The retina and brain share embryological origins; therefore, the retina can be thought of an extension of the central nervous system. Located at the back of the eye, it is the first structure in the visual pathway and is made up of photoreceptors (rods and cones) and neural cells that work together to capture visual information that is sent to the brain. 

The visual signal is sent from the retina to the brain via the optic nerve, a bundle of fiber made up from the axons of ganglion cells (retinal output neurons). The optic nerve mainly projects to the lateral geniculate nucleus of the thalamus, which sends the visual signal to the primary visual cortex (V1). Here, processing of visual information begins and continues as V1 sends information to higher visual cortices (V2, V5/MT, etc.).

Optical coherence tomography (OCT) utilizes light to create high-resolution images of the retina and its layers (image on left). Retinal vasculature can also be visualized through optical coherence tomography angiography (OCT-A, image on right). 

A plethora of measures can be extracted from these images: layer thickness, capillary blood flow (via Doppler OCT), vessel diameter, vessel density, and capillary complexity. In psychosis, layer atrophy (particularly in the inner retina), increased vein diameter, and decreased artery diameter have been observed. 

Retinal function can be assessed using functional electroretinography (fERG). This assessment stimulates the photoreceptors and ganglion cells and measures the cells’ electrical response and amplitude (image to the right). Different waves (A and B-wave shown on the right) correspond to different cell responses, and through this, we can understand how well retinal cells are working. We can also investigate whether there are specific structural and vascular changes that are associated with changes in retinal function.