Why we need to better understand the cortical neurophysiology of impaired postural responses with age, disease, or injury

INTRODUCTION AND PURPOSE The ability to maintain standing balance and orientation is crucial to mobility and independence. Standing balance is generally maintained by anticipatory postural adjustments associated with voluntary actions. However, inaccurate judgment or impaired anticipatory processes, as well as extrinsic postural perturbations– external forces not generated intrinsically due to voluntary movement (e.g., due to a slip, trip, push, etc.)–can render the need for reactive postural control to recover orientation and balance (Fasano et al., 2012). For people with disorders of posture and balance associated with aging, neurodegeneration, or injury, an appropriately timed and coordinated postural response to extrinsically perturbed balancemay represent the crucial difference between a harmless balance recovery and an aggravated pain condition or injurious fall. Indeed, postural responses to slips and sudden changes in load are common circumstances for incurring an episode of low back pain (LBP), a worldwide leading cause of disability (Manning et al., 1984; Andersson, 1999). In addition, falls associated with aging or neurological disorders are leading causes of injury, decreased activity participation, morbidity, andmortality (Grimbergen et al., 2004; Finlayson and Peterson, 2010; Batchelor et al., 2012). For these reasons research must detail the neurophysiology responsible for producing both healthy and impaired human postural responses to an extrinsically induced perturbation of standing balance. A recently proposed neurophysiologic model of extrinsically induced postural responses reviewed initial evidence that the cerebral cortex influences these postural responses by (a) priming the most contextually accurate response during preparation, and (b) modifying late response phases (Jacobs and Horak, 2007). The involvement of the cerebral cortex during early response phases thus appears indirect and limited to priming sub-cortically generated synergies based on contextual features known prior to the perturbation, but the cortex can then directly participate in modifying the late response phases to improve response efficacy. Cortical functions associated with priming contextually appropriate responses are thus represented through pre-perturbation measures of cortical activity such as pre-movement potentials, and cortical functions associated with online modifications to late-phase responses are represented by measures of cortical activity following perturbation onset, such as perturbation evoked potentials (PEPs) (Adkin et al., 2008; Jacobs et al., 2008; Mochizuki et al., 2008, 2010). Further research remains needed, however, due to a paucity of data on neural mechanisms of human postural responses with disease or injury. Understanding cortical function associated with impaired postural responses is essential given (a) the important role of the cerebral cortex for generating postural responses, (b) the potential that its influence may be enhanced to compensate for impaired automated processes of sub-cortical control, and (c) the accessibility of cortex for neuroplastic change with intervention. Unfortunately, because postural responses to extrinsic perturbations were historically considered reflexive and indicative of sub-cortical processing, the literature regarding cortical influence on extrinsically induced postural responses is not well developed. Whereas a more longstanding and developed literature exists on the role of the cerebral cortex for generating anticipatory postural adjustments with voluntary movement—both in the use of many methods of neurophysiologic recording and in the evaluation of people with health conditions (Gurfinkel and El’ner, 1988; Massion, 1992; Saitou et al., 1996; MacKinnon et al., 2007; Tsao et al., 2008; Jacobs et al., 2009a, 2010; Ng et al., 2012; Lomond et al., 2013; Papegaaij et al., 2014), the literature on postural responses to extrinsically induced perturbations is less extensive. Therefore, the purpose of this opinion article is to focus on the cortical neurophysiology of impaired human postural responses to extrinsic perturbations of upright stance. This article will highlight the insights provided from rare studies of cortical function in people with impaired standing postural responses in order to demonstrate the need and potential value of future research focused on the cortical neurophysiology of impaired human postural responses to extrinsic perturbations.