Animals control contact with surfaces when locomoting, catching prey, etc. This requires sensorily guiding the rate of closure of gaps between effectors such as the hands, feet or jaws and destinations such as a ball, the ground and a prey. Control is generally rapid, reliable and robust, even with small nervous systems: the sensorimotor processes are therefore probably rather simple. We tested a hypothesis, based on general τ theory, that closing two gaps simultaneously, as required in many actions, might be achieved simply by keeping the taus of the gaps coupled in constant ratio. τ of a changing gap is defined as the time-to-closure of the gap at the current closure-rate. General τ theory shows that τ of a gap could, in principle, be directly sensed without needing to sense either the gap size or its rate of closure. In our experiment, subjects moved an effector (computer cursor) to a destination zone indicated on the computer monitor, to stop in the zone just as a moving target cursor reached it. The results indicated the subjects achieved the task by keeping τ of the gap between effector and target coupled to τ of the gap between the effector and the destination zone. Evidence of τ-coupling has also been found, for example, in bats guiding landing using echolocation. Thus, it appears that a sensorimotor process used by different species for coordinating the closure of two or more gaps between effectors and destinations entails constantly sensing the τs of the gaps and moving so as to keep the τs coupled in constant ratio.
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Acknowledgements Thanks are due to Tim Hely for help with the computer analyses. This research was supported by United States Public Health Service Grant PSMH48185, the US Department of Veterans Affairs, the American Legion Brain Sciences Chair and UK Joint Council Initiative grant G9212693 from the MRC, SERC and ESRC.