Quantitative comparisons of corticothalamic topography within the ventrobasal complex and the posterior nucleus of the rodent thalamus

To compare the topographic precision of corticothalamic projections to the ventrobasal (VB) complex and the medial part of the posterior (POm) complex, different anterograde tracers were placed in neighboring parts of the primary (SI) and secondary (SII) somatosensory cortical areas. The location of labeled corticothalamic terminals and their beaded varicosities were plotted, and the digital reconstructions were analyzed quantitatively to determine the extent of overlapping projections from the cortical injection sites. Among animals that received all tracer injections in SI cortex, tracer overlap in the thalamus varied according to the proximity of the cortical injection sites. Regardless of which combination of somatic representations were injected in SI, within each animal the amount of tracer overlap in POm was similar to that observed in VB, and a matched-sample statistical analysis failed to reveal significant differences in the proportion of the labeled regions that contained overlapping projections from the injected cortical sites. Among those animals in which the tracers were injected into the whisker representations of SI and SII, the amount of tracer overlap in the thalamus was not affected by the proximity of the cortical injection sites. Instead, tracer overlap appeared to be related to the degree of somatotopic correspondence. Furthermore, within each of these animals, the amount of tracer overlap in POm was similar to that found in the VB complex. These results indicate that POm has a well-defined topographic organization that is comparable to the degree of topography observed in the VB complex.