Corticogenesis: younger neurons migrate past older ones using radial glia as a scaffolding.Figure from https://en.wikipedia.org/wiki/Neural_development#/media/File:Corticogenesis_in_a_wild-type_mouse.png
Poor Billy would have been long forgotten, were it not for the fact that he died suddenly shortly after he had undergone extensive assessment for his specific learning difficulties. An autopsy found that death was due to a brain haemorrhage caused by an angioma in the cerebellum, but the neuropathologist also remarked on some unusual features elsewhere in his brain:
"In the cerebral hemispheres, anomalies were noted in the convolutional pattern of the parietal lobe bilaterally. The cortical pattern was disrupted by penetrating deep gyri that appeared disconnected. Related areas of the corpus callosum appeared thin (Figure 2). Microscopic examination revealed the cause of the hemorrrage to be a cerebellar angioma of the type known as capillary telangiectases (Figure 3). The cerebral cortex was more massive than normal, the lamination tended to be columnar, the nerve cells were spindle-shaped, and there were numerous ectopic neurons in the white matter that were not collected into distinct heterotopias (Figure 4)." p. 496*
I had tracked down this article in the course of writing a paper with colleagues on the neuronal migration account of dyslexia – a topic I have blogged about previously The 'ectopic neurons' referred to by Drake are essentially misplaced neurons that, because of disruptions of very early development, have failed to migrate to their usual location in the brain.
I realised that my hazy memory of this paper was quite different from the reality: I had thought the location of the ectopic neurons was consistent with those reported in later post mortem studies by Galaburda and colleagues. In fact, Drake says nothing about their location, other than that it is in white matter – which contrasts with the later reports.
This made me curious to see how this work had been reported by others. This was not a comprehensive exercise: I did this by identifying from Web of Science all papers that cited Drake's article, and then checking what they said about the results if I could locate an online version of the article easily. Here's what I found:
Out of a total of 45 papers, 18 were excluded: they were behind a paywall or not readily traceable online, or (1 case) did not mention neuroanatomical findings A further 10 papers included the Drake study in a bunch of references referring to neuroanatomical abnormalities in dyslexia, without singling out any specific results. Thus they were not inaccurate, but just vague.
The remaining 17 could be divided up as follows:
Seven papers gave a broadly accurate account of the neuroanatomical findings. The most detailed accurate account was by Galaburda et al (1985) who noted:
"Drake published neuropathological findings in a well-documented case of developmental dyslexia. He described a thinned corpus callosum particularly involving the parietal connections, abnormal cortical folding in the parietal regions, and, on microscopical examination, excessive numbers of neurons in the subcortical white matter. The illustrations provided did not show the parietal lobe, and the portion of the corpus callosum that could be seen appeared normal. No mention was made as to whether the anomalies were asymmetrically distributed."p. 227.
Four (three of them from the same research group) cited Drake as though there were two patients, rather than one, and focussed only on the the corpus callosum, without mentioning ectopias.
Six gave an inaccurate account of the findings. The commonest error was to be specific about the location of the ectopias, which (as is clear from the Galaburda quote above), was not apparent in the text or figures of the original paper. Five of these articles located the ectopias in the left parietal lobe, one more generally in the parietal lobe, and one in the cerebellum (where the patient's stroke had been).
So, if we discount those available articles that just gave a rather general reference to Drake's study, over half of the remainder got some information wrong – and the bias was in the direction of making this early study consistent with later research.
The paper is hard to get hold of**, and when you do track it down, it is rather long-winded. It is largely concerned with the psychological evaluation of the patient, including aspects, such as Oedipal conflicts, that seem fanciful to modern eyes, and the organisation of material is not easy to follow. Perhaps it is not so surprising that people make errors when reporting the findings. But if nothing else, this exercise reminded me of the need to check sources when you cite them. It is all too easy to think you know what is in a paper – or to rely on someone else's summary. In fact, these days I am often dismayed to discover I have a false memory of what is in my own old papers, let alone those by other people. But once in the literature, errors can propagate, and we need to be vigilant to prevent a gradual process of distortion over time. It is all too easy to hurriedly read a secondary source or an abstract: we (and I include myself here) need to slow down.
Drake, W. E. (1968). Clinical and pathological findings in a child with a developmental learning disability Journal of Learning Disabilities, 1(9), 486-502.
Galaburda, A. M., Sherman, G. F., Rosen, G. D., Aboitiz, F., & Geschwind, N. (1985). Developmental dyslexia: four consecutive cases with cortical anomalies. Annals of Neurology, 18, 222-233.
* I assume the figures are copyrighted so am not reproducing them here
**I thank Michelle Dawson for pointing out that the article can be downloaded from this site: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.949.4021&rep=rep1&type=pdf