Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, PC257

Poster Communications

Interpreting exafferent vestibular information during active head movement

R. F. Reynolds1, C. J. Osler1

1. School of Sport & Exercise Sciences, University of Birminhgam, Birmingham, United Kingdom.

  • Figure 1. Representative data depicting the relationship between head pitch and turn velocity. Trials where head pitch was static (red) are compared to those where active pitch movements were made (black=0.025; blue=0.05Hz; turquoise=0.1Hz). Positive values of Yaw Velocity represent movement in the anodal direction. Continuous lines show the ongoing time-series trace for movement trials. Circles show time-binned data for these time-series. Linear regression lines were fitted to the data to estimate response gain. It can be seen that the magnitude of turn velocity reduces as movement frequency increases. (Note: a shallower slope signifies greater gain).

  • Figure 2. Mean response gain is shown for all frequencies of head movement (+/-SEM). Values were derived from the slope of the regression lines shown in figure 1.

To maintain balance and orientation the brain must discriminate head motion caused by voluntary movement from that due to external disturbances. Animal data shows that this discrimination process occurs at the earliest stage of vestibular processing where brainstem neurons are uniquely responsive to exafferent information, cancelling out reafference(1). However, indirect evidence from humans suggests that active head movement interferes with the ability to maintain balance(2), raising the possibility that the separation of vestibular exafference from reafference is an imperfect process. To address this we measured the ability of human subjects to transduce exafferent vestibular information into motor responses while performing active head movement. Ten volunteers (6 male, 19-24yrs) were exposed to Galvanic Vestibular Stimulation (GVS; 1.5mA) for 60s while marching on the spot at 80bpm. Subjects attempted to maintain orientation within the laboratory while visual and acoustic cues were abolished by a blindfold and white-noise headphones. GVS is known to evoke turning responses in such situations, with the rate of turn depending upon the pitch orientation of the head. We therefore asked subjects to adopt various head orientations between +/-45 degrees pitch. In a static condition, head orientation was maintained throughout the trial. Then we examined the effect of voluntary movement upon the GVS response by asking people to produce head movements in time with a sinusoidally-modulated acoustic tone. To determine if the rate of head movement had any influence, three frequencies of modulation were employed: 0.025, 0.05 and 0.1Hz. Head pitch and whole-body turning velocity were measured using motion tracking sensors placed on the head and trunk, respectively(Polhemus Fastrak). During the static condition GVS evoked turning responses in all subjects. In accordance with previous research(3), the rate of turn was maximal with the head pitched down, minimal with head level, and reversed direction with the head up (Fig 1; F4,36=10.51; p<0.001). During active head movement turning also occurred. The magnitude and direction of turn velocity was continuously modulated throughout the duration of each trial. However, response gain was systematically reduced with increasing frequency of head movement (Fig 2; F2,18=10.46; p=0.001). These results show that humans can transform exafferent vestibular input into motor output even during active head movement. This suggests that the brain successfully discriminates between exafferent and reafferent vestibular input. The reduced response gain with increasing movement frequency may be due to sluggish motor responses. However, it could be due to a breakdown in the discrimination process as reafferent signals become stronger. Future experiments will address this directly by comparing GVS responses during active and passive head pitch movement.

Where applicable, experiments conform with Society ethical requirements