A Reflection on Handedness and Lateralization

Is there a list of considerations regarding test findings, observations and treatment guidelines for those with left hand preference?

A couple of things that helped generate the question include:

  • I think I heard Ron say in a PRIVY episode that the Standing Lateral Shift Occlusal Test might remain positive in “left-handed” folks.
  • In the recent Human Evolution course, Ron made comments about case study subject “John” (page 120) to the effect that he wondered if he was left-handed. I think the observations included the proximity of the left hand and left hip when centering himself to initiate gait and the fact that he didn’t fall to the right during static left single leg balance despite a significant list.

Lateralization is an ongoing interest of mine and has been for years. There is so much I could comment and reflect on regarding handedness, (which is not a single phenomenon); there are degrees of handedness in different individuals for different activities and there are different footedness, earedness and eyedness as McGilchrist points out in his book ‘The Master and his Emissary’. As I said, I have probably not researched, nor read material on any other subject, more than material on limb dominance and lateralization, as related to functional or structural asymmetries of the primary cortex and descending pathways. So, I say this in earnest…handedness and other neural control patterning is usually traced to asymmetric neurodevelopment influences that occurred in the prenatal environment. Researchers like Previc, Bagesteiro, and McManus, and so many recent articles in Nature, have helped me understand the relationships of handedness to psychology and embryology.

11% of humans are considered broadly left-handed. And many are ambidextrous or do some activities with one hand and other activity with the other hand. The majority, or about 75% (of this 11 %) still have their speech centers in the left hemisphere and would appear to follow the standard definition of “left-handed”. 5% of the population overall who are known not to lateralize for speech in the left hemisphere, have a simple inversion of the hemispheres. They would have to read ‘right’ to ‘left and ‘left’ to ‘right’. What normally happens in the right hemisphere is happening in the left and vice versa. Small number of people.

A subset of left handers, as well as some people with other conditions, irrespective of handedness, such as probably schizophrenia and dyslexia individuals, those with some forms of autism, Aspergers, savant conditions, etc. may have a partial inversion, leading to brain functions being lateralized in unconventional combinations. For them, the normal partitioning of motor and sensory function breaks down, and making sense of conscious and non-conscious thought and delivery can be challenging. This may confer special benefits, or lead to disadvantages and confusion, in carrying out of different activities, as McGilchrist points out.

I believe, after much thought and understanding of multi-theorists, that left handers (as well as other people with non-standard lateralization) are living with the abilities to pattern activities like painting, running, talking, planning, etc. etc. with bilateral parts of each cortical hemisphere that either make their world of decision making and behavior, less or more challenging when asked to use one arm, one leg, one eye, etc. They are all less alike than those who are “standardized” and fall into the 89% norm of being left cortically dominant and consistently operating with the left cerebral hemisphere integrating right-sidedness, right handedess/footedness, speech production and language processing into successful expression and confidence. This is theoretically, why for me, that ‘normal’ PRI testing results will reflect this 89%, but not necessarily the 11% of unconventional cerebral processing. Left-handed people are not necessarily right hemispheric dominant nor left hemispheric dominant. They have neuro tendencies for both, or either one, depending on the processes used for consistency or creativity.

Therefore, in the Standing Lateral Shift Occlusal Test, a ‘left handed person’ might not follow conventional cerebral hemispheric organization, and the management of mass and environmental forces are possibly patterned in organization that requires dissociation from other vestibular processes that are typically witnessed in the 89% of those who depend on the left cerebral hemisphere for postural organization and conventional sensory combinations to empower the right thoracic diaphragm. Some of the most creative, broad thinking, artistic, organized leaders also may have challenging moments with expression, dyslexic behavior, and concepts that are confusing to the listener, but not to the deliverer. The accuracy of the discussion usually is higher in the deliverer. These same people usually will not fall, even when they have a significant list. Their asymmetrical behavior is so engrained by gravity, that they appear, look and demonstrate more of an unstable method to offset gravity or force demands, but their tensegrity is more stable overall than someone who was just repositioned with a 90-90 left hemibridge activity.     – Ron Hruska, MPA, PT