The thalamus, a region located deep inside the brain, is a major communication hub for sensory information propagating to cortex and consciousness. One of the thalamic nuclei, the dorsal lateral geniculate nucleus (dLGN), is often referred to as the visual “relay” nucleus, even though it not only relays visual information, but is the first brain region in which visual information coming from the retina and traveling towards visual cortex is integrated and modified. Yet, how exactly information is modified at this stage is not fully understood.
In addition to retinal information, the dLGN integrates top-down information and is modulated by behavioral state, arousal, and attention. Recent studies have shown that attentional modulation of visual processing is mediated by an interplay between inhibition and excitation in the dLGN. Inhibition in the dLGN is provided by a local population of inhibitory interneurons, as well as inputs from inhibitory neurons located in the thalamic reticular nucleus (TRN).
Understanding visual information processing in the dLGN is of great clinical relevance. In patients with retinal or optic nerve damage, the dLGN is largely preserved, and therefore thalamic visual prostheses could, by stimulating dLGN neurons, potentially mimic the no longer available visual information. Moreover, schizophrenic patients show abnormalities in early visual processing and deficits in visual attention. The TRN has been implicated with these clinical symptoms of schizophrenia, and a possible role of the dLGN is debated. A better understanding of the neuronal circuits underlying function and malfunction of early visual processing would be needed to approach an understanding of the neurobiological disease mechanisms.
The aim of “Thalamic Circuits” was to describe the neuronal circuits underlying early-stage thalamic visual processing in the mouse dLGN and its modulation by long-range projections. For gaining a deeper biophysical understanding of information processing in the LGN and the basic rules of signal integration in LGN neurons, two methods needed to be established: deep brain monosynaptic rabies tracing (to resolve functional connectivity with monosynaptic resolution in vivo) and deep 2-photon calcium imaging (to achieve in vivo recordings of both cellular activity and subcellular signals). Applying these methods, we find that dLGN interneurons play an unexpectedly complex role for information processing.
