Olfactory Functioning in the Autistic Spectrum

Background: Starting with the first descriptions by Kanner and Asperger, atypical sensory processing has been recognized as an essential feature of the autistic spectrum. Considerable clinical and experimental evidence on atypical sensory processing and autistic strengths in low level perception has been gathered. The Enhanced Perceptual Functioning (EPF) model by Mottron and colleagues (2006) describes these phenomena by stating that higher-order processing which is mandatory in typically developing individuals is more optional in autistic individuals, while low-level perceptual processing, such as feature extraction, is intrinsically superior. The combination of these two principles results in enhanced detection, discrimination, and categorization of perceptual stimuli in the visual and auditory modalities and, to a lesser extent, in the tactile modality. Despite clinical indications pointing towards atypical chemosensory processing in autism, and the theoretical importance of investigating all sensory modalities, the chemosensory modalities have received little scientific interest. Conclusive data on olfactory functioning in autism are missing.

Objectives: We aimed to compare olfactory functioning in individuals with a diagnosis of autism or Asperger syndrome to typically developing control subjects. The investigated olfactory functions were odor detection, discrimination, and identification. Subjective ratings of perceived odor pleasantness, familiarity, and intensity, as well as self-reported chemical sensitivity, were collected to investigate differences in subjective odor perception.

Methods: Participants were 5 Asperger, 5 autistic, and 5 non-autistic adult males, matched on age (18-35 years) and Wechsler IQ. To measure odor detection thresholds we used an adaptive 3-alternative forced-choice ascending staircase method for 2 different odors: phenyl ethyl alcohol and n-butanol. Both thresholds were measured 3 times. Odor discrimination was investigated with a same-different paradigm, using odors selected according to their rated similarity. The University of Pennsylvania Smell Identification Test was used to assess the identification of 40 common odorants, embedded in scratch and sniff labels, using a 4-alternative forced-choice method. Participants rated the perceived pleasantness, intensity and familiarity of 8 different odorants on a visual analogue scale with verbal labels ranging from very unpleasant, weak or unfamiliar to very pleasant, strong or familiar. Participants rated their sensitivity to odorant substances in daily life on the 21 statements of the Chemical Sensitivity Scale.

Results: Preliminary results suggest impaired olfactory identification in autistic spectrum individuals compared to typically developing controls. There were no indications of group differences in detection and discrimination of odors.

Conclusions: Preliminary results do not show evidence of superior olfactory perceptual traces in the autistic spectrum. Typical performance on odor detection and identification suggests functional integrity of the medial temporal lobe structures implicated in olfactory processing, whereas diminished olfactory identification either is suggestive of functional atypicalities of olfactory orbito-frontal structures, or points to difficulties in labeling subjective information in the autistic spectrum.