Extended Data Figure 10. A matrix of major connections between functionally distinct cortical regions and thalamic nuclei, corresponding to Fig. 6.

Upper and lower panels show projections from cortex (source) to thalamus (target) or from thalamus (source) to cortex (target), respectively (ipsilateral projections only). The label ‘pc’ indicates that cortico-thalamic and thalamo-cortical projections in the gustatory/visceral pathway are between GU/VISC cortical areas and VPMpc/VPLpc nuclei. The number of pluses denotes relative connectivity strength and corresponds to the thickness of arrows in Fig. 6. All the connections described here were found in our data set. Connections labelled in red are previously known, whereas those labelled in black are not previously described in the rodent literature to our knowledge. There are also cases in which a connection was described in the literature but is excluded here because we could not find solid evidence in our data set to support it. All references that we have used to compare with our data are listed^{33,39,47–127}. Specifically, the cortico-thalamic system can be divided into six functional pathways: visual, somatosensory, auditory, motor, limbic and prefrontal. The visual pathway is composed of primary and associational visual cortical areas (VISp, VISam, VISal/l, and TEa) and thalamic nuclei LGd, LGv, and LP^100,103,109, with LGd and VISp playing primary roles in processing incoming visual sensory information, visual associational areas involved in higher-order information processing and LP potentially modulating the function of all visual cortical areas (thus similar to the pulvinar in primates). LGv does not project back to cortex. Similarly, the somatosensory pathway is composed of primary and secondary somatosensory cortical areas (SSp and SSs) and thalamic nuclei VPM, VPL and PO^48,74, with SSp and VPM/VPL playing primary roles in processing incoming somatosensory information, SSs in higher-order information processing and PO modulating the function of all somatosensory cortical areas. The gustatory and visceral pathway (involving GU/VISC cortical areas and VPMpc/VPLpc nuclei)^55,108 and the auditory pathway (involving primary and secondary AUD areas and different MG nuclei)^82,98 also have similar organizations, although our current data do not have sufficient resolution to resolve fine details. The motor pathway is composed of primary and secondary motor cortical areas (MOp and MOs) and the VAL nucleus^92,94. The limbic pathway (which is closely integrated with the hippocampal formation system not discussed here) is composed of the retrosplenial (RSP) and anterior cingulate (ACA) cortical areas and thalamic nuclei AV, AD and LD^107,114. The prefrontal pathway, which is considered to play major roles in cognitive and executive functions, is composed of the medial, orbital and lateral prefrontal cortical areas (including PL, ILA, ORB and AI) and many of the medial, midline, and intralaminar nuclei of the thalamus (including MD, VM, AM, PVT, CM, RH, RE and PF)¹¹¹. The reticular nucleus (RT) is unique in that it is a relay nucleus for all these pathways, receiving collaterals from both cortico-thalamic and thalamo-cortical projections although itself only projecting within the thalamus. Between these pathways, we have observed cross-talks, mediated by specific associational cortical areas and thalamic nuclei that may be considered to play integrative functions. For example, the anterior cingulate cortex (ACA) appears to bridge the prefrontal and the limbic pathways, interconnecting extensively with both. The posterior parietal cortex (PTLp) and the LD nucleus may relay information between the visual and the limbic pathways. PTLp, while hardly receiving any inputs from the thalamus, projects strongly to both LP and LD. On the other hand, LD, while projecting quite exclusively to the limbic cortical areas, receives strong projections from all visual cortical areas. There is also extensive cross-talk between the motor pathway and the prefrontal pathway, with both MOp and MOs receiving strong inputs from VM and sending strong projections to MD, and additionally with MOs projecting widely into many medial, midline and intralaminar nuclei. Finally, the thalamic nuclei PO, VM, CM, RH and RE all send out widely distributed, albeit weak, projections to many cortical areas in different pathways, thus potentially capable of modulating activities in large cortical fields.