Tuesday 7 November 2017

Crossing the Midline

More and more these days, people are interested in the neuroscience underlying our behaviours and our ability to learn. This is fantastic, but the downside to this enthusiasm for neuroscience is that there is a lot of pseudo-neuroscience making the rounds. A few weeks back, when someone on a piano pedagogy Facebook group mentioned exercises for “crossing the midline”, a warning sign immediately flashed in my mind, and I decided to investigate a little.

There is a fairly common neuro idea that exercises that involve body parts crossing the midline (for example, touching your right elbow to your left knee) are good for encouraging neuroplasticity, especially interhemispheric communication (the two sides of the brain talking to each other) and bilateral sensory integration (putting together sensory information from the two sides of the body). Midline-crossing exercises are touted to improve how the two sides of the brain talk to each other and have all sorts of other benefits. See, for example, this article entitled “Why crossing the midline activities helped this child listen to his teacher”.

 My gut feeling was that these claims are questionable, and I wanted to know whether there is any research to support them.

A quick search of the scientific literature found exactly zero studies investigating this effect. There is no evidence to directly support the idea that crossing-the-midline exercises improve interhemispheric communication. A Google search of the same topic turned up a 2013 “Ask a neuroscientist” blog post, which confirmed that there is no research to directly support the effectiveness of crossing-the-midline exercises.  In fact, that blog post suggested that the best way to increase connections between the two hemispheres of the brain was to learn a musical instrument.

It’s tempting to just conclude that this idea is pseudoscience and leave it at that. But given the prevalence of this idea, I’d like to dig just a little deeper and talk about where this idea comes from. There is actually some logic to it. To start with, let me explain how these types of exercises work both sides of the brain. Imagine you’re using your right hand to reach over and touch a target in front of the left side of your body. The right hand is controlled by the left motor cortex. Your awareness of space on the left side of your body happens in the right side of your brain, in the posterior parietal lobe.

If we move our right hand into the left side of our personal space, then in order to coordinate where our hand is in space, the left motor cortex must communicate with the right parietal lobe, using fibres that travel through the corpus callosum, the big fibre bundle connecting the two sides of the brain. 

Presumably, if both sides of the brain are active and talking to each other, this increases the strength of their connection, but I should reiterate that it’s not clear that simply performing exercises that cross the midline will lead to increased connectivity.

When I delved into the literature about crossing the midline, I found that researchers study the development of midline reaching as part of the development of handedness.  If you put an object in front of a baby, but put it a little to one side of the midline, she will almost always reach with the closest arm.  As children age, they become more likely to reach with the dominant arm, which means that if the object is placed on the non-dominant side, they must reach across the midline to pick it up.  The development of this behaviour seems to parallel the development of the corpus callosum, the big bundle of nerve fibers which communicates between the two hemispheres.  This doesn’t necessarily mean that the cross-midline reaching causes interhemispheric communication; it’s more likely that the increase in communication between the sides allows the hemispheres to specialize and this leads to hand dominance.

Children with developmental delays often don’t automatically reach across the midline.  They are more likely to reach with whichever hand is closest to the object. This is correlated with delayed dominance and decreased laterality of the brain.  So occupational therapists do test for a child’s tendency to cross the midline, and if the child doesn’t reach across the midline in a normal fashion, the therapist will recommend midline-crossing activities to try to help develop a more dominant hand. This doesn’t seem to be an evidence-based therapy, seeing as there aren’t any studies to support it.  However, therapists may see improvement based on these exercises in individual cases and this justifies their use.

Even if midline-crossing exercises do help develop hand dominance and bimanual interaction in children with developmental delays, that doesn’t mean there is any benefit to these exercises in normally developing children.  In children who have developed strong hand dominance, midline-crossing exercises probably aren’t doing much, in my opinion. 

In short, there is no evidence that these exercises are useful for people with normal development, and even for people who are not neurotypical, the main relevance seems to be in the development of handedness, not some miraculous creation of connections in the brain.

Provine, R.R., and Westerman, J.A. (1979). Crossing the midline: limits of early eye-hand behavior. Child Dev 50, 437–441.

Schofield, W.N. (1976). Do children find movements which cross the body midline difficult? Quarterly Journal of Experimental Psychology 28, 571–582.

Schofield, W.N. (1976). Hand Movements Which Cross the Body Midline: Findings Relating Age Differences to Handedness. Perceptual and Motor Skills 42, 643–646.

Surburg, P.R., and Eason, B. (1999). Midline-crossing inhibition: an indicator of developmental delay. Laterality 4, 333–343.

Surburg, P.R., Johnston, J., and Eason, B.L. (1994). Effects of midline crossing on response processing of adults with mental retardation. Journal of Developmental and Physical Disabilities 6, 327–338.