Trends in Cognitive Sciences
OpinionDegeneracy and cognitive anatomy
Section snippets
The lesion-deficit model
Until the first PET studies of language processing were reported [10], the primary method for establishing cognitive anatomy was the lesion-deficit model. The principle behind this approach is that if a physiological lesion results in a cognitive deficit, then part of the lesioned area must be necessary for the lost cognitive process. Throughout the last century several methodological developments enabled the lesion-deficit approach in humans. Delineation of permanent (pathophysiological and
Functional imaging studies of normal subjects
It is generally assumed that functional imaging techniques will overcome many of the limitations associated with the lesion-deficit model. The main advantages that functional imaging offers are that it can identify the set of regions that are engaged for one task relative to another and it is not limited to ‘lesioned’ cortex. For example, when normal subjects are instructed to match written words on the basis of their meaning, they activate a distributed set of areas (the semantic-retrieval
Combining the lesion-deficit approach with functional imaging studies of normal subjects
Functional imaging can, in principle, identify the set of regions that are sufficient for a cognitive operation and the lesion-deficit model identifies which of these areas are necessary. We have previously argued 24., 25. that a combination of functional imaging and the lesion-deficit model should enable us to identify ‘necessary and sufficient’ brain systems. We were wrong – there might be no single necessary and sufficient system because the existence of two or more degenerate systems that
Motivation for functional imaging studies of neurologically damaged patients
We have argued that functional imaging results can be used to guide neuropsychological investigations and that neuropsychological investigations can be used to test predictions generated from functional imaging data. It is possible, however, that the ability to complete the task might not be impaired following any combination of lesions to areas activated in functional imaging studies of normal subjects. This could occur if there are latent systems that are either untrained or ‘inhibited’ in
Conclusion
In this article we have discussed the concept of degeneracy at the level of cognitive anatomy. The key point is that a full understanding of the degenerate sets of neural systems that underlie any given cognitive task can only be revealed by systematically combining data from neuropsychological and neuroimaging studies of normal subjects and neurological patents (see Fig. 6). We believe that an understanding of degeneracy will provide insights into intersubject variability and the mechanisms
Acknowledgements
This work was funded by the Wellcome Trust. The data reported in Fig. 3, Fig. 4 and Fig. 5 were collected by C.J.P. and co-workers, and originally reported in a different format in Rik Vandenberghe et al. [22] and Cath Mummery et al. [23].
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