Greg Gdowski - Current Research
Human models of head stabilization: effects of inertia on reflexive movements
We are studying human subjects to reveal that these reflexes are active during linear translation and are sensitive to inertial changes.
How active are vestibular reflexes that control lateral flexion in humans and are they sensitive to inertia?
Several groups have studied head movements during whole body translation because of the putative relationship between such movements and whiplash injuries that occur during automobile accidents (Kumar et al. 2004; Kumar et al. 2000). Head movements during controlled translations of the body in the fore-aft and side-to side directions have also been investigated (Keshner 2003; 2004; Vibert et al. 2001). These studies prompt two main questions:
- How does the vestibular system contribute to the control of translation-evoked head movements?
- Do the vestibulocollic reflexes (VCR) produce or prevent head movement?
The answers to these questions are important because whether the VCR prevents or inadvertently exacerbates injury during whiplash remains unclear (Vibert et al. 2001).
If the function of these reflexes were to reduce head movement, is the headâs inertia important?
We conducted experiments with human subjects to determine how sensitive these reflexes were to changes in the headâs inertia (Brown et al. 2009a; b). We used an apparatus with a sled that was mounted between two rotational motors. This configuration allowed us to reposition the sled while preventing the subjectâs knowledge of the sledâs orientation. In complete darkness, step translational stimuli (0.25g, 20 cm/s) were produced in random directions while the subjects attended to trivia questions. Head movements were measured using a Vicon Motion System. Electromyographic activity was recorded from the splenius muscles during several stimulus repetitions. When the sled was moved directly leftward, the head counter-rotated primarily in roll and yaw directions and to a lesser extent in pitch. Without the EMG activity, we might have assumed that, during translation, the VCR produced counter-rotation of the head. This was not the case, because the left splenius muscle was activated during rightward head movements. This pattern of activity appeared to prevent head rotation that presumably results from the headâs inertia.
We tested this hypothesis by having subjects wear a football helmet that increased the headâs inertia by almost 50%. Although the head movements produced during translations were statistically indistinguishable from those without the helmet, the magnitude of the EMG activation of the left splenius muscle nearly doubled. The timing of EMG activity provided insight into how the vestibular pathways orchestrate this function. This was particularly important, because it became clear that it was the movement of the head relative to the trunk, rather than the stimulus itself that produced reflex activation of the neck muscles, even though both activated the vestibular system. This suggests that the vestibular system must be able to perform two critical tasks in controlling head movement reflexes during linear translation:
- To distinguish between sensory signals that arise as a consequence of head-on-trunk movements and those produced by whole body movements.
- To independently adjust these signals so that only those that are related to head-on-trunk movement are scaled to compensate for changes in inertia.