Gradiental approach to neocortical functional organization.

Examination of neuropsychological syndromes suggests strong congruence between neuroanatomical and functional neocortical metrics. Two cortical syndromes disrupt cognitively close functions if and only if their neuroanatomical territories are close. To capture this relationship, the concept of a cognitive gradient is introduced as a basic unit of macroscopic neocortical brain-behavioral analysis. A cognitive gradient is a continuous distribution of related functions along an axis defined at its extremes by a pair of sensory projection or motor areas. Cortical gradiental structure is viewed as a fundamental symmetric organization on which hemispheric elaborations are superimposed. The gradiental structure of the left hemisphere is presented in detail as an illustration of the concept. It is presumed that a similar set of elaborations can be uncovered for the right hemisphere. The gradiental model implies that at least from the macroscopic standpoint, functional neocortical organization is to a substantial degree continuous, interactive, and emergent, as opposed to mosaic, modular, and prededicated. It is proposed that a shift from latter to former properties signified a major development in the evolution of the brain. Thalamo-cortical integration may represent coexistence of evolutionally early and recent principles of cerebral organization.