Proximate mechanisms of control of locomotion in horses: corrective input in central pattern generators

On a proximate level, rhythmic motor patterns are controlled by central pattern generators (CPGs), neural networks that can endogenously (i.e. without sensory or central input) produce rhythmic patterned outputs. With the corrective input of sensory feedback, these motor patterns can be altered to deal with environmental information. We tested the extent to which horses show evidence of underlying CPG-activity and corrective central input when dealing with visual information on obstacles. Four different tests were performed in an arena on 40 sport horses. 1) When horses were halted after having stepped with the front legs over a short pole, and the pole was subsequently visibly pulled from under the horse, we saw that in 89% of the tests, they lifted the first moved hind foot, futilely. This illustrates that a CPG had started and unfolded till the end, without correction by central information on the sudden absence of the obstacle. A minority of the horses did not lift a hind foot, showing the effect of corrective central input. 2) When horses were halted with a short pole just in front of the front legs, with reduced vision by eyepatches to block their vision of the pole, we saw that in the majority of the tests they correctly lifted the front legs after 10 seconds (73%), 20 seconds (68%) and 30 seconds (74%). There was no difference in lifting between these time lengths (p=0.329). The centrally stored information that can regulate the start of a new CPG can span a time interval that is longer than the reputedly 8 seconds short term memory. 3) When horses were halted, after stepping over a short pole with the front legs, with reduced vision to block sight on the pole under their belly, we found that the horses correctly lifted the back legs after 10 seconds (71%), 20 seconds (76%) and 30 seconds (77%) and this time effect was not significant (p=0.356). The CPG that was started by the front legs easily lasted at least 30 seconds. 4) In another test-type, the horses were halted and a wooden pole was slowly but visibly shoved under the belly. The horse had to lift the hind leg in order to avoid the pole. In 85% of the tests, the horses did not lift the hind leg and touched the pole. Possibly, corrective input is more difficult when the motion starts with the hind legs. Simple tests can show how sensory information can modulate centrally generated endogenous patterns in horses. We also looked at the modulating effect of age, individual and sporttype. This information on the basic proximate regulation of locomotory behaviour can help to understand biological limitations of voluntary control of horse locomotion. Riders need to realize that horses may differ from humans in their ability to modulate every leg movement.status: publishe