The Master and his Emissary
In chapter 2, "What do the two hemispheres 'do'?", McGilchrist writes about physical differences between the two hemispheres.
1. SIZE DIFFERENCE: (p. 33)
- The right hemisphere is "longer, wider, and generally larger, as well as heavier, than the left", something true, apparently, "of social mammals in general."
- The right hemisphere is wider everywhere, but for one place; the left is wider in the posterior parieto-occipital region.
- This right-side-bigger-than-left asymmetry is consistent from childhood to adulthood.
2. OTHER DIFFERENCES IN STRUCTURE
"As well as differing in the size and shape of a number of defined brain areas (Galaburda 1995), the hemispheres differ in the number of neurons (Galaburda, Aboitiz, Rosen 1986), neuronal size (the size of individual nerve cells) (Hayes & Lewis 1993), and the extent of dendritic branching (the number of connective processes put out by each nerve cell) within areas asymmetrically (Scheibel, Paul, Fried et al 1985). There is greater dendritic overlap in cortical columns in the right hemisphere, which has been posited as a mechanism for greater interconnectivity compared with the left (Seldon 1982). The ratio of grey to white matter also differs (Allen, Damasio, Grabowski et al 2003; Gur, Turetsky, Matsui et al 1999; Gur, Packer, Hungerbühler et al. 1980; Galaburda 1995). The finding that there is more white matter in the right hemisphere, facilitating transfer across regions, also reflects its attention to the global picture, where the left hemisphere prioritizes local communication, transfer of information between regions."- p. 33
3. DIFFERENCES IN NEUROCHEMISTRY
- Right hemisphere is more sensitive to testosterone (Lewis & Diamond 1995)
- Right hemisphere is more sensitive to pharmacological agents (Glick, Carlson, Drew et al 1987)
- Left hemisphere relies mainly on dopamine; right hemisphere relies mainly on noradrenaline (Glick, Ross & Hough 1982; Tucker & Williamson 1984; Wagner, Burns, Dannals et al. 1983; Fride and Weinstock 1988)
No wonder this book is taking me so long to read/digest. Every page is crammed with fascinating side tracks. The most pertinent one on this page, in my opinion, for PT and other movement therapies, is the difference in neurochemistry in Tucker and Williamson 1984 (abstract):
Reviews the literature on the neurotransmitter substrates controlling motor readiness, showing that these substrates produce qualitative changes in the flow of information in the brain: Dopaminergic activation increases informational redundancy, whereas noradrenergic arousal facilitates orienting to novelty. Evidence that these neurotransmitter pathways are lateralized in the human brain is consistent with the left hemisphere's specialization for complex motor operations and the right hemisphere's integration of bilateral perceptual input. Principles of attentional control are suggested by the operational characteristics of neural control systems. The affective features of the activation and arousal systems are integral to their adaptive roles and may suggest how specific emotional processes dynamically regulate cognitive function.(See #12 below.)
1. Galaburda 1995
2. Galaburda, A. M.; Aboitiz, F.; Rosen, G. D.; Sherman, G. F.; Histological asymmetry in the primary visual cortex of the rat: Implications for mechanisms of cerebral asymmetry. Cortex: A Journal Devoted to the Study of the Nervous System and Behavior, Vol 22(1), Mar 1986, 151-160.
3. Tamara L. Hayes, MS; David A. Lewis, MD; Hemispheric Differences in Layer III Pyramidal Neurons of the Anterior Language Area. Arch Neurol. 1993; 50(5):501-505
4. Scheibel AB, Paul LA, Fried I, Forsythe AB, Tomiyasu U, Wechsler A, Kao A, Slotnick J;
Dendritic organization of the anterior speech area. Experimental Neurology. Volume 87, Issue 1, January 1985, Pages 109–117
5. Seldon HL; Structure of human auditory cortex. III. Statistical analysis of dendritic trees. Brain Research Volume 249, Issue 2, 14 October 1982, Pages 211–221
6. Allen JS, Damasio H, Grabowski TJ, Bruss J, Zhang W; Sexual dimorphism and asymmetries in the gray–white composition of the human cerebrum. NeuroImage Volume 18, Issue 4, April 2003, 880–894
7. Gur RC, Turetsky BI, Matsui M, Yan M, Bilker W, Hughett P, Gur RE;
Sex Differences in Brain Gray and White Matter in Healthy Young Adults: Correlations with Cognitive Performance. The Journal of Neuroscience, 15 May 1999, 19(10): 4065-4072
8. RC Gur, IK Packer, JP Hungerbuhler, M. Reivich, WD Obrist, WS Amernek, HA Sackeim; Differences in the distribution of gray and white matter in human cerebral hemispheres. Science, 207 (1980), pp. 1226–1228.
9. Lewis & Diamond 1995
10. Glick, S. D., Carlson, J. N., Drew, K. L., & Shapiro, R. M. (1987). Functional and neurochemical asymmetry in the corpus striatum. Duality and Unity in the Brain. New York: Macmillan, 3-16.
11. Glick SD, Ross DA, Hough LB; Lateral asymmetry of neurotransmitters in human brain. Brain Research. Volume 234, Issue 1, 18 February 1982, Pages 53–63
12. Tucker DM; Williamson PA; Asymmetric neural control systems in human self-regulation. Psychological Review, Vol 91(2), Apr 1984, 185-215
13. Wagner HN, Burns HD, Dannals RF, Wong DF, Langstrom B, Duelfer T, Frost JJ, Ravert HT, Links JM, Rosenbloom SB, Lukas SE, Kramer AV, Kuhar MJ; Imaging Dopamine Receptors in the Human Brain by Positron Tomography. Science, New Series, Vol. 221, No. 4617 (Sep. 23, 1983), pp. 1264-1266
14. Fride E, Weinstock M; Prenatal stress increase anxiety related behavior and alters cerebral lateralization of dopamine activity. Life Sciences Volume 42, Issue 10, 1988, Pages 1059–1065