The authors present physiological evidence, from monitoring supra- and subthreshold activations in rat barrel cortex and adjacent areas after single whisker activation, for a concurrent dual organization of neocortex, with both topographic and non-topographic components. They further raise the interesting possibility that the more extensively investigated topographic specificity of evoked sensory activity could be extracted from the large-scale non-specific activation.

Research on sensory cortical organization has overwhelmingly emphasized topographic organization and its putative basis in "driving" "bottom-up" signalling from the periphery via the thalamus. A contrast is then made with the great "modulatory" systems (serotonergic, dopaminergic, noradrenergic, cholinergic) as well as, to some extent, with "top-down" cortical feedback connections, which are more spatially divergent and by implication more "modulatory". In this study, the authors take a different view, which gives new importance to non-topographic connections. They show cortical activations as "spilling over" across conventional areas and suggest that the activity spread is due to sparse long-range horizontal connections. The experimental documentation for this claim -- of labeled fibers invading neighboring territories -- is plausible, but immediately raises the question of whether there is a specialized subset of pyramidal neurons that give rise to these particular long-range, non-topographical connections. While the proposal appears novel and/or an outlier view of cortical organization, the authors accurately relate their findings to other work on multisensory processing and to early thinking of Pavlov on activity spreads as a potential underlying mechanism for plasticity and associative learning.

Evaluated 12 Dec 2008