Graziano - 2007 - Rethinking cortical organization

Citation

Graziano MS, Aflalo TN Rethinking cortical organization: moving away from discrete areas arranged in hierarchies. Neuroscientist. 2007 Apr;13(2):138-47. PUBMED

10 Word Summary

Cortical organization occurs based on "like attracts like".

Abstract

One way to understand the topography of the cerebral cortex is that "like attracts like." The cortex is organized to maximize nearest neighbor similarity. This principle can explain the separation of the cortex into discrete areas that emphasize different information domains. It can also explain the maps that form within cortical areas. However, because the cortex is two-dimensional, when a parameter space of much higher dimensionality is reduced onto the cortical sheet while optimizing nearest neighbor relationships, the result may lack an obvious global ordering into separate areas. Instead, the topography may consist of partial gradients, fractures, swirls, regions that resemble separate areas in some ways but not others, and in not a lack of topographic maps but an excess of maps overlaid on each other, no one of which seems to be entirely correct. Like a canvas in a gallery of modern art that no two observers interpret the same way, this lack of obvious ordering of high-dimensional spaces onto the cortex might then result in some scientific controversy over the true organization. In this review, the authors suggest that at least some sectors of the cortex do not have a simple global ordering and are better understood as a result of a reduction of a high-dimensional space onto the cortical sheet. The cortical motor system may be an example of this phenomenon. The authors discuss a model of the lateral motor cortex in which a reduction of many parameters onto a simulated cortical sheet results in a complex topographic pattern that matches the actual monkey motor cortex in surprising detail. Some of the ambiguities of topography and areal boundaries that have plagued the attempt to systematize the lateral motor cortex are explained by the model.

Notes

• Check out notes from his talk at SFN.
• The cortical sheet is assumed to be more-or-less 2D while the information that it deals with is 3D or even higher dimensional.
• It is argued that the mapping of high-dimensionality into physically lower-dimensional space is accomplished with a nearest neighbor mapping that may produce organization that is not readily understandable without observing the functional implications of the brain.
• Topography of the monkey motor cortex is explained using this approach.
• The excepted theses of this paper are:
  • The cerebral cortex is separated into distinct areas
  • Some areas have a finer organization such as somatotopic or retinotopic maps
  • Cortical areas do not function individually but operate as interconnected functional networks
  • The areas appear to be ordered hierarchically from abstract->periphery
• History of cortical motor areas. This is useful for referencing back to.
• This work uses an (Kohonen 2001) algorithm to create a competing nearest neighbor mapping of the motor cortex.
• Results from the Kohonen Map
  • The map produced overlapping functional areas
  • Anterior and Posterior regions of the somatotopic map were recreated
  • The hand map was divided into three areas corresponding to functional space
  • Behaviorally relevant areas formed with very little overlap