Goal-dependent modulation of reflex responses during movement

Open Access
Mutha, Pratik K
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
December 12, 2008
Committee Members:
  • Robert L Sainburg, Dissertation Advisor
  • Robert L Sainburg, Committee Chair
  • Stephen Jacob Piazza, Committee Member
  • Jinger Gottschall, Committee Member
  • Byron C Jones, Committee Member
  • Movement
  • Reflex
  • Modulation
  • Humans
  • Sensory information
The role that feedback mechanisms such as reflexes play in the control of movement has been a topic of extensive research for several years. The study of reflexes in humans has focused on the idea that these circuits serve as a mechanism for regulating the mechanical properties of the limb while performing tasks in various dynamical environments. Consistent with this idea, task-dependent modulation of reflex responses has been documented over multiple studies. The general interpretation of these findings has been that modulation of reflex responses is one way in which the nervous system alters limb impedance that is presented to various loads that might act on it. However, most of these studies have examined conditions in which the maintenance of a stable posture is emphasized, perturbations used to elicit reflexes are predictable and the goal of the task remains fixed. Such predictability is rarely a feature of everyday movements, and whether and how reflex responses can be modulated during movement under unpredictable task conditions remains poorly understood. Moreover, whether reflex modulation is limited to occur within a limb, or whether reflexes can be flexibly organized across limbs remains an open question. In a series of three experiments, we investigate whether modulation of upper-limb reflex responses can occur during the course of movement. Our results show that reflex responses can be significantly tuned during movement. This modulation occurs in response to changes in task goals and is sensitive to the direction but not the amplitude of the change in goal. Moreover, reflex modulation can occur not just within a limb, but also across limbs when achievement of the task goal requires cooperation between them. Importantly, the modulation is such that reflex responses are enhanced when they assist in achievement of the task goal, but reduced when they resist goal achievement. Thus reflex circuits appear to be tuned in a goal-optimal manner. These results significantly enhance our understanding of the modulation of reflex responses and demonstrate the immense flexibility available within the central nervous system even as a movement is being performed.