Figure 1.
The subjects were exposed to a counterclockwise rotation of a textured tactile disk under their right hands (upper right) and/or a counterclockwise rotation of a visual scene projected via a video projector (bottom right) and/or a vibratory stimulation applied to the tendons of the pollicis longus muscle of their right wrists. The subjects held a potentiometer in their left hands to copy any illusory sensation perceived in their right hands. Potentiometric traces and EMG signals from the right extensor carpi ulnaris and pollicis longus muscles were recorded.
Figure 2.
Typical recordings of clockwise angular deviations (°) copied by a participant and raw EMG activity (mV) in the extensor carpi ulnaris (ECU) and pollicis longus (PL) muscles during the three unimodal conditions.
A- Muscle proprioceptive vibration at 80 Hz. B- Counterclockwise tactile disk rotation at 40°/s. C- Counterclockwise visual scene rotation at 40°/s.
Figure 3.
The effects of 5 levels of stimulation intensity on the perceptual and motor responses elicited by isolated proprioceptive (left column), tactile (central column) or visual (right column) stimulation.
From top to bottom: Mean velocities (± S.D.) of the kinaesthetic illusions reproduced by the participants; Mean RMS (± S.D.) of EMG activity in the right extensor carpi ulnaris (ECU) and pollicis longus (PL) muscles;- Mean latencies (± S.D.) of the motor and perceptual responses; Mean gains (a.u.) of the perceptual responses (± S.D.), i.e., the ratio between the illusion velocity and the stimulation intensity.
Figure 4.
Comparison between the mean illusion velocities (± SEM) induced by unimodal, bimodal and trimodal stimulation applied at Low, Medium or High stimulation intensities.
The unimodal stimulation conditions are represented by empty geometric figures: yellow triangle for tactile (T), red diamond for muscle proprioception (P) and blue circle for visual (V) conditions. The bimodal conditions are represented by solid geometric squares: purple squares for proprio-tactile (TP), orange squares for proprio-visual (PV) and green squares for visuo-tactile (TV) co-stimulation. The trimodal conditions are represented by solid black squares (TPV). Statistical data are p-values obtained by two-way ANOVAs (post-hoc tests,*p<0.05, **p<0.01; NS: Not significant).
Figure 5.
Proportional enhancement of the illusion velocities induced by bi- and trimodal stimulation over the fastest unisensory illusion.
Histograms are the mean (± SEM) “multisensory index” MSI = [(multisensory velocity – highest unisensory velocity )/highest unisensory velocity ]×100.
Table 1.
Mean Latencies (ms) ±S.D. of kinaesthetic illusions induced by uni-, bi- and tri-modal stimulation applied at Low, Medium or High intensity level.