In 1985 Albert Galaburda and colleagues dissected the brains of four males who ranged in age between 14 and 32 years, and had been diagnosed as dyslexic during life. Non-righthandedness and several autoimmune and other illnesses were present in the personal and family histories.
All brains showed developmental anomalies of the cerebral cortex. These consisted of neuronal ectopias (cells in the wrong place) and other anomalies affecting predominantly the left hemisphere. Furthermore, all brains showed a deviation from the standard pattern of cerebral asymmetry instead being characterized by symmetry of the planum temporale, part of Wernicke’s area, a functional area for language. It lies under the Sylvian fissure which separates the frontal and parietal lobes from the temporal lobe.
In a later study in 2006 Galaburda and co-workers reproduced ectopias in rats.
Galaburda et al. (2006) ‘Developmental dyslexia: Four consecutive patients with cortical anomalies, Annals of neurology, 1985, Volume 18; Galburda et al., ‘From genes to behavior in developmental dyslexia’, Nature Neuroscience, Volume 9 (10)
A second example from anatomy uses a fairly new imaging technique called ‘diffusion tensor imaging’ (DTI).
It’s obvious that the channels of communication, the bundles of axons between areas of the brain, called fascicles, inside the brain and below the cortex, have to be correctly formed in order to work well and convey information fast and reliably.
Daniela Perani and colleagues have produced tractrographic images from 2-day old babies and contrasted them with adults, showing that some neural networks relevant to language are present from birth.
You can see that the lower pathway, in green, is present at birth, although more developed in adults. Then there are two upper tracts, one of which displays a noticeable difference between babies and adults: the path in blue is only present in adults, which implies development through exposure to language. There is also a difference between the hemispheres in both babies and adults (LH and RH).
(A is anterior; S superior; F fasciculus; L longitudinal)
Perani, D. et al. (2011). ‘Neural language networks at birth’, Proceedings of the National Academy of Sciences, Vol 108, no. 38, pp 16056-16061, September 2011 (reproduced with permission)
DTI is revealing something of these structures because it can highlight with a three-dimensional image the strength of orientation and coherence along one particular axis. Where there is a predominant axis the image is said to be ‘anisotropic’. Torkel Klingberg and colleagues write that anisotropy is determined by a number of factors related to the microstructure of the nerves: integrity of cell membranes, amount and integrity of myelin, coherence of axonal orientation, and number and size of axons. Increased myelination will show greater anisotropy.
This suggests that high-speed communications between areas of the brain may be impaired and this in turn could affect its integrating function.
Klingberg, T., Hedehus, M., Temple, E., Salz, T. Gabrieli, J.D.E., Moseley, M.E. and Poldrack, R.A. (2000). ‘Microstructure of Tempero-Parietal White Matter as a Basis for Reading Ability: Evidence from Diffusion Tensor Magnetic Resonance Imaging’,Neuron, 25, pp 493 – 500
Problems do not always lie solely in the cerebral cortex. The brain has to control the fine tracking of the central most sensitive part of the eye when reading and one of the pathways, the ‘magnocellular’, may also be impaired. Similarly, auditory paths may be weak. These factors would affect the speed of the processing which is instrumental in integrating the information from our senses. There is evidence that other subcortical structures like the cerebellum (‘little brain’) are implicated.