It is a scientific fact, confirmed at thousands of Christmas parties every year, that drinking alcohol affects our balance. Observant drinkers will note that alcohol induces a sensation of rotation, in a direction that depends on the position of the head. If you drink heavy water instead of alcohol the spin will reverse direction.

The explanation of this spinning sensation highlights the exquisite sensitivity of the sensors and control circuits in the brain that enable us to feel stable as we totter soberly about our daily business. Three sources of information are used: accelerations of the head, detected by the vestibular organ in the inner ear; movements of the neck and body, signalled by the muscles and joints; and movements of the objects about us, signalled by the visual system. Alcohol exerts its effect on the vestibular organ.

Information from the different sources is rigorously cross-checked before it reaches consciousness, and before we take corrective action. Nodding or shaking our heads causes violent acceleration signals from the vestibular organ, but these are consistent with the signals from the neck, so we feel stable. We hardly notice the movement.

We also move our eyes so that head movements do not generate visual signals. We take for granted the speed and accuracy of the vestibulo-ocular reflex, which controls these eye movements.

Any inaccuracy in the VOR shows up as an instability in the visual world. A common example is the vertigo we experience when we change our spectacle prescription. Changing spectacles changes the magnification of the eye, and hence the size of the eye movement that perfectly compensates for a given head movement. As we get used to the spectacles we adjust the VOR and the world becomes stable again.

Because the input (head movement) and output (eye movement) of the VOR can be measured precisely it is one of the best understood control systems in the brain. But even now it is throwing up new surprises. One of the latest is that the compensation for the visual distortion produced by spectacles is learned and remembered. When we change spectacles we adjust the VOR instantly. We can even learn to change the size of the VOR depending on whether we look up or down to compensate for bifocals.

You can test your VOR by shaking your head rapidly from side to side while reading this. You can see clearly while you shake or nod your head, but not if you keep your head still and move the paper. When your head shakes, the VOR moves your eyes at exactly the right velocity to keep them pointing in the same direction in space. When you shake the paper, visually controlled corrective eye movements are too slow to compensate.

Head rotation is detected by a part of the vestibular organ called the semicircular canal, a loop of fluid-filled tubing embedded in the skull bone. The tube is blocked by a barrier called the cupula. When the head rotates in the plane of the canal the fluid tends to lag behind, pressing against the cupula. The pressure, detected by special neurones called hair cells, gives you the signal that your head is spinning. We have three semicircular canals on each side of the head. Each is positioned to signal rotation in a different direction.

The VOR responds to rotation with an oscillating pattern of eye-movements called nystagmus. The eye moves in the opposite direction to the rotation at exactly the same speed, and then flicks rapidly forwards. Nystagmus gives an exact indication of the state of the VOR.

The commonest clinical test of the VOR is ingenious, but crude. Syringing the ear with hot or cold water stimulates the semicircular canals by causing expansion or contraction of the fluid in them. The caloric nystagmus that results is easy to detect. We used to think that caloric nystagmus was caused by convection currents, but experiments on Skylab showed that this cannot be so. Caloric nystagmus occurs in space, where there is no gravity to produce convection currents

In contrast, although it has not been tested in space, the spinning sensation induced by alcohol depends on gravity. Otherwise its direction would not depend on the position of the head. Alcohol stimulates the semicircular canal because alcohol in the bloodstread makes the cupula lighter than the surrounding fluid, causing it to float. The resulting upward pressure on the cupula is identical to that caused by rotary acceleration of the head, and is interpreted by the brain as such.

Mixing two parts of heavy water with one part alcohol neutralises this effect. Unfortunately, the high price of heavy water and the effects of alcohol on the nervous system make this an unrealistic way of keeping a clear head.