Part 1: The Prevalence of Human Asymmetry and Lateralization
The human body is asymmetrical. This concept is popularly accepted from an internal organ perspective. The heart, stomach, spleen, and descending colon are on the left while the liver, gallbladder, and ascending colon are on the right. We have 3 lobes of lung on the right and 2 on the left. The right vagus nerve is longer than the left with different innervation patterns. The right gives rise to the celiac branch supplying the ascending colon and 1st 2/3rds of transverse colon while the left forms the hepatic branch innervating the liver and part of duodenum.
It is also commonly acknowledged that the human brain is lateralized with dominant speech and motor centers on the left with visuospatial and emotional centers on the right. It has been found that some parallel areas of the cortex develop at different rates and correspond to variations in asymmetrical gene transcription measured as early as 12 weeks old. (Zaidi, 2011)
Below is an outline of structural and functional brain asymmetries:
Brain Structure Asymmetries
Pattern of Increased Size on the Left
Pattern of Increased Size on the Right
Stated Correlation with Handedness
|Central Sulcus (became apparent as early as 29 weeks gestation)||X||Yes|
|Hippocampus||X or no asymmetry|
|Wernicke’s Area in temporal sulcus||X||Yes|
|Broca’s Area (differences apparent as early as 1 year of age)||X||Yes|
|Somatomotor Cortex (7% increase in dominant hemisphere)||X||Yes|
|Corticospinal Tract||X (originating from the L hemisphere|
|Uncinate fascicle (connects inferior-frontal with anterior-temporal cortex)||27% larger with 33% more fibers|
|Arcuate fasciculus (showed up at 4 months of age)||X|
|Lateral Ventricles (can show up as a neonate or within weeks after birth)||X|
Functional Brain Asymmetries
|Learning, analytical and sequential processing; execution and coordination of movements; language organization (syntax, decoding, producing); discrimination; categorizations and local dynamics||X|
|Emotional systems; visuospatial processing; relational aspects; perceiving body signals; and global dynamics (taking in the big picture and intuitive reasoning)||X|
|Handedness||X Cortex||X Body|
|Learned or conditioned fearful stimuli||X Amygdala (associated with increased cerebral blood flow to R and ANS activation)|
|Perception of innately fear related items such as photographs of threatening stimuli or faces||X Amygdala|
|Infant head turning (begins at 10 weeks gestational age)||X Turning to R (2/1 ratio)|
|Smell Identification||Nostril and Hemisphere|
|Flavor identification||Nostril and Hemisphere|
|Sexual Involuntary Response||X|
|Recognizing and Remembering Melodies||X|
|Rhythm and Pitch Functions||X|
In addition to visceral and brain asymmetries, the following table describes skeletal and vascular left to right differences.
|Fetal bones||Been shown to be stronger and heavier in the dominant limb|
|Facial (higher asymmetry associated with poorer health)||Varying reports on dominance. Even aesthetically pleasing shaped faces showed asymmetry. There may be a trend that at a younger age the right side is larger but shifts to the left side as one ages.|
|Frontal Sinus||Considered normal to have asymmetry between them.|
|Pelvic asymmetry||Common in both normal and populations with pain/dysfunction|
|Clavicle||Increased length||Increased diaphyseal breadth|
|Upper limb bones||Heavier, humerus is the most asymmetrical upper extremity bone|
|Femur||Heavier and Longer per some sources||Heavier per some sources|
|Tibia and fibula||Longer||Heavier|
|Extremities||Upper extremity demonstrates more variable asymmetry than the lower with the right upper extremity being longer. The more proximal bones of both upper and lower have more asymmetry than distal.|
|Carotid Artery||Bifurcates more cranially||Bifurcates more caudally|
|Vertebral Artery (cranial)||Larger|
Studies have documented asymmetries in sex characteristics in both males and females. Prenatal testosterone exposure influences the development of extragenital sexual dimorphism and may also slow growth in some areas of the left hemisphere while promoting growth in areas of the right. This may influence handedness. Testosterone exposure in utero has been correlated to autism, dyslexia, migraines, stammering, autoimmune disease, sexual preferences, spatial, language, music, and mathematical abilities. A low ratio of length between the 2nd digit and the 4th digit is associated with lower testosterone and sperm levels in men and higher oestrogen and lutenizing hormone (LH) levels in women.(Zaidi, 2011)
The following describes findings related to genital asymmetries:
|Testicles-size location related to handedness and cognitive skills. Ex-larger right associated with better spatial skills||Lower||Higher|
|Breasts||Higher asymmetry associated with increased risk of cancer (higher rate in the larger breast), decreased fecundity, and higher pre-term delivery rate|
Environmental factors that can influence asymmetry are increased asymmetrical manual labor or sports. There will be even more of an impact if the epiphyses have not been fused yet. Trauma or toxins in fetal or early life may influence asymmetry. Protein deficiency can diminish asymmetry. Industrial groups have less asymmetry than pre-industrial as well as decreased dimorphism asymmetry. (Zaidi, 2011)
Newborns exhibit a right head turning preference. There exists a theory that the right head turning preference is the result of the asymmetrical development of the vestibular system. The most common birth position is a left vertex position, where the fetus lies with its back to the mother’s left ilium with the right side of the head facing outwards. This position can potentially facilitate right arm-hand movement and induce asymmetrical forces. The left-otolithic dominance hypothesis suggests that in this position the left otolith receives more stimulation via the hair cells as it detects the sharper backward deceleration as compared to forward acceleration when the mother walks (due to the braking effect). This, in turn, leads to more impulses to the brain stem terminating on the ipsilateral vestibulospinal tract connecting with the neck extensors and SCM thus creating a biased right head turning response. There may be a critical time period in the 3rd trimester when the cortico-vestibular pathways are being formed when this bias develops. Furthermore, research has shown a correlation between an asymmetrical Moro reflex (right biased) also hypothesized to be related to the vestibular imbalance. In addition to a lateralized vestibular system, there are likely other asymmetrical neural, genetic, and epigenetic influences on the head turning as well. (Rönnqvist, Hopkins, Van Emmerik, & De Groot, 1998)(Joganic, Lynch, Littlefield, & Verrelli, 2009)(Zaidi, 2011)(Domellöf, 2006)
The visual processing system is not symmetrical. The left visual field corresponds to the right hemisphere while the right visual field is analogous to the left hemisphere. The right hemisphere is dominant for processing global visual information (neuronal lower spatial frequencies) while the left is more adept for local (neuronal higher spatial frequencies). It has been hypothesized that the brain computes at least two kinds of spatial relation representations. The right hemisphere is more efficient at processing coordinate or metric distance information. Regarding interpretation of relational spatial characteristics such as “connected to” or “above” there is no asymmetry or a left dominance. There are also differences between the right dominant magnocellular visual processing (sensitive to low spatial frequencies, contrast, perception of motion and depth, localization of visual stimuli in space, global analysis of visual scenes, high temporal resolution, responds quickly and transiently to moving targets) and the left dominant parvocellular visual processing (sensitive to high spatial frequencies, color, acuity, and visual patterns of stationary targets.) (Joseph B. Hellige., 1996)
Additional significant findings regarding lateralization (Zaidi, 2011):
- Increased asymmetry is associated with increased aggression.
- Belief in the paranormal is associated with more asymmetry of finger length and is found more often in women. Paranormal belief is not correlated with education levels.
- Situs Inversus shows a reversal of the frontal and occipital petalia dominance patterns but the planum temporale and inferior frontal gyrus sizes were the same as non situs inversus.
- Higher asymmetry of the upper and lower limbs is associated with an increased chance of low back pain.
- There is a 7% increase in size of the dominant somatomotor cortex.
- Although both ears have bilateral cortical contributions, there is a stronger connection contralaterally vs. ipsilaterally. Typically, the left auditory cortex is larger.
- Choline acetyltransverase acitivity is higher in the left that right.
Based on all of the data that exists supporting the widespread prevalence of asymmetry within the human body one must ask whether or not it is beneficial to human beings and what is the significance of an asymmetrical system? Part 2 of “Asymmetry, Lateralization, and Alternating Rhythms of the Human Body” will provide an answer.
Arun, C. P. (2004). The importance of being asymmetric: The physiology of digesta propulsion and earth and in space. Annals New York Academy of Sciences. doi:10.1097/JAC.0b013e3181e62bf3
De Theije, C. G. M., Wu, J., Da Silva, S. L., Kamphuis, P. J., Garssen, J., Korte, S. M., & Kraneveld, A. D. (2011). Pathways underlying the gut-to-brain connection in autism spectrum disorders as future targets for disease management. European Journal of Pharmacology, 668. doi:10.1016/j.ejphar.2011.07.013
Domellöf, E. (2006). Development of functional asymmetries in young infants: A sensory-motor approach. Umeå University, Sweden. Retrieved from http://www.diva-portal.org/smash/get/diva2:144424/FULLTEXT01.pdf
Fehmi, L. (2008). The Open Focus Brain.
Horvath, K., & Perman, J. a. (2002). Autism and gastrointestinal symptoms. Current Gastroenterology Reports, 4, 251–258. doi:10.1007/s11894-002-0071-6
Joganic, J. L., Lynch, J. M., Littlefield, T. R., & Verrelli, B. C. (2009). Risk factors associated with deformational plagiocephaly. Pediatrics, 124, e1126–e1133. doi:10.1542/peds.2008-2969
Joseph B. Hellige. (1996). “Hemispheric asymmetry for visual information processing.” Acta Neurobiol. Exp., 56, 485–497.
McManus, C. (2003). Right Hand Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms, and Cultures.
Nichols, W. J. (2014). Blue Mind: The Surprising Science That Shows How Being Near, In, On, or Under Water Can Make You Happier, Healthier, More Connected, and Better at What You Do.
Pottenger, F. (1945). Effect of Heat-Processed Foods and Metabolized Vitamin D Milk.
Price, W. A. (2009). Nutrition and Physical Degeneration.
Rönnqvist, L., Hopkins, B., Van Emmerik, R., & De Groot, L. (1998). Lateral biases in head turning and the Moro response in the human newborn: Are they both vestibular in origin? Developmental Psychobiology, 33, 339–349. doi:10.1002/(SICI)1098-2302(199812)33:4<339::AID-DEV5>3.0.CO;2-R
Shannahoff-Khalsa, D. S. (1991). Lateralized rhythms of the central and autonomic nervous systems: Implications for stress, adaptation and psychopathology. International Journal of Psychophysiology, 11, 76. doi:10.1016/0167-8760(91)90317-Q
Vallortigara, G., & Rogers, L. J. (2005). Survival with an asymmetrical brain: advantages and disadvantages of cerebral lateralization. The Behavioral and Brain Sciences. doi:10.1017/S0140525X05000105
Wolpert, L. (2005). Development of the asymmetric human. doi:10.1017/S1062798705000682
Zaidi, Z. F. (2011). Body Asymmetries : Incidence , Etiology and Clinical Implications. Australian Journal of Basic and Applied Sciences, 5(9), 2157–2191.
Postural Restoration Institute. https://www.posturalrestoration.com/
Wikipedia. Suprachiasmic Nucleus. http://en.wikipedia.org/wiki/Suprachiasmatic_nucleus