Stick insects can usually be seen doing one of two unexciting things:standing still or walking. During walking, stepping movements are generally considered to be governed by one to several control centres associated with the joints that make up each leg. In order to walk in a coordinated fashion,the activity of these control centres must be synchronised, so that all the joints in one leg are doing the right thing at the right time to produce a`step.' This is not a simple task because stick insects have to walk in many ways, including backwards and even sideways to cross uneven terrain or avoid obstacles. It would be very complicated to have a different set of rules for each walking direction, yet this is what most previous research has proposed. Setting out to find a sensory signal that stick insects could use to synchronise the activity of their leg-control centres, U. Bässler and colleagues tested the effect that vibration might have on the insect's locomotion control. Although they don't feel vibrations while they walk, so vibration isn't a signal that the insects use in their daily comings and goings, the team have discovered that vibration signals applied by the experimenter are capable of reversing the direction of movement in all leg joints, no matter what the joints are doing. This is an optimistic result;having found a single coordinating stimulus that works in principle (if not in reality), there may be other stimuli that the insect uses in normal behaviour.
To determine how the control of walking behaviour is affected by vibrations, Bässler and colleagues monitored the responses of the nerve cells that control the muscles of the leg joints by using recording electrodes, whilst stimulating the knee's vibration-sensitive organ. This was done in several ingenious ways: either by attaching the organ in restrained animals to a wiggling pole, or by looping it up on a vibrating hook through a hole in the knee of freely moving animals, or by manually vibrating one leg in animals walking on a treadmill.
The results showed that in moving animals, if the knee joint was in the process of extending, vibration would cause the leg to flex, and vice versa. Similarly at other joints, if the leg was being raised, vibration would cause it to be lowered and vice versa. For walking animals this means that vibration makes it more likely that the leg will be lifted off the ground in a stepping movement.
Since stepping behaviour is determined by leg-control centres, these findings show that vibration signals are capable of directing the activity of leg-control centres in a coordinated way. Because the reversal behaviour is also bidirectional, vibration could work as a synchronising signal for any walking direction.
Although vibration is not a sensory signal that the insect uses in practice, the team have shown that it is possible for a sensory signal to synchronise appropriate movements in more than one joint whilst still being independent of walking direction. Until now, this was not thought to be possible. Perhaps this finding will lead to the discovery of the real coordinating signal, and point the future of locomotor control in a new direction.
