Please could you outline the effects of stroke that were previously known about?
Information provided by tactile receptors about objects we touch and hold in the hand is essential for our ability to use our hands and perform even simple everyday tasks. For example, would you be able to pick a raspberry or hold an egg with numb fingertips? You would have a lot of difficulty eating with a fork and knife, regardless of whether you have full muscle strength. Our sense of touch is crucial to control our hand actions. Usually we don’t realise the complicated computational tasks our brain performs “behind the scenes” and only when something goes wrong it becomes evident how important sensory information is.
Stroke often affects both our ability of our hands to feel and our ability to contract muscles. Sensory and motor functions are tightly linked. Yet, most rehabilitation strategies aimed at regaining dexterity after stroke largely focus only on motor recovery.
How did your research originate and what new effect of stroke did your research find?
As a neuroscientist investigating how sensory information is used to control the hands, the current situation that tactile sense is largely neglected didn’t feel right. In my opinion, little is known and too little attention has been devoted to tactile sensory dysfunction and mechanisms of sensory recovery after stroke. Standard clinical tests can identify functional deficits, but they often fall short explaining what the underlying mechanism is and thus what the most efficient rehabilitation strategy would be.
By thoroughly testing one stroke patient we discovered that the spatial (somatotopic) map of the hand has changed and orderly representation has been lost in the brain: the brain still receives the signal, but can't make sense of it. This is like trying to recover an image from a digital camera using pixels that have been scattered randomly.
How did your research discover this effect?
When patients learn what I do or read my work, it is not unusual that they, or their relatives and friends, ask whether research can help. Symptoms of one such stroke patient intrigued me and I decided to investigate. Teamed up with the brilliant clinical neurologists Prof Gunnar Wasner and Prof Inara Logina, who also share my passion and curiosity, we considered various tests. Just by chance we noticed that when the patient’s eyes were closed they felt touch in a different part of the hand than was actually touched. This is called referred sensation. Such a sensory disorder would usually remain undetected during routine clinical examination, and our patient wasn’t aware of it even 10 years after suffering stroke.
To document this scientifically, we had to design a whole new testing procedure using non-painful touch and that hid the stimulus and the hand from the patient’s view. We had to be able to distinguish referred sensations from the inability to localise stimuli and guesswork. Our tests showed that of the 25 locations tested, the patient correctly identified the location of the touch in one instance only. When one of the test points on the finger was touched, the patient could feel it, but in different location, for example, on the palm or on another finger.
What do you think is the underlying mechanism behind this effect?
Stroke patients may lose a considerable number of inputs and nerve cell connections in the brain. When stroke affects nerve centres and pathways involved in processing sensory information from the arm and hand, the loss of sensation initially can be quite severe, however, some recovery is possible.
We think that one contributing mechanism is that connections and pathways between nerve cells, which in a healthy brain were too weak or were inhibited, now - in the absence of normal activation pattern - become reinforced. New connections may be created. Unfortunately, these “backdoor” connections do not necessarily obey orderly organisation obtained during development and training after we are born. This can create the situation whereby the patient can feel the stimulus, but distorted circuits in the brain cannot provide adequate interpretation of this information.
Why do you think researchers had never previously described this effect?
Clinicians and researchers haven’t specifically looked for it. This is another reminder that many hypothesis and discoveries have origins in our ability to observe and pay attention to detail. When I tell clinicians about our findings, their reactions are quite interesting – some are puzzled, some are unsurprised and think they ‘knew’ this already, and some remain a bit sceptical.
What impact do you think this discovery will have?
As a researcher I don’t want to overstate the meaning of these findings just yet. We need to test more patients and find out how typical or unusual this effect from stroke is. However, I believe this is just the first step towards a wide range of studies in this direction eventually leading to the development of new and more efficient rehabilitation strategies for stroke survivors.
Where can readers find more information?
I suggest this very interesting book which certainly inspired me: “The Body Has a Mind of Its Own. How Body Maps in Your Brain Help You Do (Most) Everything Better” by Sandra and Matthew Blakeslee: http://www.sandrablakeslee.com/books/body-has-mind-of-its-own.php
About Dr Ingvars Birznieks
Dr Ingvars Birznieks is a senior research fellow at Neuroscience Research Australia and senior lecturer at The University of Western Sydney.
Dr Birznieks received PhD training in the world’s leading laboratories in the field of somatosensory research led by Prof Roland S Johansson at Umeå University in Sweden and Prof Antony W Goodwin at Melbourne University, Australia.
He has long standing teaching experience which begun at the Department of Human and Animal Physiology, University of Latvia where he was known for his passion teaching sensory physiology and regulation principles in biological systems.
In 2005 he received a prestigious Fellowship from the Swedish Medical Research Council to conduct postdoctoral research at Neuroscience Research Australia with Prof Vaughan Macefield. During his postdoctoral studies he broadened his research competence to study the physiological mechanisms underlying development of chronic pain and the function of the autonomic nervous system.
After postdoctoral studies he established his own cross-disciplinary research network centred around the study of neuronal information encoding mechanisms in the somatosensory system. Dr Birznieks now focuses on projects that link neuroscience, clinical neurology, and biomedical engineering.