Visually induced involuntary arm, head, and torso movements.

Area of Science:

• Human motor control
• Perception and sensory processing
• Vestibular and visual-vestibular interactions

Background:

• Illusory self-motion, such as visually induced self-rotation and displacement (SR&D), can affect motor responses.
• The manual following response (MFR) is a known mechanism influencing passive arm control and target maintenance.
• Understanding how visual-vestibular information influences body posture and arm movements is crucial for explaining perceptual-motor phenomena.

Purpose of the Study:

• To investigate the effects of SR&D on the arm, head, and torso posture in standing subjects.
• To examine how actively pointing towards a target influences these deviations during SR&D.
• To relate the observed deviations to existing models of motor control, including the MFR and visual-vestibular convergence.

Main Methods:

• 12 standing subjects participated in 75-second trials involving visually induced self-rotation and displacement.
• Arm movements were assessed under 'Free' conditions and a 'Tool condition' where subjects held a rod.
• Arm deviations were measured when the arm was extended freely or pointing at a proprioceptively specified target.

Main Results:

• Subjects showed significant unintentional arm deviations (averaging 55°) in the direction opposite to illusory self-motion in Free conditions.
• Pointing at a target reduced these arm deviations by approximately 80%, indicating a corrective mechanism.
• Deviations in head and torso positions also occurred, preceding the subjective experience of SR&D, and were partially correctable by pointing.

Conclusions:

• The manual following response (MFR) mechanism appears to extend to passive arm control and target maintenance during SR&D.
• Visual-vestibular convergence in neural pathways likely underlies the observed body and arm deviations.
• Findings have implications for vehicle control in environments prone to inducing illusory self-motion and displacement.