## 3 Methodology
### Analysis and Structure of the Bio-Inspired Retinal Model  
#### 3.1.1 Global Structure  
The retina is a light-sensitive multilayer located at the back of the eye. Each layer of the retina is made up of neuronal cells with specialized functions that, by working in concert, can convert light signals into electrical impulses for transmission to the brain. Figure 1 illustrates the distribution of the layers that make up the retina.  
The RPE attaches to the choroid via Bruch's membrane and maintains the health of adjacent rod and cone photoreceptors through various processes. The tight junctions between individual RPE cells facilitate the formation of the blood-retinal barrier, maintaining the immune-privileged status of the eye and regulating ion exchange.  
Photoreceptors, the main type of light-sensitive cell in the retina, traverse the next three layers of the retina in a highly polarized manner. The inner and outer segments of the rod- and cone-shaped photoreceptors form the photoreceptor layer adjacent to the RPE, followed by the outer limiting membrane (OLM), outer nuclear layer (ONL), and outer plexiform layer (OPL), as shown in Figure 2.  
The human retina contains approximately 6 million cones and 120 million rods, with cones concentrated in the macula and fovea, providing central vision and color perception. Rods are located along the periphery of the retina and enable vision in poorly lit environments.  
Via visual phototransduction, trapped light causes dissociation of retinal molecules from opsin proteins in cones and rods, leading to hyperpolarization of photoreceptors and inhibition of glutamate release. In the dark, glutamate suppresses the activity of bipolar and horizontal cells, while its absence relieves the inhibition of retinal neurons, leading to their activation. Bipolar cells amplify and transmit the electrical signal downstream to  
Figure 1: Representation of human retinal anatomy [27].  
Figure 2: Schematic representation of the biological connectivity utilized for modeling the outer plexiform layer.  
amacrine cells, while horizontal cells help optimize performance and provide feedback to photoreceptors.  
The nuclei of secondary neurons--horizontal, amacrine, and bipolar cells--are located within the inner nuclear layer (INL), as shown in Figure 3. Amacrine cell dendrites synapse with retinal ganglion cell (RGC) dendrites in the inner plexiform layer (IPL). RGCs send action potentials through their axons, which form the optic nerve and carry visual messages to the brain. The internal limiting membrane (ILM) is the innermost layer of the retina, separating the layer of nerve fibers from the vitreous.  
The outer and inner plexiform layers can be considered a set of biological low-pass and high-pass filters, forming a spatiotemporal filter. Processing is mostly performed in the frequency domain, facilitating the parvocellular and magnocellular pathways. The parvocellular pathway highlights contours to answer \"What is it?\" while the magnocellular pathway highlights movement and answers \"Where is it?\" Both pathways connect with the lateral geniculate nucleus (LGN) and relay information to different areas of visual processing. This study focuses on the parvocellular responses of bipolar cells, which highlight image contours and facilitate image processing.