Morphology of visual sector thalamic reticular neurons in the macaque monkey suggests retinotopically specialized, parallel stream‐mixed input to the lateral geniculate nucleus
While it is clear that the TRN is important for a number of broadly defined functions such as sensory selection, attention, and arousal, how circuits connecting TRN neurons with their thalamic targets mediate these functions remains unknown. The specific organization and function of TRN neurons is especially intriguing in the context of highly specialized sensory systems, such as the visual system of the primate. Neurons in the primate retina and dorsal lateral geniculate nucleus of the thalamus (dLGN) are optimized for acuity and color vision—unique specializations among mammals—and also form distinct parallel processing streams to encode a rich representation of the visual world. Primate retinal and dLGN neurons in the magnocellular (M), parvocellular (P), and koniocellular (K) streams are morphologically and physiologically distinct in order to convey information about visual motion, form/acuity, and color in parallel (Kaplan, 2004). M, P, and K neurons are physically segregated into separate layers in the dLGN and synapse in specific laminar compartments within the visual cortex. Given the strict segregation of feedforward visual signals into parallel streams, it remains an open question whether TRN inputs to the dLGN in the primate maintain this stream‐specific segregation or provide a more global input that is not stream‐specific. More is known about the morphology, physiology, and organization of TRN neurons in nonprimate species; these findings provide clues about possible primate TRN‐dLGN connectivity schemes, discussed below.
Neurons in the visual sector of the TRN receive inputs from and project axons to the dLGN (Sherman and Guillery, 2006). These afferent inputs and efferent projections are retinotopically organized, consistent with the topographic organization of afferent and efferent connections between TRN sectors and their associated sensory thalamic nuclei (Montero et al., 1977; Crabtree and Killackey, 1989; Conley and Diamond, 1990; Uhlrich et al., 2003; Fitzgibbon et al., 2007). Evidence from carnivores, rodents, and Galagos suggests that sectors of the TRN, including the visual sector, are organized into adjacent stripes or tiers including “inner” and “outer” tiers that receive input from higher‐order and first‐order thalamic nuclei, respectively (Conley and Diamond, 1990; Sherman and Guillery, 2006; Baldauf, 2010; Lam and Sherman, 2011).