Distinct olfactory cross-modal effects on the human motor system.
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.
Rossi
Simone
S
Dipartimento di Neuroscienze, Sezione Neurologia, University of Siena, Siena, Italy. Rossisimo@unisi.it
De Capua
Alberto
A
Pasqualetti
Patrizio
P
Ulivelli
Monica
M
Fadiga
Luciano
L
Falzarano
Vincenzo
V
Bartalini
Sabina
S
Passero
Stefano
S
Nuti
Daniele
D
Rossini
Paolo M
PM
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