Parallel processing streams in the rodent barrel cortex and their role in sensorimotor integratio

The rodent whisker system contains ascending, parallel processing streams that are characterized by different response properties. The lemniscal system, which contains the thalamic nucleus VPM, has neurons with sharply-tuned receptive fields, whereas the paralemniscal system, which contains the nucleus POm, has large multi-whisker receptive fields. In layer IV of the SI whisker representation, cellular aggregates called barrels are separated from each other by zones known as septa. Barrels receive thalamic information from the lemniscal VPM, whereas the septa receive input from the paralemniscal POm. In view of their segregated thalamic inputs, the barrel and septal compartments are thought to represent the cortical extensions of the lemniscal and paralemniscal processing streams, respectively.;In the first project, we used a retrograde tracing paradigm to characterize SI projections to MI and SII, with respect to the barrel and septal compartments. Following MI tracer injections, the labeled neurons in SI were predominantly aligned with the layer IV septa. By contrast, tracer injections in SII revealed neurons that were aligned with both barrel and septal compartments. Therefore, MI receives information mainly from the paralemniscal system, whereas SII receives information from both lemniscal and paralemniscal pathways.;In the second project, single unit activity was recorded from neurons in the layer IV barrel and septal compartments. During multi-whisker stimulation at progressively higher frequencies, septal neurons showed increasing latencies as a function of stimulus frequency, but the response latencies of the barrel neurons remained constant.;In the third project, we recorded simultaneously from SI and MI neurons during multi-whisker stimulation. A laminar analysis of MI revealed the strongest and fastest sensory responses occurred in the deeper layers of cortex. These responses always occurred after SI discharges and also showed increasing latencies as a function of stimulus frequency. Furthermore, MI sensory responses decreased by 80% upon reversible inactivation of SI. Finally, cross-correlation analysis revealed dynamic shifts in SI-MI correlations as a function of stimulation frequency. These results show that sensory responses in MI are elicited by projections from the paralemniscal pathway.