When reaching for an object we must compensate for any body movement that may take the arm off-target. For example, reaching for a handrail when standing on a bus that suddenly accelerates requires detection and compensation for body movement relative to the handrail. A key sensory system which can detect body motion is the vestibular system. The vestibular apparatus is located in the inner ear, and provides signals of head rotation (head movement about axes of rotation, e.g. nodding or shaking head) and translation (head movement that occurs as a result of body movement) from which we can derive a sense of whole-body movement relative to our surroundings. It is well known that vestibular sensory input plays an important role in balance control when standing. We wanted to find out if this vestibular input is also used to maintain reaching accuracy during unexpected body movement.
To investigate vestibular control of reaching we used a technique called Galvanic vestibular stimulation (GVS). GVS involves passing a small electrical current (typically less than 5 mA) between electrodes behind each ear. This artificially stimulates the vestibular nerve without actual body movement, and evokes a sensation of illusory whole-body movement (for a review of GVS see Fitzpatrick & Day, 2004). When standing or walking, GVS evokes compensatory responses, involving changes in orientation and sway, respectively (see ‘remote controlled’ man video). Previous research has also shown changes in arm trajectory evoked by GVS when reaching (Bresciani et al., 2002). We examined these responses in further detail to determine their function. In particular, we wanted to know if the responses are intended to maintain reaching accuracy by compensating for body motion. For this to be true, the mechanism should only operate when reaching in an earth-fixed reference frame (e.g. picking up a cup from a table), as this requires compensation for body movement relative to the target. In contrast, moving the arm in a body-fixed reference frame (e.g. scratching your nose) does not require any compensation for whole-body movement.
We rotated participants using a rotating chair to generate unexpected body movement. With their eyes closed, the participants were completely reliant on vestibular signals to detect their movement. We also applied GVS during chair rotation to alter perception of body movement, such that participants falsely perceived they had moved further or less than they actually had, depending on stimulation polarity (Day & Fitzpatrick, 2005). During rotation, participants were asked to control their arm trajectory either in an earth-fixed or body-fixed reference frame. This was achieved by asking them to reach either for memorized targets situated on the laboratory wall (earth-fixed) or targets situated on their body (body-fixed).
WHAT DID WE FIND?
When reaching for an earth-fixed target during body movement, the participants made arm movements to compensate for chair rotation, thus maintaining reach accuracy. When GVS was applied, they compensated more or less depending on the polarity of the GVS stimulus, even though the actual body movement was always the same. In contrast, when reaching in a body-centred reference frame, there was no effect of GVS on the arm movement. Our findings demonstrate that vestibular-evoked changes in arm movement only occur when reaching in an earth-fixed reference frame. Only under these circumstances is there any need to compensate for body motion. We conclude that vestibular input plays an important role in maintaining reaching accuracy during unexpected body movement.
SIGNIFICANCE AND IMPLICATIONS
We have shown that vestibular input plays an important role in maintaining reaching accuracy during body movement. As we get older, vestibular detection of body movement is impaired. Based on our findings, this age related vestibular dysfunction could impair our ability to reach for a handrail during an unexpected body motion, a factor that may contribute to a higher incidence of falls in the elderly.
Smith CP, Reynolds RF (2016). Vestibular feedback maintains reaching accuracy during body movement. J Physiol. DOI: 10.1113/JP273125
Bresciani JP, Blouin J, Popov K, Bourdin C, Sarlegna F, Vercher JL, Gauthier GM (2002). Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets. Neurosci Lett 318, 34-38.
Day BL, Fitzpatrick RC (2005). Virtual head rotation reveals a process of route reconstruction from human vestibular signals. J Physiol 567, 591–597.
Fitzpatrick RC, Day BL (2004). Probing the human vestibular system with galvanic stimulation. J Appl Physiol 96, 2301-2316.
AUTHORS OF BLOG POST
Craig Smith and Raymond Reynolds