Basal ganglia-thalamocortical circuits: parallel substrates for motor, oculomotor, "prefrontal" and "limbic"...

The central theme of the "segregated circuits" hypothesis is that structural convergence and functional integration occurs within, rather than between, each of the identified circuits. Admittedly, the anatomical evidence upon which this scheme is based remains incomplete. The hypothesis continues to be predicated largely on comparisons of anterograde and retrograde labeling studies carried out in different sets of animals. Only in the case of the "motor" circuit has evidence for the continuity of the loop been demonstrated directly in individual subjects; for the other circuits, such continuity is inferred from comparisons of data on different components of each circuit obtained in separate experiments. Because of the marked compression of pathways leading from cortex through basal ganglia to thalamus, comparisons of projection topography across experimental subjects may be hazardous. Definitive tests of the hypothesis of maintained segregation await additional double- and multiple-label tract-tracing experiments wherein the continuity of one circuit, or the segregation of adjacent circuits, can be examined directly in individual subjects. It is worthy of note, however, that the few studies to date that have employed this methodology have generated results consistent with the segregated circuits hypothesis. Moreover, single cell recordings in behaving animals have shown striking preservation of functional specificity at the level of individual neurons throughout the "motor" and "oculomotor" circuits. It is difficult to imagine how such functional specificity could be maintained in the absence of strict topographic specificity within the sequential projections that comprise these two circuits. This is not to say, however, that we expect the internal structure of functional channels (e.g., the "arm" channel within the "motor" circuit) to have cable-like, point-to-point topography. When the grain of analysis is sufficiently fine, anatomical studies have shown repeatedly that the terminal fields of internuclear projections (e.g., to striatum, pallidum, nigra, thalamus, etc.) often appear patchy and highly divergent, suggesting that neighboring groups of projection cells tend to influence interdigitating clusters of postsynaptic neurons. While more intricate and complex than simple point-to-point topography, however, this type arrangement should also be capable of maintaining functional specificity. As discussed briefly above, it is not yet clear to what extent the inputs to the "motor" circuit from the different precentral motor fields (e.g., MC, SMA, APA) are integrated in their passage through the circuit. It now appears that at the level of the putamen such inputs remain segregated.(ABSTRACT TRUNCATED AT 400 WORDS)