The primary visual cortex is the first processing center for visual signals within the brain, which is divided into six functionally distinct layers. Shown here are feline stellate cells (A,B) from Layer 4 of the primary visual cortex. Stellate cells receive the majority of their input from the lateral geniculate nucleus (LGN) of the thalamus, then send signals to pyramidal cells in layer 6 (H,F) for further processing. While studies of visual system gross anatomy dating from the 1940s indicated that information travels from the eye through the LGN and then into the cortex, more recent work has revealed that only 5-10% of the excitatory synapses onto stellate cells actually come from the LGN; most input comes from deeper cortical layers. The mechanism by which so few LGN inputs control stellate cells remains unknown – synapses from the LGN not appreciably larger than the others, nor are they clustered on stellate cell dendrites. One possible explanation is that signals from the LGN are highly synchronized with each other, whereas other inputs are more temporally dispersed.

3D print by Jeremy Swan based on an original illustration by Santiago Ramon y Cajal. Courtesy of the Cajal Institute, Spanish National Research Council or CSIC©