How the vestibular system modulates tactile perception in normal subjects: a behavioural and physiological study

Caloric vestibular stimulation (CVS) is a physiological technique demonstrated to transiently improve hemianaesthesia in right brain-damaged patients (Bottini et al. in Exp Brain Res 99(1):164-169, 1994, Nature 376:778-781, 1995, Neurology 65(8):1278-1283, 2005). Recent studies suggest that these effects are based on the anatomical overlapping between vestibular and tactile projections (Bottini et al. in Nature 376:778-781, 1995) in the human brain. However, much less is known about behavioural effects of this manipulation on normal subjects. We aimed to explore tactile perception during left ear CVS in normal subjects. We administered seventeen right-handed normal subjects with different types of tactile stimuli (above and below threshold) during left ear CVS. To further ensure standardized procedure, tactile stimulation was delivered through a tool-developed ad hoc for the experiment. The experiment was divided in 3 conditions: (1) Baseline, (2) PostCVS and (3) Delayed CVS. We found a main effect of stimulus type (F ((2,32)) = 907.712; P = 0.000) and condition (F ((2,32)) = 55.505; P = 0.000). Moreover, post hoc comparisons revealed that below threshold stimuli are most affected by CVS (t ((16)) = -11.213; P = 0.000). Left ear CVS modulates tactile perception also in normal subjects. Moreover, this modulation seems to be selective for below threshold stimuli and not caused by attentive processes. A multisensory phenomenon is possibly the best explanation for this interaction between touch and vestibular systems, corroborated also by the anatomical evidence and by the previous knowledge about interaction with the environment.