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Balancing Act


Moving effortlessly through the world and maintaining balance requires the use of specific areas in the brain that specialise in the processing of the information required for us to be able to do them.

By Mark Edwards and Michael Ibbotson

What can an earthquake simulator tell us about how our visual and vestibular systems communicate with each other to help us balance?

Moving effortlessly through the world and maintaining balance are two activities that we are exceptionally good at. While these tasks may appear simple, given that we can typically perform them with no apparent conscious effort, they are extremely complex and demanding and require the use of specific areas in the brain that specialise in the processing of the information required for us to be able to do them.

Vision clearly provides a rich source of information about our movement. As we move, the images reaching our eyes undergo complex patterns of motion called optic-flow patterns, and the shape of the pattern indicates the type of motion that we are making. For example, a radially-expanding pattern, so famously used in the Star Wars movies when a spacecraft jumped to light speed, indicates forward motion while a contracting pattern results in a sense of backwards motion.

Similarly, our vestibular system is clearly important in maintaining our “sixth” sense of balance. The vestibular system is part of the middle ear and consists of fluid-filled chambers. It responds to the inertial forces produced by changes in our speed and direction of movement.

Under conditions of normal postural control, we sway back and forth slightly, and this swaying produces weak optic-flow and vestibular signals. The contribution of both the optic-flow and vestibular systems in maintaining balance can be shown by two simple demonstrations.

For optic-flow, if you stand on one foot, maintaining balance is much harder with your eyes closed (no optic-flow information) than with your eyes open. To test the vestibular system, if you spin around in a circle you set the fluid in your vestibular chambers moving in that direction, and it continues to do so for a while after you have stopped spinning. This continued movement of the fluid results in false vestibular signals (telling you that you are rotating in the opposite direction to your original rotation), and hence it is difficult to maintain balance.

This involvement of both the optic-flow and vestibular systems is characteristic of most tasks that involve movement. While each system can, in isolation, tell us about the nature of our movement, we were interested in determining whether they actively interact with each other. That is, does activity in one of them, say the vestibular system, influence how well information is processed in the optic-flow system?

The possibility of such an interaction is suggested by a number of observations. For example, areas of the brain thought to be involved in the processing of optic-flow information also appear to receive input from the vestibular system. Additionally, anecdotal evidence suggests that when children are affected by motion sickness, closing their eyes (thus removing optic-flow input and hence removing any possibility of a vestibular/optic-flow interaction) reduces the intensity of the motion sickness.

Similarly, people at sea often feel sick when they are below deck. Under this condition they see the cabin and the objects inside it are stationary, so the optic-flow information is telling the brain that they are stationary. However, in reality, they are being moved around by the oscillations of the vessel on the waves. The vestibular system is sensitive to this self-motion so there is a mismatch in the information being provided by the two systems, with the optic-flow system telling them that they are not moving and the vestibular system telling them that we are.

A common way to overcome seasickness is to go up onto the upper deck and look out at the horizon. This results in the appropriate optic-flow signal on the eyes, and hence consistent information from the optic-flow and vestibular systems.

While these observations suggest a vestibular/optic-flow interaction, there has been, to date, no direct evidence for such an interaction. To investigate this issue, we determined the sensitivity of people to optic-flow patterns while also physically moving them in a manner that was either consistent or inconsistent with that optic-flow pattern. For example, we determined sensitivity to an expanding optic-flow pattern (indicating forward motion) while either moving the person forward, so the optic-flow and vestibular signals were consistent, or moving them backwards to create an inconsistent pairing. Our logic was that if vestibular signals do directly feed into the optic-flow system, then sensitivity to optic-flow patterns should be greatest when the two signals are consistent rather then when they are inconsistent. In the consistent case, the two signals could facilitate one another, hence leading to greater sensitivity.

We determined optic-flow sensitivity by using images consisting of a number of bright dots moving on a large screen. There were two types of dots: signal and noise. The signal dots moved in the correct direction for that motion pattern (e.g. radially away from the centre of the image) while noise dots moved in random directions. When all of the dots are signal dots (i.e. 100% signal intensity) the direction of motion can be easily seen. When none of the dots are signal dots (i.e. 0% signal intensity) then only random motion is perceived.

Sensitivity can be established by sequentially presenting people with two motion sequences, with one of the sequences always containing 0% signal intensity and the other a variable signal intensity. The temporal order of the different sequences varies from presentation to presentation and the task is to indicate which sequence, first or second, contained the motion signal.

The signal intensity is varied until the level at which the person can just make that judgement is established. That level is an indication of their sensitivity to the optic-flow stimulus. People are very sensitive to such stimuli, typically needing signal intensities of around only 10%.

To be able to move the person in a controlled manner while they were doing this task, we conducted the experiments in the earthquake simulator at Questacon, the National Science and Technology Centre in Canberra. This machine consists of a platform that can be moved to simulate the effects of an earthquake. We placed the experimental equipment and observer on that platform and moved them either consistently or inconsistently with the signal condition being presented.

We found that people were indeed more sensitive to optic-flow patterns when they were moved in a manner consistent with the pattern. That is, a lower number of signal dots was required to detect a radially-expanding pattern when the person was moved consistently (i.e. moved forward) rather than when they were moved inconsistently (i.e. moved backwards).

As far as we are aware, these results are the first to show a direct effect of vestibular-based stimulation on the sensitivity to optic-flow information. This facilitatory cross-talk between the two systems makes sense given their relative sensitivities to the different aspects of our movement.

While both systems are very sensitive to motion in depth, the optic-flow system is substantially insensitive to changes in speed, while the vestibular system is optimally tuned to detect changes in speed. Hence, combining the two inputs would enhance the ability of the brain to represent information about our movement. This facilitation would be of greatest use under conditions where both systems are being weakly driven, for example while trying to maintain our balance while standing.

Finally, this cross-talk between the vestibular and optic-flow systems may also be of importance in generating the feeling of motion sickness. An interesting possibility is that, if the strength of the cross-talk varies between people, a particular person’s susceptibility to motion sickness may be linked to their particular cross-talk strength. Given that motion sickness typically occurs when a person is experiencing conflicting optic-flow and vestibular signals, it is possible that the greater a person’s cross-talk strength, the greater will be their susceptibility to motion sickness.

Mark Edwards is an Associate Professor at the Australian National University’s Department of Psychology. Michael Ibbotson is a Professor in the ARC Centre of Excellence in Vision Science at the Australian National University.