Hatchling Xenopus laevis embryos usually swim when the skin is touched with a fine hair. Less common are small, local V-flexions and more general C-flexions. Simple flexions or the initial flexion at the start of swimming occur predominantly on the opposite side to the stimulus to direct the animal away from the stimulus. Strokes to the midline lead to random sidedness of responses.
The reliability of the sidedness of flexions and the first flexions of swimming decreases the more rostrally the stimuli are given. The range of directions of swimming paths are larger with more rostral stimuli so responses to head stimuli are unpredictable in direction.
In animals immobilized in α-bungarotoxin, strokes to the skin produce electrically recorded motor output which corresponds to: V-flexions, C-flexions and swimming. Fictive activity generally starts on the side opposite to the stimulus. The fictive responses suggest that the three basic behaviour patterns observed can be generated entirely within the central nervous system without any sensory feedback.
We discuss possible mechanisms for the generation of ‘protean’ responses to head stimulation which are unpredictable in direction.