the salience of one's own name may elicit activity in non-auditory brain regions.…”