The connectional organization of the cortico-thalamic system of the cat

Data on connections between the areas of the cerebral cortex and nuclei of the thalamus are too complicated to analyse with naked intuition. indeed, t he complexity of connection data is one of the major challenges facing neur oanatomy. Recently, systematic methods have been developed and applied to t he analysis of the connectivity in the cerebral cortex. These approaches ha ve shed light on the gross organization of the cortical network, have made it possible to test systematically theories of cortical organization, and h ave guided new electrophysiological studies. This paper extends the approac h to investigate the organization of the entire corticothalamic network. An extensive collation of connection tracing studies revealed similar to 1500 extrinsic connections between the cortical areas and thalamic nuclei of th e cat cerebral hemisphere. Around 850 connections linked 53 cortical areas with each other, and around 650 connections linked the cortical areas with 42 thalamic nuclei. Non metric multidimensional scaling, optimal set analys is and non-parametric cluster analysis were used to study global connectivi ty and the 'place' of individual structures within the overall scheme. Thal amic nuclei and cortical areas were in intimate connectional association. C onnectivity defined four major thalamocortical systems. These included thre e broadly hierarchical sensory or sensory/motor systems (visual and auditor y systems and a single system containing both somatosensory and motor struc tures). The highest stations of these sensory/motor systems were associated with a fourth processing system composed of prefrontal, cingulate, insular and parahippocampal cortex and associated thalamic nuclei (the 'fronto-lim bic system'). The association between fronto-limbic and somato-motor system s was particularly close.