While ridding the Metro subway into work this morning I watched a pretty college student grooming herself. I was at the advantage of being a distance away. From this perspective I was able to see her directly in addition to the reflected window image on her left with another window reflection on the opposite side of the car, left and right reflections as she keep verifying her profile pivoting her head to compare her mirror reflections sweeping back her long hair in an exaggerated fashion. I got to thinking about if we are designed in symmetry ? I offer some reflections on the brain’s left versus right mirror image of itself.
I was thinking along this topic wondering if congenitally deaf people have the same pattern of disruptions within their different brain anatomy. I am currently reading a great first book by author Scott McCredie on Balance: In Search of the Lost Sense. The author described how deaf people don’t respond to nausea inducing motion-sickness the way normal people do. Needless to say I found this totally fascinating. Basically, with deafness since birth, you might expect the specific speech areas of the brain to be not as prominent in terms of their brain network volume compared with of the hearing networks that are our language zones. In the first illustration the speech area is green plus the comprehension area zone is blue on the transparent surface model. Modern imaging studies use designated anatomical subdivisions of the brain surface that were originally based on cells just below the surface for different cellular arrangements according to their actual staining properties. So cerebral cortex zones according to Korbinian Broadmann (1868-1918) got mapped into subdivisions as if they were neighborhoods within the city of our brain, like green color coded Broca speech territory and Wernicke’s blue zone. Just like a city with all its below ground interconnectedness including fiber optic cables , electrical conduits , plumbing runs, subway lines plus all the other service links our brains are even more interconnected with the mirror image city on the opposite side hemisphere running through the midline yellow tinted corpus callosum. So is the opposite side really a mirror image with same place same neighborhood except that everything is reversed ? Are the language areas behaving like over crowded mufti-tongued immigrant neighborhoods, volume wise, creating a asymmetric brain ?
I am going to concentrate on language brain lopsidedness since I live in Montreal where we are perpetually bombarded on a daily basis by our newly elected minority government obsessed with language issues. We are told endlessly, “We must preserve to protect French at the exclusion of all foreign language which mainly includes English. I love French I’m fluently bilingual, my sons are trilingual but for an education minister to say,” We can’t expose young children to English too early, they may be diluting their French skills.” Numerous psychology reports from McGill University refute this political bias to demonstrate the complete opposite effect that a second fluent language, in fact, improves the primary language as a spill over effect. Can I interpret the intentions of our dear education minister to be promoting and advocating non-learning skills ? En tout cas, bilingual Montrealais certainly have very asymmetric brains with all these acquired extra language skills filling their brain volumes.
Back in the nineteenth century Broca and Wernicke both described impaired language skills in response to tumors or strokes in the left hemisphere. Language production with some components of syntactic processing including the pars triangularis and pars opercularis within Broca’s area of the inferior frontal gyrus. Language comprehension like listening to unintelligible squawking as garbled subway notifications have been localized including Wernike’s area in the posterior temporal-parietal region (Brodmann area 39, 40, posterior 21, and 22 with part of 37). Our handedness correlates more strongly with structural and functional asymmetries in language processing zones, implicating the planum temporale and other association labeled areas surrounding the Sylvian fissure. Typically but not always right handed people, 97% reveal more left sided specialization for both speech and language comprehension. A small percentage of right handed people 3% show either bilateral language representation or on the opposite side representation. Left handed people have 70% of language representation on the left side with 30% exhibiting bilateral or opposite side for language representation. Many dynamic variables affect such handedness with language representation involving genetic predisposition developments within the embryonic environment, signalling asymmetries with life experiences and disease mixing representations even further. Impressions into cerebral cranium bone caused by asymmetric bulging, termed petulias, reveal the asymmetry confirming a non-mirror image volume difference 3-dimension relief seen in the impression of the bone, as if from a dental impression of a bulging tooth. Petalias are a type of cerebral asymmetry, with greater protrusion of the surface of one hemisphere beyond that of the opposite hemisphere. The most typical configuration in modern humans is for the combination of a right frontal lobe petalia and a left occipital lobe petalia. Anthropologists have repeated these impression techniques in fossilized cranial bones from older primates also revealing asymmetries, to signal the evolution of the expansion of the pre-frontal cortex as speech became a more predominant attribute for survival.
” This three dimensional rendering of the inferior surface of a human brain is derived from an in vivo magnetic resonance imaging (MRI) scan that has been exaggerated to illustrate prominent asymmetries found in the gross anatomy of the two brain hemispheres. Noticeable protrusions of the hemispheres, anteriorly and posteriorly, are observed, as well as differences in the widths of the frontal (F) and occipital (O) lobes. These protrusions produce imprints on the inner skull surface, known as petalia. A twisting effect is also observed, known as Yakovlevian torque, in which structures surrounding the right Sylvian fissure are ‘torqued forward’ relative to their counterparts on the left. The left occipital lobe is also splayed across the midline and skews the inter hemispheric fissure in a rightward direction.” The Yakovlevian asymmetries were first published in 1976 entitled: Morphological cerebral asymmetries of modern man, and non-human primates Ann NY Acad. Sci. , 280, 349-366 by LeMay, M, & Kido, D.K.
” Dendritic arborization
A further provocative finding came in 1985 when Scheibel et al reported that the extent of high-order dendritic branching (high-order branches are thin branches that lie far away from the main dendrite) was greater in the left-hemisphere speech areas (including Broca’s area) than in their homologues on the right.However, lower-order dendrites were longer in the right hemisphere.The authors also noted that the right hemisphere develops faster in the first year of postnatal life, but is eventually surpassed by the left hemisphere. In the first postnatal year, left-hemisphere language regions consistently lag behind their right-hemisphere homologues in their state of development, perhaps to await speech development. The hemispheres might follow separate developmental programmes,with a variety of physical asymmetries emerging in utero, in childhood and in the teenage years.”
Further asymmetry has been described as, ” The asymmetrical trajectory of the Sylvian fissure was one of the first anatomical asymmetries to be described. At its posterior limit, the right Sylvian fissure curves upwards more anteriorly than the left, and the left has a gentler slope. The height of the end-point of the Sylvian fissure is also negatively correlated with the volume of the planum temporale. This region, in the posterior superior temporal lobe, is important for phonological encoding and speech perception, and is the epicenter of a mosaic of left-hemisphere language regions. It analyses the amplitude and frequency of sounds, as well as other acoustic information involved in speech perception. The planum temporale shows a marked leftward volume asymmetry that is related to the degree of right-handedness. Using an asymmetry index (AI) that corrects for total planum temporale size (AI = (right – left)/0.5(right + left)), Steinmetz analyzed 154 MRI scans, and found that righthanders have greater planum temporale asymmetry (mean AI = –0.30 ± 0.28; n = 121), whereas lefthanders show a weaker but still leftward asymmetry (mean AI = –0.16 ± 0.31; n = 33). In this study, no gender effects or gender-by-handedness interactions were found, indicating that these are probably subtle, if present.
Although the left planum temporale is an extension of Wernicke’s posterior receptive language area, the planum temporale asymmetry also appears in higher non-human primates (including chimpanzees). Its marked increase in humans points to a link with the evolution of language. In humans, the left planum temporale is up to ten times larger than its right-hemisphere counterpart; this is perhaps the most prominent and functionally significant human brain asymmetry. Broca’s speech area is also larger in volume than its homologue in the right hemisphere.”
“The greatest asymmetries of structure are clearly localized to the perisylvian language area. Hochberg and LeMay studied the location of the posterior tip of the Sylvian fissure; they found that it was higher on the right in 67 of the 100 right-handers that they studied, but in only 6 of 28 non-right-handers (21%). HESCHL’S GYRUS is also larger on the left side, a feature that can be attributed to greater amounts of underlying white matter on the left. These asymmetries are also found in children. Their magnitude increases throughout childhood and the teenage years, even after adjusting for developmental increases in brain volume. This indicates that there might be hemispheric differences in white matter maturation, perhaps during the many regional growth spurts in myelination that occur in
childhood. In addition, exposure to gonadal steroid hormones during critical developmental periods might differentially affect the growth of each side of the brain. The anatomical connectivity of the anterior temporal and inferior frontal lobes is also thought to be more highly developed in the right hemisphere. The uncinate fasciculus, which connects these two regions, has been found to be asymmetrical in both sexes, being 27% larger and containing 33% more fibers in the right hemisphere.”
It can be very difficult to make comparisons within individuals especially with our bilingual Montrealais population since the variations of language representation are so wide. These authors have remarked on this sort of group language representation as potential studies emphasizing with,” Group studies of functional anatomy rarely stratify their samples into groups with different normal anatomical variations, but such studies are needed to elucidate how these normal variants affect functional organization and cerebral asymmetries.”
” The Sylvian fissure is, in general, longer in the left hemisphere. Strikingly, some right hemisphere structures are ‘torqued forward’ relative to the left. This is consistent with the direction of the petalia (FIG. 2), in which the right frontal lobe juts forward relative to the left (see illustration). Nonetheless, the effect is comparatively localized, and perisylvian structures show the strongest asymmetries. Other studies have evaluated the incidence of sulci in one hemisphere relative to the other, compiling stereotaxic maps for the planum temporale in standardized atlas coordinates. Paus et al. generated a probabilistic map to describe the location of the cingulate and paracingulate sulci (when present) in each brain hemisphere. In MRI data from 247 healthy young volunteers, the paracingulate sulcus occurred more frequently in the left hemisphere, a feature that is thought to be linked to the participation of the left anterior cingulate cortex in language tasks. Subsequent functional MRI (fMRI) studies revealed that task-related brain activation during a word-generation task rarely extended into the cingulate sulcus when a prominent paracingulate sulcus was present; however, if no paracingulate sulcus was present, these activations spread into the cingulate sulcus.”
Why are our brains asymmetric ?
” Functional asymmetries in the brain were initially thought to be uniquely human, reflecting unique processing demands required to produce and comprehend language. However, functional and structural asymmetries have been identified in non-human primates and in many other species. Passerine birds produce song primarily under left-hemisphere control, and Japanese Macaques have a right-ear advantage for processing auditory stimuli. Language is commonly lateralized to the left hemisphere, and some argue that this is advantageous.First, it avoids competition between hemispheres for control of the muscles involved in speech. Second, it might be more efficient to transfer language information between a collection of focal areas in a single hemisphere. Asymmetrical brains, for example, have a corpus callosum with a reduced midsagittal area relative to more symmetrical ones. This might reflect fewer and/or thinner fibers connecting the two hemispheres, perhaps owing to differences in axonal pruning. The massive evolutionary expansion of the brain might have resulted in a level of complexity in which the duplication of structures was no longer efficient compared with the specialization of functions within a hemisphere. Time limits in callosal transfer of information between the brain hemispheres, in larger brains, might also have favored the development of unilateral networks.”
” Research on indigenous gestural languages invented by children in Taiwan and in Nicaragua provides some evidence for the innate relationship between gesture and language. Functional neuroimaging studies also indicate that deaf subjects using a gestural sign language might activate many of the systems involved in verbal language production.These congruences in functional anatomy seem to support the hypothesis that verbal language evolved from gestural language as an outgrowth of the already asymmetrical motor control system.”
Now onto the guts of this essay. I will be referring to Paul McCrory’s speculative hypothesis on The nature of Concussions released in the British Journal of Sports Medicine 2001 35: 146-147. It’s worth quoting extensively from this short article because there is enormous observational wisdom in his assessment. I will try to tie the Yakovlevian torque into McCrory’s hypothesis speculation.
“Concussion is one of the commonest forms of neurological injury seen throughout the world. Although common in sport, this condition has parallels in the types of injury suffered in motor vehicle crashes, falls, and other forms of brain trauma. Despite the fact that the effects of brain injuries have been recognized for at least 3000 years and the clinical state of concussion initially described over 1000 years ago, the understanding of the pathophysiology of these injuries remains limited. One issue that remains difficult to reconcile is the absence of consistent neuroimaging abnormalities in the face of dramatic symptoms. This may in part be because concussion is due to a functional rather than structural lesion but also that the anatomical locus may be not cortical as is often assumed.”
” The acute symptoms of concussion are described in detail in many published studies. Prospectively validated signs and symptoms include amnesia, loss of consciousness, headache, dizziness, blurred vision, attentional deficit, and nausea. The attentional deficit is often loosely described by clinicians as “confusion” or “disorientation”. While these terms are often seen as sine qua non of concussion, it is more scientifically appropriate to use the correct terminology. Other recently documented clinical features of acute concussion include convulsive and motor phenomena.
Most of these symptoms are protean and non-specific in terms of cerebral localization. The only symptom complex that is more likely to represent cortical or sub cortical dysfunction is memory disturbance. Traditional neurological thinking would suggest that the anatomical locus for such symptoms should be in the temporal lobes or orbitotemporal region.
Biomechanical theories of concussion
In 1974, Ommaya et al developed the centripetal theory of cerebral “concussion”. This theory invokes the geometric structural and material properties of the cranium and its contents. In this theory, the diffuse effects of the rotational component of inertial loading are produced by a centripetal progression of strains from the outer surfaces to the core of the brain (coinciding with the mid brain and basal diencephalon). At low levels of inertial loading, injurious levels of shear strain would not extend deeper than the cortex, while strains large enough to reach the well protected mesencephalic part of the brain-stem would result in loss of consciousness.
Such bio mechanical concepts are based on primate research and the authors readily allow that mild brain injury may not necessarily follow these principles. Nevertheless based upon these concepts, concussion severity grading scales and management strategies for sporting concussion have been proposed. If the underlying theory is incorrect then it follows that most if not all the existing grading scales are brought into question.
The “centripetal theory” revisited
The association of putative brain-stem phenomena—for example, loss of consciousness, convulsive phenomena—in the setting of concussion raises some challenging conceptual issues. If this centripetal theory holds true, then in milder concussive injury, “cortical” symptoms such as memory disturbance should predominate whereas only more severe injuries should manifest mesencephalic or brain-stem symptoms such as loss of consciousness or motor phenomena. The observation that brain-stem signs can occur in the absence of significant “cortical” symptomatology suggests that the clinical symptomatology of concussion may be more complex than the previously held view, and that the “centripetal theory” does not hold for all cases
The traumatic coma of experimental cerebral “concussion” has also been associated with failure of activity in the mesencephalic reticular formation and with loss of brain-stem reflex response without evidence of cortical involvement. Several studies in different animal models of experimental concussion have also demonstrated ultra structural and biochemical alterations in the brain-stem structures.
Although the symptoms of amnesia may be due to cortical dysfunction, some authors have postulated that amnesic symptoms may also be due to isolated brain-stem disturbance where ascending cortical projections are disturbed in the absence of cortical or sub-cortical pathology .Neuroimaging studies similarly have been unsuccessful in determining either consistent evidence of structural pathology in mild head injury or evidence of a “centripetal” gradient of injury severity, which would be predicted by the Ommaya theory.
Speculation on the nature of concussion
In contrast to concussion, our clinical understanding of severe brain injury is underpinned by firm experimental data. Extrapolation of the conceptual understanding of severe injuries to mild brain injury may be inappropriate given the structural rather than functional nature of neurological dysfunction in these cases.
Could there be a “brain- stem concussion” as distinct from “cortical concussion” ? We speculate that the nature of the clinical symptoms of concussion, the existence of motor and convulsive movements, the experimental animal evidence of brain-stem signs in concussion, and the absence of neuroimaging evidence of structural brain injury would be in keeping with this hypothesis.
Similarly the constellation of symptoms known as the “post-concussive syndrome” (including vacant stare, irritability, emotional lability, impaired coordination, sleep disturbance, noise/light intolerance, lethargy, behavioral disturbance, and altered sense of taste/smell) are equally difficult to localize and may reflect a global activation or attentional deficit rather than focal injury.
From a clinical stand point, management strategies may need to be rethought. Does a “brain-stem concussion” differ in severity or prognosis from a cortical concussion—that is, amnesiac concussion? Is there likely to be a different outcome for the athlete who wakes up cognitively intact after a concussive loss of consciousness as compared to an athlete with prolonged amnesiac symptoms? Should the management guidelines reflect such situations?
It is likely that the entity of concussion reflects a functional membrane dysfunction with the bulk of the anatomical focus in the brain-stem. The presence of memory disturbance however, is likely to reflect at least some cortical pathology, although brain-stem mechanisms do exist to account for such phenomena. This hypothesis would thus explain the absence of structural cortical pathology and the protean nature of the clinical symptoms more specifically than the existing centripetal hypothesis of Ommaya. The simplistic view of head injury being a linear spectrum from mild to severe with neuropathological accompaniments explaining clinical symptoms does not accord with clinical practice. This in turn has important implications for our conceptual understanding of concussion in relation to the spectrum of head injury and the development of appropriate management strategies.”
Dr McCrory poses the question: is a brain-stem concussion distinct from a cortical concussion? Since the brain is moving within its trajectory there will be a strong tendency to deform and reabsorb part of the kinetic energy within the confines of the skulls dimensional shape. Since that shape holds extended asymmetrical portions that have been demonstrated earlier in this essay the brain will have to torque itself back into its equilibrium position, correct? In doing so the link is established between the periphery with the central hub as if a tensegrity wheel is rotated back into its position of a floating tension net, a tensio natat of equilibrium. A continuum of tension changes will have been invoked in the entire tension network zone, both zones at the cortical areas connected with the brain-stem areas are perturbed including all tensio natat networks in between.
I introduce the term: Snelson tension twist within the transverse plane to describe this repositioning event of the tensio natat. The kinetic energy of the concussive forces are dissipating after the original coup/contre-coup within the cranial bone vault affecting both cortical and brain stem networks at the same time within a Snelson twisting floating tension network.
It is my extreme honor to dedicate this term : Snelson tension twist to artist Kenneth Snelson who in 1948 brought forth the first model for a floating tension configuration. This discovery is becoming a biological link between form to function. Snelson’s visionary creation has established a break through concept that we are still learning about to this day.