Visual and vestibular contributions to prelanding EMG during jump-downs in cats.

Prelanding EMG responses in elbow flexors and extensors were assessed during landing from jump-downs (0.6, 0.8, and 1.0 m) in normal blindfolded cats and labyrinthectomized cats with and without vision occluded. Jump-down conditions determined the strategy of response elicited in the normal cat. When the height could be anticipated by the blindfolded animals, a response typical of a jump in the presence of visual cues occurred; extensor activity began an average of 73 +/- 12 ms before landing, while flexor activity was minimal. When the animal was 'tricked' by an unexpected change in jump height, it displayed a pattern of EMG activity appropriate for the jump height just previously experienced, not for the actual height. If the jump height was uncertain, the cat commonly exhibited continuous extensor activity that began soon (100-150 ms) after both forepaws left the platform. In the presence of visual cues, labyrinthectomized cats were able to execute jump-downs at 0.6 m. Onset of extensor EMG activity was normal during the first postoperative jumps, although the typical two-burst pattern was absent, and average prelanding extensor IEMG was less than that of the control jumps. In addition, landing was usually awkward, as the forelimbs collapsed and the ventral surface of the trunk contacted the landing pad. During subsequent sessions, the two-burst pattern reappeared, average prelanding extensor IEMG increased, and flight position improved, so that landing occurred without the forelimbs collapsing. Without visual cues, the labyrinthectomized cats were unable to execute a jump-down. These results suggest that visual input may normally regulate timing of the extensor prelanding motor program; however, without visual input, prelanding responses can be preprogrammed. When visual input is present, vestibular input during the flight phase may be primarily manifested as gain in extensor activity rather than in the temporal sequencing. When visual input is absent and jump height is uncertain, vestibular input may become more influential in determining the pattern of prelanding activity.