The story of Grey Level Index: forty years of misleading consistency?

3. Application of GLI profiles comparison for detection of cortical area borders

A substantial modification of GLI profiling approach was published 20 years tater in the paper by A. Schleicher, K. Amunts, S. Geyer, T. Kowalski and K. Zilles entitled “An observer-independent cytoarchitectonic mapping of the human cortex using a stereological approach” (Acta Stereologica, 1998, 17/1, pp. 75-82). More precisely, this method was apparently reported for the first time in 1995 (Schleicher, A., Amunts, K., Geyer, S., Simon, U., Zilles, K., and Roland, P. E., “A method of observer-independent cytoarchitectonic mapping of the human cortex”, Human Brain Mapping. Proceedings of the First International Conference on Functional Mapping of the Human Brain, Vol. 1. (suppl.), 77), but this presentation is now available as one-page abstract, so we cannot analyze it.

Practically at the same time a very similar paper was published by  A. Schleicher, K. Amunts, S. Geyer,  and K. Zilles with P. Morosan instead of T. Kowalski: “Observer-Independent Method for Microstructural Parcellation of Cerebral Cortex: A Quantitative Approach to Cytoarchitectonics” (NeuroImage, 1999, 9, pp.165-177). The same technique was re-published again in 2000, in “A stereological approach to human cortical architecture: Identification and delineation of cortical areas”, by A.Schleicher, K. Amunts, S.Geyer, T.Kovalski, T.Schormann, N.Palomeo-Gallagher and K.Zilles (J. of Chemical Neuroanatomy, 20, 2000, 31-47).

Modifications included: (1) GLI image is created in computer memory, (2) profiles are measured using pre-calculated non-linear grid, which follows the shape of the cortical surface in horizontal direction, and the orientation of the cortical columns in vertical direction, (3) a procedure of profiles comparison using Mahalanobis distance was developed to detect the position of the borders between cortical areas. Also, they re-iterated the definition of GLI: “The areal fraction of cellular profiles in each (measuring) field is an estimate of the volume density”.

All these improvements were very significant. Non-linear grid permits measuring curved regions of the section, as opposed to only “flat” parts measured in the first version of the method. The application to detection of areal boundaries is even more substantial, because it actually defines a revolutionary milestone: the beginning of the era of computerized mapping of human cortex at microscopic level. Unfortunately, a number of substantial omissions and misleading claims diminishes the value of these papers.

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