Research Review By Dr. Demetry Assimakopoulos©

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Date Posted:

April 2016

Study Title:

Neural Representation and the Cortical Body Matrix: Implications for Sports Medicine and Future Directions

Authors:

Wallwork SB, Bellan V, Catley MJ & Moseley GL

Author's Affiliations:

Sansom Institute for Health Research and Pain, Adelaide University of South Australia, Australia; Neuroscience Research, Australia, Sydney, New South Wales, Australia.

Publication Information:

British Journal of Sports Medicine 2016; 50: 990-996. doi:10.1136/bjsports-2015-095356

Background Information:

Let’s face it – sports injuries happen. Sometimes, they’re even bad enough to significantly reduce movement and sport participation. Movement reduction can occur for a number of reasons, such as reduced articular range of motion (ROM) or muscle strength secondary to injury, external splinting/bracing, kinesiophobia, or simply because the patient was instructed not to move by their health care provider. Typically, the mainstay for recovery is physical rehabilitation. The rehabilitation process often focuses on progressive tissue loading, and terminates once the athlete believes they can withstand the loads imparted on the injured area during competition.

The neural mechanisms underpinning movement preparation, planning and execution are also very important to address during injury rehabilitation. Take for example, an NFL running back who suffers a terrible non-traumatic ACL injury while cutting during a game. The athlete undergoes a successful reconstruction, heals well post-operatively, and the knee is stable. However, you observe that when he returned to practice, he was hesitant to cut in a similar way because of pain and fear of re-injury. In scenarios such as this, it is important to help the athlete recover by improving their motor control and their ability to bear load, while also addressing kinesiophobia. Your treatment in these cases must truly be biopsychosocial. In this clinical commentary, the authors describe the neural mechanisms underpinning movement execution, and how to clinically audit them. They then propose an appropriate treatment to optimize recovery.

Summary:

The Complexities of Motor Performance

Skilled motor performance requires utilization and integration of multiple neural representations, including visual, proprioceptive, spatial and tactile areas. External sensory information is constantly integrated with our default sensory-motor system to create smooth, efficient and accurate movement. It is important to fine tune this system post-injury to restore the brain’s capacity to rapidly integrate sensory information in a constantly changing sensory-motor environment.

Neural Representations or ‘Neurotags’

Neurotags are large groups of brain cells that work together to evoke a specific output. Neurotags are thought to be distributed across multiple cortical and subcortical areas. Each neurotag consists of multiple member brain cells that are networked across multiple brain regions. Member brain cells can be part of multiple neurotags. WRITER’S ASIDE: Cells throughout the brain are interconnected structurally and functionally. For instance, brain areas in the frontal cortex (motor programming/function, problem solving) are networked with both the temporal lobes (auditory perception) and occipital lobes (visual perception). These multiple areas combine and collectively process incoming sensory information to make complex decisions. A pain neurotag might involve cells from the frontal, occipital and somatosensory cortices, and amygdala, among others. Because of the very similar neuroanatomy involved, the pain neurotag might overlap considerably with fear and/or depression neurotags. So when the member brain cells which create the pain neurotag collectively activate, they create the OUTPUT of pain. More on how this works below.

Neuronal mass, neuronal precision and neuroplasticity direct the operation of neurotags. Neuronal mass is defined as the number of member brain cells within a neurotag, and the efficiency of synapatic communication between them. Neuronal precision refers to the specificity of member brain cell activation, and inhibition of non-member brain cells. In simpler terms, neuronal precision considers the accuracy with which brain cells can activate and communicate with intended neighbouring cells. Communicating cells will also inhibit activation of uninvolved adjacent brain cells. The strength of any given neurotag depends on both neuronal mass and precision. Typically, larger neurotags will dominate over smaller ones, and more precise neurotags dominate imprecise ones.

Neurotags are subcategorized as either primary or secondary, depending on their effect. Primary neurotags affect an action at an end organ, leading to a tangible and noticeable outcome. For example, the primary neurotag for motor units can evoke a perception of pain, or a sensation of muscular weakness. On the other hand, secondary neurotags have an effect through modulating the neuronal mass and precision of primary neurotags, and thus influence the likelihood of primary neurotag activation. Sport specific skills can be conceptualized as activation of primary neurotags that are modulated and refined by secondary neurotag activation.

Proprioception is an excellent example of how this complex sensory inter-relationship works. Proprioceptive awareness, defined (generally) as ‘where’ you feel a body part in space, can be considered a primary neurotag output. Sensory information from the external environment such as vision and mechanoception can affect the primary proprioception neurotag. In this case, vision and mechanoception are secondary neurotags, which can modulate the primary proprioception neurotag. The nervous system additionally combines and prioritizes afferent input. For example, when visual and proprioceptive information are available at the same time, vision dominates the resultant output. Essentially, the brain ‘trusts’ or prioritizes vision more so than proprioception, and thus carries greater influence because of its greater relative neuronal mass and precision.

The third principle of neurotags is neuroplasticity, or the property of the nervous system to undergo structural and/or functional change in response to reinforcement and activity. The likelihood of neurotag activation can be altered by modulating neurotag strength and precision through reinforcement or activity. The less active a specific neurotag is, the weaker and less precise it becomes. On the other hand, the more active a neurotag, the stronger and more precise it becomes.

The Cortical Body Matrix (CBM)

The CBM is defined as a network of neurotags that perceptually and physiologically regulate, control and protect the body and the immediate space around it. Consider a study by Moseley et al. (1), where subjects with Chronic Regional Pain Syndrome (CRPS) looked at their affected hand through binoculars. When the hand was magnified, or perceived as larger, pain and swelling increased. When observed through the wrong end of the binoculars, and the limb appeared smaller, pain perception and swelling decreased. Perception is an important quality to pain. It is important to consider the CBM for a number of reasons:
  1. The CBM model integrates complex proprioceptive, motor and spatial representations that are involved in sport, particularly those requiring equipment, such as a ball or inter-athlete interaction.
  2. It specifies that the neurotags representing end organs (muscles, blood vessels, etc.) are closely integrated with neurotags representing sensation (warmth, pain etc.). This is why muscle pain or weakness is often combined with altered sensation, such as allodynea, parasthesiae or dysesthesiae.
  3. It also implies that external and internal situations are mapped spatially according to multiple frames of reference, such as around oneself and/or an external object such as a ball or an opponent’s hand/foot. Spatial tasks that include both frames of reference should be incorporated into sports rehabilitation.
Understanding neurotags and the CBM present three important implications for sports rehabilitation:
  1. Secondary neurotags that are activated by either real or implied danger can influence primary neurotag motor output.
  2. Integrating virtual rehabilitation with physical rehabilitation allows for neuroplastic change, while balancing protection and return to full performance.
  3. Neurotags related to body-related outputs and feeling/sensation-related outputs are bi-directionally linked. Clinicians can manipulate one to modulate the other.
The authors suggest that pain and motor control are outputs from their own primary neurotags. They also suggest that motor control and pain neurotags are intimately linked, and modulated by secondary neurotags. This argument is strengthened by the large body of evidence showing that nociception is neither sufficient, nor necessary for the perception of pain. Nociceptors are simply one contributor to the pain experience. They also suggest that a similar relationship exists for motor outputs, whereby nociception can contribute to the production of protective motor output.

The Effect of Injury & Inactivity on the Cortical Body Matrix

The effects of inactivity and injury can have dramatic effects on the CBM. Neuroplasticity and secondary neurotag activation can influence motor output and perception. Any information that implies danger to body tissue is represented by a secondary neurotag that can modulate the primary pain neurotag, making it more likely to fire. Protective outputs such as fatigue, anxiety, fear, dyspnea, stiffness and weakness can all modulate the primary pain or motor control neurotags. Similarly, up-regulated inflammatory responses, increased cortisol production and elevated heart rate are considered protective CBM responses. This implies that pain, emotion, immune and autonomic factors are functionally connected, neurophysiologically. WRITER’S ASIDE: Trauma and injury can be psychologically impactful. Let’s again take the example of the NFL running back who tore their ACL while cutting. They might have developed a “what if my ACL tears again” or kinesiophobia secondary neurotag. This secondary neurotag might modulate the neuronal cell mass and precision of the primary pain neurotag, thus making it activate more readily. This elegantly explains why fear and pain are persistently linked. This relationship can be extended to a variety of scenarios, such as a “my injury happened at work” neurotag or a “driving is bloody scary” neurotag. If these scenarios imply a perception of danger or further tissue damage, they can elicit a pain response to discontinue activity. The patient’s lifestyle, continued painful movements, and even your treatment may continually activate secondary neurotags that represent danger to body tissue.

On the other hand, any information that implies safety to body tissue is also represented by secondary neurotags, which make the pain neurotag less likely to fire. Therapies such as reconceptualization of pain and pain education are now considered central tenants of chronic pain rehabilitation, and should be involved in sports rehabilitation.

Assessments

Motor Assessments:
A deficit in one’s ability to perform imagined movements is related to pain. It has also been shown that motor imagery training can lead to improvement in pain ratings. Motor imagery can be either explicit or implicit. Imagining one’s own body or body parts moving is a form of explicit motor imagery. On the other hand, implicit motor imagery involves unwittingly activating motor neurotags, by having a patient determine whether a pictured body part belongs to the left or right side of the body. Such an assessment involves an initial ‘automatic’ judgement, followed by a confirmation process, whereby the subject mentally manoeuvres their own hand from its current position to the position of the pictured hand. This process engages secondary proprioceptive and spatial neurotags, which modulate the primary motor neurotag, but fail to activate it. The left/right differentiation ability is region specific. For example, those with low back pain perform badly on left-right trunk rotation tasks (2), but perform normally on hand judgement tasks. Similarly, those with hand CRPS perform poorly on left/right hand judgement tasks, but not on knee judgement tasks (3-5). Similar region specific deficits have been shown in patients with neck pain, knee osteoarthritis and leg pain. Left/right judgement tasks provide both accuracy of left/right judgement and reaction time, which are thought to reflect different neurotags. Accuracy deficits imply decreased neuronal mass or precision of proprioceptive neurons used for movement, while reaction time deficits imply similar disruptions in spatial neurotags (6, 7). Left/right discrimination can be assessed using commercially available software, such as the Recognize App.

Tactile Acuity:
Tactile neurotags are thought to represent a sensation of touch on the body. Impaired tactile performance has been implicated in pain disorders, and training tactile performance is associated with pain reduction (8, 9). Impaired tactile acuity is region specific. Such deficits are associated with decreased neuronal mass and precision of subserving neurotags, and have been documented in people with CRPS (10), LBP (11), facial pain (12) and arthritis (13).

The Potential Role of Neurotag Rehabilitation in Return to Sport

Typically, treatments which target secondary motor or tactile neurotags help to achieve clinical recovery in those with chronic pain. A similar approach can be used in sport rehabilitation. The spectrum of Graded Motor Imagery (GMI) can be used to reinstate normal motor neurotags. GMI is a 3 stage process, starting with implicit motor imagery (left/right discrimination using the Recognize App), explicit imagery (imagined movements) and finally, mirror therapy (4). The order is important in severely disabling painful conditions, such as CRPS. GMI should be performed during individually specific exposures to cues that signal danger to body tissue. Such cues include time of day, location, noise, competition, stress, fatigue, weather conditions, or other player/teams involved to name a few. GMI can also be done well in advance of training. The hope is that performance of GMI can modulate neurotag output on movement, immune responses and feelings by modification of neuronal mass and precision through neuroplasticity.

Another component of neurotag rehabilitation is tactile discrimination training. Such training involves forced choice between at least two different tactile stimuli, relying on somatosensory information to make the choice. The most common protocol is to stimulate one location on the patient’s body and ask them to identify which location was stimulated (12). The nature of the stimulus is not important, but the requirement to differentiate it from similar stimuli is (i.e. Is it sharp or dull, and where did I stimulate?). Still, evidence for this particular intervention is lacking, and remains conjecture.

Unfortunately, there is a paucity of information for these interventions for acute sporting injuries. However, they could logically be extended to sports rehabilitation from the chronic pain literature.

Clinical Application & Conclusions:

The authors of this paper offer a very sophisticated neuroplastic perspective of sports rehabilitation. They begin by discussing the nature of modern pain-related theories, including neurotags, neuronal mass, neuronal precision and neuroplasticity. They then discussed how Graded Motor Imagery (GMI) can modulate neurotags, which create pain, weakness, and altered proprioception. GMI can optimize sport rehabilitation, and prevent negative neuroplastic changes for developing post-injury.

The authors assert that altered motor output and other protective outputs can be modulated by the perception of danger. They also contend that incorporating neurotag assessment and retraining as a component of sport rehabilitation will limit the harmful effects of pain and inactivity on the brain.

The concept of neuroplasticity is gaining traction in the medical and manual therapy fields. We as clinicians typically describe threats of re-injury as loads and postures that can possibly provoke tissue damage. The authors argue that fear, setting and context can independently lead to chronicity and re-injury. Let’s take for example the NFL running back described above. If they were initially fearful of cutting and no intervention was done, they might injure themselves again. The hope is, by having the patient perform imagined movements in different contexts (such as at home, before the game, or the on the sideline), they might deactivate the fear and knee pain neurotag, and optimize their expression of movement. Remember, pain is in the brain! Nociception is in the periphery, and is neither needed nor required to create pain. Sometimes, changing perception is important!

Study Methods:

This was a clinical review. No statistical analysis or search strategy were discussed

Study Strengths / Weaknesses:

Strengths: The authors provided a structural framework for why to integrate these therapeutic methods into practice. They also thoroughly discuss how to properly assess these central neuronal mechanisms, and provide methods of treatment.

Weaknesses:
  • The authors did not discuss how Graded Motor Imagery (GMI) can be integrated into a typical sports rehabilitation setting. Manual therapy, soft tissue therapy and typical rehabilitation strategies are still valid and have been shown to work. Using this rehabilitation strategy does not preclude clinicians from prescribing traditional methods. Sometimes, patients need to rest. Other times, they need to be mobilized. Injured tissues should still be progressively and optimally loaded. GMI should ideally be used in addition to these more traditional therapies. For instance, a manual therapy/rehabilitation session can start with left/right discrimination, and progress to manual therapy and stress loading. A patient can perform motor imagery multiple times at home to facilitate brain changes. Mirror therapy could also be used post-therapy to decrease the possible threat associated with manual therapy. The combinations are endless. But traditional therapies should not be summarily abandoned.
  • The authors also did not discuss dosage. Should these exercises be done multiple times per day, or only once daily? This probably depends on the patient to a large degree. Likely, to stimulate neuroplastic change, the patient should perform these tasks as many times as possible through the day, without symptom provocation.

Additional References:

  1. Moseley GL, Parsons TJ, Spence C. Visual distortion of a limb modulates the pain and swelling evoked by movement. Curr Biol 2008; 18: R1047–8.
  2. Bowering KJ, Butler DS, Fulton IJ, et al. Motor imagery in people with a history of back pain, current pain, both, or neither. Clin J Pain 2014; 30: 1070–5.
  3. Schwoebel J, Coslett HB, Bradt J, et al. Pain and the body schema: effects of pain severity on mental representations of movement. Neurology 2002; 59: 775–7.
  4. Schwoebel J, Friedman R, Duda N, et al. Pain and the body schema: evidence for peripheral effects on mental representations of movement. Brain 2001; 124: 2098–104.
  5. Moseley G, Butler D, Beames T, et al. The graded motor imagery handbook. Adelaide: NOI group publishing, 2012.
  6. Bellan V, Gilpin HR, Stanton TR, et al. Untangling visual and proprioceptive contributions to hand localisation over time. Exp Brain Res 2015; 233: 1689–701.
  7. Moseley GL, Wiech K. The effect of tactile discrimination training is enhanced when patients watch the reflected image of their unaffected limb during training. Pain 2009; 144: 314–19.
  8. Moseley GL, Zalucki NM, Wiech K. Tactile discrimination, but not tactile stimulation alone, reduces chronic limb pain. Pain 2008; 137: 600–8.
  9. Reiswich J, Krumova EK, David M, et al. Intact 2D-form recognition despite impaired tactile spatial acuity in complex regional pain syndrome type I. Pain 2012; 153: 1484–94.
  10. Luomajoki H, Moseley GL. Tactile acuity and lumbopelvic motor control in patients with back pain and healthy controls. Br J Sports Med 2011; 45: 437–40.
  11. von Piekartz H, Wallwork SB, Mohr G, et al. People with chronic facial pain perform worse than controls at a facial emotion recognition task, but it is not all about the emotion. J Oral Rehabil 2015; 42: 243–50.
  12. Stanton TR, Lin CW, Bray H, et al. Tactile acuity is disrupted in osteoarthritis but is unrelated to disruptions in motor imagery performance. Rheumatology (Oxford) 2013; 52: 1509–19.
  13. Moseley GL, Wiech K. The effect of tactile discrimination training is enhanced when patients watch the reflected image of their unaffected limb during training. Pain 2009; 144: 314–19.