I just recently finished 8 weeks at an excellent neuro-rehab facility. During my time there I presented an inservice proposing visual dominance as an indication for mirror therapy, but as I suggested during my talk–I think this sensory preference could go much further than indicating the use of a mirror during stroke rehab if we take advantage of it. Below is an adaptation from my inservice.
A reflection from rehabilitation:
Ever heard of the McGurk effect?(1) Maybe…maybe not. Essentially, the McGurk effect (when performed) displays the tendency for our visual stimuli to over-rule our auditory stimuli. It can be performed in different ways, but the simplest is the one I’m about to describe. The effect is conducted by having an individual be observed saying “ba ba ba.” The viewer will observe (see, visualize, etc.) a man’s mouth saying “ba ba ba,” as well as hear his voice saying “ba ba ba.” The trick (or effect) comes when the man begins to mouth the sound “fa fa fa,” but the auditory stimulus provided is still “ba ba ba.” This is where the effect or illusion comes into play. The person receiving the auditory stimulus of “ba ba ba,” but also receiving the visual stimulus of “fa fa fa” will inevitably hear the sound “fa fa fa.”
Let me reiterate without all the nonspecific pronouns and confusion: people will hear “fa fa fa” while “ba ba ba” is actually the auditory stimulus being provided because they see “fa fa fa.”
Check out this video for an excellent demonstration:
Disclaimer: this man’s voice may linger in your mind for extended periods.
The McGurk effect is a prime example of our (humans) visual sensory dominance, and its only one example. Visual dominance can be defined as (my definition):
“The tendency of the visual system to substitute or override other sensory stimuli in their respective absence or simultaneous presentation.”(2)
This has huge implications for all of life; but right now, most pertinent to this blog post, is the importance for your stroke rehabilitation patient. Visual dominance occurs in many situations and is already utilized in stroke rehabilitation. Mirrors and visual feedback have been used for decades to help those with pusher syndrome to reorient their internal alignment.(3) Motor imagery/external cuing are now used in stroke rehab vs previous non-visual based treatment approaches.(4)
Overwhelmingly, visual sensory stimulus presides when in competition with auditory stimulus and this is most clearly displayed in another fancy effect called the Colavita effect.(5) The Colavita effect has been established since 1974, and repeatedly tested and demonstrated since then in various forms. One of which is below:
Original work on the Colavita effect demonstrates the occurrence of auditory extinction in the presence of bimodal sensory stimulus (i.e. auditory and visual). Essentially, when subjects were asked to respond to either auditory or visual stimuli separately, they pass with flying colors. When they are asked to respond to both auditory and visual stimuli presented simultaneously (i.e. bimodal sensory stimulation), they neglect the auditory stimulus…only responding to the visual. In the graphic above, another test was performed on reaction times (RT) with bimodal stimuli. That study demonstrated some slightly more impressive results. Subjects were asked to respond to unimodal stimuli and RTs were recorded. All subjects on average responded significantly faster to auditory stimulus in isolation when compared with visual stimulus in isolation. Again…the interesting part…when presented with bimodal stimulation, subjects responded first to visual stimuli…then auditory. Even more damning for any audiophiles out there…the visual RTs decreased significantly from their baseline isolation attempts. The implication being (in my viewpoint): that our brains so prefer to respond to visual stimuli over auditory they are capable of ramping up some amount of latent processing speed in order to first respond to the image displayed over the noise produced. Basically, an image is just more stimulating than a sound.
There is also good evidence supporting the tendency of visual dominance over other sensory modalities…notably, our proprioceptive sense. Studies have been around since the mid to late 90s demonstrating this.(6,7) Again, it’s essentially the same thing, when presented with visual and proprioceptive input: we prefer the visual. The assertion being that while proprioception is a valid and necessary sensory modality, it is ultimately vague and imprecise. As a result, we (humans) will rely on visual observation to determine the exact position of our limbs or body in space when asked to do so without caveats.
I think a strong case has been made for the preference of visual sensory input over other sensory modalities, but where does this visual dominance end? Well, it would seem as though those limits are not yet known. In fact, visual input (albeit modified visual input) can influence pain and even inflammation.(8) In a study conducted by Moseley et al., subjects with CRPS were asked to observe their own hand motions with the naked eye. Baseline pain levels and girth measurements for swelling were taken. The subjects were then asked to perform the same movements while viewing through a set of binoculars, with the magnification lenses removed. No surprise, equivalent results were recorded. Next the same subjects viewed their hand movements through binoculars with the magnification lenses intact…the results: increases in pain and swelling. Woah…crazy I know, but that’s not all. The next test in this study involved the same subjects, with the same movements, but with the binoculars turned around (backwards). The effect being that the observed hand appears smaller or further away. Under these situations, pain and inflammation measurements dropped below baseline by a statistically significant amount.
These individuals simply viewed their hands as smaller or further away and a drastic change was had not only on their subjective pain experience, but also their objective swelling measurements. Moseley cites other studies he’s performed using visual illusion to modify individuals pain experience, some causing subjects to report pain at a higher level based on colors associated with a noxious stimuli. Others even able to cause subjects to report feeling pain in locations outside their own bodies (like in the arm of the person with whom they are shaking hands).
Our visual system is so powerful in determining our perception of the world around us, but it would also seem its just as critical for moderating the reality and perception of the world within us.
So finally, we get to the question of ,”why is this important for my CVA patient?” With all of this evidence indicating the weight of our visual sensory dominance, the indication is also made for capitalizing on our visual dominance via mirror therapy (MT) in the subacute stage of stroke rehabilitation…and the physiologic and outcome based studies on MT suggest it as well.
If you are unfamiliar with MT, the simple explanation is that patients who have suffered a stroke utilize a mirror at midsagittal, either between the upper or lower extremities. The patient then performs movements with their uninvolved (non-hemiparetic) side, and observes these movements in the mirror. The effect is that the patient is given the visual feedback that their involved (hemiparetic) side is now moving.
Mirror therapy has been used for reduction of phantom limb pain for quite some time (click here for an excellent Ted Talk with lots of rolling Rs), the thought being that often a phantom limb becomes fixated in a uncomfortable and painful position. Subsequently, with a lack of proprioceptive, tactile, temperature, and any other type of feedback, the patient is stuck in that position–with no relief. VS Ramachandran, in the Ted Talk above, describes how the visual input activates the patient’s M1 (primary motor cortex) and allows the patient to actually control the phantom limb…via the contralateral limb. The result, for the man he discusses in the video at least, is alleviation of the stuck phantom limb and no more pain.
Mirror therapy uses the same mirror neuron stimulation to activate the ipsilesional M1, which is the same neurological pathway utilized by motor imagery training.(9) MT, however, is also shown to reduce something called “interhemispheric inhibition” (IHI). Interhemispheric inhibition is described as:
“The neurophysiological mechanism whereby the M1 of one hemisphere of the brain inhibits the opposite M1, and arises from pyramidal neurons of cortical layer III in one M1 (transcallosal cells) projecting, via the corpus callosum, onto GABAergic inhibitory interneurons in the other M1, which in turn modulate the excitability of corticospinal pyramidal neurons of cortical layer V located in the same hemisphere (Ferbert et al., 1992).”(10)
Our cerebral hemispheres are in constant communication. Normally, when we move one hand cortical activity occurs in the responsible M1 area, but also in the opposite primary motor cortex. The activation that occurs in the opposite M1 is actually inhibitory to the contrahemispheric activation (or the original activation), but because our hemispheres are healthy and working normally this inhibition is relatively insignificant. In the stroke affected brain there is an overactivity of the contralesional hemisphere during attempted movement of the hemiparetic side; which results in a dampening of the ipsilesional signals attempt to produce an action potential and initiate movement.
This IHI is actually closely correlated to the degree or amount of motor impairment in subacute CVA patients. MT has been demonstrated to decrease abnormal IHI following training, thus potentially increasing motor control of the affected extremities. While the number of studies on this topic are few, the existing evidence leans in favor of using MT in stroke rehabilitation.
Outcome based studies have been conducted, and a Cochrane review supports the use of mirror therapy for the upper extremity.(11) Only one study conducted on the lower extremities was reviewed, and though it barely reached statistical significance..it did. From the outside, it would seem there is no big difference from brain activation of an arm vs a leg. So, the results from the lower extremity study suggest we need more research in this area and better methods for conducting MT on the lower extremities because the evidence both physiologically based and outcome measure based is in favor of mirror therapy for the uppers.
Now, I’m not making the case for MT revolutionizing physical therapy in rehabilitation centers, but I do think it would make a difference. Perhaps a significant one. Its inexpensive, harmless, easy to implement, and people regain more function with MT than those who use standard therapy alone. Moreover, the central focus of this entire blog post being visual dominance, I think that deep thinking and sound scientific research aimed at capitalizing on this sensory preference does have the potential for dramatically improving stroke rehab in the future. To my knowledge, there are no studies which investigate the effect of the intensity of an external cue to aid in movement initiation. Do we know if lit floor panels or moving targets would be better at aiding in motor planning than a simple colored disc or piece of tape? Or if we create a better mirror implement that makes a more convincing illusion during MT, would there be a larger decrease in IHI and better long term outcomes? To answer these questions, I think we just need to look ahead.
- Nath A, Beauchamp M. A Neural Basis for Interindividual Differences in the McGurk Effect, a Multisensory Speech Illusion. Neuroimage. 2012 January 2; 59(1): 781–787.
- Muro S. Visual Dominance: An Indication for Mirror Therapy in Stroke Rehabilitation. 2014. Unpublished.
- Hans-Otto Karnath and Doris Broetz. Understanding and Treating “Pusher Syndrome.” PHYS THER. 2003; 83:1119-1125.
- Santos-Couto-Paz CC, Teixeira-Salmela LF, Tierra-Criollo CJ. The addition of functional task-oriented mental practice to conventional physical therapy improves motor skills in daily functions after stroke. Braz J PhysTher. 2013 Nov-Dec; 17(6):564-571.
- Spence C, Parise C, Chen YC. The Colavita Visual Dominance Effect. In: Murray MM, Wallace MT, editors. The Neural Bases of Multisensory Processes. Boca Raton (FL): CRC Press; 2012. Chapter 27.
- van Beers RJ, Sittig AC, Denier van der Gon JJ. How humans combine simultaneous proprioceptive and visual position information. ExpBrain Res. 1996. 111:253–261.
- van Beers RJ, Sittig AC, Denier van der Gon JJ. The precision of proprioceptive position sense. Exp Brain Res. 1998. 122:367–377.
- Moseley GL, Parsons T, et al. Visual distortion ofa limb modulates the pain and swelling evoked by movement. Current Biology. 2008. 18(22).
- Carvalho D, Teixeira S, Lucas M, et al. The mirror neuron system in post-stroke rehabilitation. International Archives of Medicine. 2013. 6(41).
- Avazino L, Raffo A, Pelosin E, et al. Training based on mirror visual feedback influences transcallosalcommunication. European Journal of Neuroscience. 2014. 1-8.
- Thieme H, Mehrholz J, Pohl M, Behrens J, Dohle C. Mirror therapy for improving motor function after stroke. Cochrane Database of Systematic Reviews. 2012, Issue 3. Art. No.: CD008449. DOI10.1002/14651858.CD008449.pub2.