Hand movement illusions show changes in sensory reliance and preservation of multisensory integration with age for kinaesthesia.

To perceive self-hand movements, the central nervous system (CNS) relies on multiple sensory inputs mainly derived from vision, touch, and muscle proprioception. However, how and to what extent the CNS relies on these sensory systems to build kinesthetic percepts as the systems decline with age remain poorly understood. Illusory sensations of right hand rotation were induced by separately stimulating these three sensory modalities at two intensity levels. A mechanical vibrator applied to the pollicis longus muscle, a textured disk for touching, and a visual pattern rotating under the participant's hand were used to activate muscle proprioception, touch, and vision, respectively. The perceptual responses of 19 healthy elderly adults (60-88 yrs) were compared to those of 12 younger adults (19-40 yrs). In the younger group, the three types of stimulation elicited similar kinesthetic illusions at each intensity level applied. The same visual and tactile stimuli elicited more salient and faster illusions in older adults than in younger adults. In contrast, the vibration-induced illusions were significantly fewer, less salient and delayed in the older adults. For the three modalities considered, increasing the intensity of stimulation resulted in smaller increases in illusion velocity in older adults than in younger adults. Lastly, a similar improvement in the perceptual responses was observed in older and younger adults when several stimulations were combined and older participants reported more salient illusions than younger participants only in the visuo-tactile condition. This study suggests that reliance on sensory inputs for kinesthetic purposes is profoundly reshaped with aging. The elderly may rely more on visual and tactile afferents for perceiving self-hand movements than younger adults likely due to relatively greater muscle proprioception degradation. In addition, multisensory integration seems preserved but not enhanced to compensate for the global decline of all sensory systems with age.

Optimal visuotactile integration for velocity discrimination of self-hand movements.

Illusory hand movements can be elicited by a textured disk or a visual pattern rotating under one's hand, while proprioceptive inputs convey immobility information (Blanchard C, Roll R, Roll JP, Kavounoudias A. PLoS One 8: e62475, 2013). Here, we investigated whether visuotactile integration can optimize velocity discrimination of illusory hand movements in line with Bayesian predictions. We induced illusory movements in 15 volunteers by visual and/or tactile stimulation delivered at six angular velocities. Participants had to compare hand illusion velocities with a 5°/s hand reference movement in an alternative forced choice paradigm. Results showed that the discrimination threshold decreased in the visuotactile condition compared with unimodal (visual or tactile) conditions, reflecting better bimodal discrimination. The perceptual strength (gain) of the illusions also increased: the stimulation required to give rise to a 5°/s illusory movement was slower in the visuotactile condition compared with each of the two unimodal conditions. The maximum likelihood estimation model satisfactorily predicted the improved discrimination threshold but not the increase in gain. When we added a zero-centered prior, reflecting immobility information, the Bayesian model did actually predict the gain increase but systematically overestimated it. Interestingly, the predicted gains better fit the visuotactile performances when a proprioceptive noise was generated by covibrating antagonist wrist muscles. These findings show that kinesthetic information of visual and tactile origins is optimally integrated to improve velocity discrimination of self-hand movements. However, a Bayesian model alone could not fully describe the illusory phenomenon pointing to the crucial importance of the omnipresent muscle proprioceptive cues with respect to other sensory cues for kinesthesia.

Passive or simulated displacement of one arm (but not its mirror reflection) modulates the involuntary motor behavior of the other arm.

Recent studies of both healthy and patient populations have cast doubt on the mirror paradigm's beneficial effect on motor behavior. Indeed, the voluntary arm displacement that accompanies reflection in the mirror may be the determining factor in terms of the motor behavior of the contralateral arm. The objective of the present study was to assess the respective effects of mirror reflection and arm displacement (whether real or simulated) on involuntary motor behavior of the contralateral arm following sustained, isometric contraction (Kohnstamm phenomenon). Our results revealed that (i) passive displacement of one arm (displacement of the left arm via a motorized manipulandum moving at 4°/s) influenced the velocity of the Kohnstamm phenomenon (forearm flexion occurring shortly after the cessation of muscle contraction) in the contralateral arm and (ii) mirror vision had no effect. Indeed, the velocity of the Kohnstamm phenomenon tended to be adjusted to match the velocity of the passive displacement of the other arm. In a second experiment, arm displacement was simulated by vibrating the triceps at 25, 50 or 75 Hz. Results showed that the velocity of the Kohnstamm phenomenon in one arm increased with the vibration frequency applied to the other arm. Our results revealed the occurrence of bimanual coupling because involuntary displacement of one arm was regulated by muscle-related information generated by the actual or simulated displacement of the other arm. In line with the literature data on voluntary motor behavior, our study failed to evidence an additional impact of mirror vision on involuntary motor behavior.