title: Distinct olfactory cross-modal effects on the human motor system.
abstract: BACKGROUND
  Converging evidence indicates that action observation and action-related sounds activate cross-modally the human motor system. Since olfaction, the most ancestral sense, may have behavioural consequences on human activities, we causally investigated by transcranial magnetic stimulation (TMS) whether food odour could additionally facilitate the human motor system during the observation of grasping objects with alimentary valence, and the degree of specificity of these effects.
  
  
  RESULTS
  In a repeated-measure block design, carried out on 24 healthy individuals participating to three different experiments, we show that sniffing alimentary odorants immediately increases the motor potentials evoked in hand muscles by TMS of the motor cortex. This effect was odorant-specific and was absent when subjects were presented with odorants including a potentially noxious trigeminal component. The smell-induced corticospinal facilitation of hand muscles during observation of grasping was an additive effect which superimposed to that induced by the mere observation of grasping actions for food or non-food objects. The odour-induced motor facilitation took place only in case of congruence between the sniffed odour and the observed grasped food, and specifically involved the muscle acting as prime mover for hand/fingers shaping in the observed action.
  
  
  CONCLUSIONS
  Complex olfactory cross-modal effects on the human corticospinal system are physiologically demonstrable. They are odorant-specific and, depending on the experimental context, muscle- and action-specific as well. This finding implies potential new diagnostic and rehabilitative applications.
authors.affiliation: Dipartimento di Neuroscienze, Sezione Neurologia, University of Siena, Siena, Italy. Rossisimo@unisi.it
authors.forename: Simone
authors.initials: S
authors.lastname: Rossi
authors.forename: Alberto
authors.initials: A
authors.lastname: De Capua
authors.forename: Patrizio
authors.initials: P
authors.lastname: Pasqualetti
authors.forename: Monica
authors.initials: M
authors.lastname: Ulivelli
authors.forename: Luciano
authors.initials: L
authors.lastname: Fadiga
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Falzarano
authors.forename: Sabina
authors.initials: S
authors.lastname: Bartalini
authors.forename: Stefano
authors.initials: S
authors.lastname: Passero
authors.forename: Daniele
authors.initials: D
authors.lastname: Nuti
authors.forename: Paolo M
authors.initials: PM
authors.lastname: Rossini
mesh_terms: D000328:Adult; D019054:Evoked Potentials, Motor; D005502:Food; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D011597:Psychomotor Performance; D012903:Smell / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation