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PeerJ. 2018 Jul 17;6:e5206. doi: 10.7717/peerj.5206. eCollection 2018.

Illusory resizing of the painful knee is analgesic in symptomatic knee osteoarthritis.

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School of Health Sciences, University of South Australia, Adelaide, South Australia, Australia.
Neuroscience Research Australia, Randwick, New South Wales, Australia.
School of Psychology, University of Nottingham, Nottingham, United Kingdom.
School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, United Kingdom.



Experimental and clinical evidence support a link between body representations and pain. This proof-of-concept study in people with painful knee osteoarthritis (OA) aimed to determine if: (i) visuotactile illusions that manipulate perceived knee size are analgesic; (ii) cumulative analgesic effects occur with sustained or repeated illusions.


Participants with knee OA underwent eight conditions (order randomised): stretch and shrink visuotactile (congruent) illusions and corresponding visual, tactile and incongruent control conditions. Knee pain intensity (0-100 numerical rating scale; 0 = no pain at all and 100 = worst pain imaginable) was assessed pre- and post-condition. Condition (visuotactile illusion vs control) × Time (pre-/post-condition) repeated measure ANOVAs evaluated the effect on pain. In each participant, the most beneficial illusion was sustained for 3 min and was repeated 10 times (each during two sessions); paired t-tests compared pain at time 0 and 180s (sustained) and between illusion 1 and illusion 10 (repeated).


Visuotactile illusions decreased pain by an average of 7.8 points (95% CI [2.0-13.5]) which corresponds to a 25% reduction in pain, but the tactile only and visual only control conditions did not (Condition × Time interaction: p = 0.028). Visuotactile illusions did not differ from incongruent control conditions where the same visual manipulation occurred, but did differ when only the same tactile input was applied. Sustained illusions prolonged analgesia, but did not increase it. Repeated illusions increased the analgesic effect with an average pain decrease of 20 points (95% CI [6.9-33.1])-corresponding to a 40% pain reduction.


Visuotactile illusions are analgesic in people with knee OA. Our results suggest that visual input plays a critical role in pain relief, but that analgesia requires multisensory input. That visual and tactile input is needed for analgesia, supports multisensory modulation processes as a possible explanatory mechanism. Further research exploring the neural underpinnings of these visuotactile illusions is needed. For potential clinical applications, future research using a greater dosage in larger samples is warranted.


Bodily illusions; Knee osteoarthritis; Mediated reality; Multisensory integration; Pain; Visuotactile illusions

Conflict of interest statement

Tasha R. Stanton received travel and accommodation support from Eli Lilly Ltd. for speaking engagements (September 2014) on these topics. G. Lorimer Moseley receives royalties for books on pain and rehabilitation and speaker’s fees for lectures on these topics. He has received support from Pfizer, workers’ compensation boards in Australia, Europe and North America, Kaiser Permanente (USA), Arsenal Football Club, the Port Adelaide Football Club, and the International Olympic Committee. Roger Newport is the creator of the MIRAGE mediated-reality systems; The University of Nottingham received equipment fees for creation of systems for external labs.

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