The Architecture of the Human Fovea

Excerpt

We summarize the development, structure, different neural types and neural circuitry in the human fovea. The foveal pit is devoid of rod photoreceptors and of secondary and tertiary neurons, allowing light to directly stimulate cones and give us maximal visual acuity. The circuitry underlying the transmission to the brain occurs at the rim of the fovea. The predominant circuitry is concerned with the ‘private’ cone to midget bipolar cell and midget ganglion cell pathways. Every cone drives two midget bipolar cells and two midget ganglion cells so that the message from a single cone is provided to the brain as a contrast between lighter signals (ON pathways) or darker signals (OFF pathways). The sharpening of this contrast message is provided by horizontal-cell feedback circuits and, in some pathways by amacrine circuitry. These midget pathways carry a concentric color and spatially opponent message from red and green cones.

Blue cones are sparse, even largely missing in the foveal center while occurring at somewhat higher density than elsewhere in the cone mosaic of the foveal slope. Signals from blue cones have different pathways to ganglion cells. The best understood is through an ON-type blue-cone-selecting bipolar cell to a non-midget, small bistratified ganglion cell. An OFF-center blue midget bipolar is known to be present in the fovea and connects to a blue OFF midget ganglion cell. Another OFF blue message is sent to a giant melanopsin ganglion cell that is present in the foveal rim area, but the circuitry driving that is less certain and possibly involves an intermediate amacrine cell. The H2 horizontal cells are thought to be feedback neurons primarily of the blue cone system.

Amacrine cells of the fovea are mostly small-field and glycinergic. The larger field GABAergic amacrines are present but more typically surround the fovea in a ring of processes, with little or no penetration into the foveal center. Thus, the small field glycinergic amacrines are important in some sort of interplay with the midget bipolar–midget ganglion cell channels. We have anatomical descriptions of their synaptology but only a few have been recorded from physiologically. Both OFF pathway and ON pathway amacrines are present in the fovea.