Feeling Stiffness in the Back: A Protective Perceptual Inference in Chronic Back Pain
SOURCE: Sci Rep. 2017 (Aug 29); 7 (1): 9681
Tasha R. Stanton, G. Lorimer Moseley,
Arnold Y. L. Wong & Gregory N. Kawchuk
The Sansom Institute for Health Research,
School of Health Sciences & Pain Adelaide Consortium,
The University of South Australia,
Adelaide, SA, Australia.
Does feeling back stiffness actually reflect having a stiff back? This research interrogates the long-held question of what informs our subjective experiences of bodily state. We propose a new hypothesis: feelings of back stiffness are a protective perceptual construct, rather than reflecting biomechanical properties of the back.
This has far-reaching implications for treatment of pain/stiffness but also for our understanding of bodily feelings. Over three experiments, we challenge the prevailing view by showing that feeling stiff does not relate to objective spinal measures of stiffness and objective back stiffness does not differ between those who report feeling stiff and those who do not. Rather, those who report feeling stiff exhibit self-protective responses: they significantly overestimate force applied to their spine, yet are better at detecting changes in this force than those who do not report feeling stiff.
This perceptual error can be manipulated: providing auditory input in synchrony to forces applied to the spine modulates prediction accuracy in both groups, without altering actual stiffness, demonstrating that feeling stiff is a multisensory perceptual inference consistent with protection. Together, this presents a compelling argument against the prevailing view that feeling stiff is an isomorphic marker of the biomechanical characteristics of the back.
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Introduction
Bodily feelings constitute a fundamental aspect of self-awareness and provide critical homeostatic functions – e.g., feeling cold makes one seek warmth [1]; feeling pain makes one seek protection [2]; feeling parched makes one drink. [3] We assume that these bodily feelings reflect the biological state of our body tissues – a ‘read-out’, so to speak, of somatosensory and visceral input – particularly when the feeling is located somewhere in the body, as it is for pain or stiffness. There is growing evidence for pain however, that it is highly modulated by a wide range of cognitive and contextual variables. [4, 5] For example, visually manipulating the perceived size of one’s hand alters the pain experienced in experimental contexts [6] and during movement of a chronically painful limb [7], and illuminating a blue or red light in synchrony with delivering a noxious cold stimulus can transform the feeling evoked from uncomfortably cold to painfully hot. [5]
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That somatic input triggered by physiological responses is important in cognitive processes – so-called ‘embodied cognition’ – has recently been countered with the idea that bodily awareness (e.g., our sense of bodily ownership) might directly modulate physiological regulation of body tissue in an anatomically specific way. [8, 9] Indeed, we now know that the sense of bodily ownership, the felt location, and the anthropometric characteristics of our body parts, are tightly linked to their physiological regulation in a bi-directional manner. [7, 10] Extensive human data from both healthy and diseased populations have led to the proposal of the cortical body matrix theory [11], the predictions of which are yielding new developments in our understanding and treatment of some pathological pain conditions. [12]
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