The thalamus is not just a relay for primary inputs, it also relays information between cortical areas.
Inputs from a particular system project to specific thalamic association nuclei. For example, visual cortical areas project to the pulvinar nucleus. Recordings in the pulvinar have found individual neurons with receptive fields as complex as any visual cortical area. So while neurons in the LGN do not have substantial orientation selectivity, there are sharply tuned neurons in the pulvinar.
Outputs from the these higher order relay nuclei go to higher cortical areas, presumably relaying inputs from one cortical area to the next higher cortical area. For example, primary visual cortex projects to neurons in the pulvinar, and these pulvinar neurons probably project to secondary visual areas.
The best defined higher order nuclear groups include
- DM – Dorsomedial nucleus, which receives input from the prefontal cortex, the olfactory cortex (eg. piriform cortex), and limbic structures.
- LP – Lateral Posterior nucleus, which receives input from the parietal lobe.
- PO – Posterior nucleus, relays information between somatosensory cortical areas.
- Pulvinar – receives input from occipital, parietal and temporal lobes, areas that process visual information.
- MGNd – dorsal Medial Geniculate nucleus, relays information between auditory cortical areas.