COGS 107B TA: Flavia Filimon

THE VISUAL SYSTEM

Note: this handout is based in part on Kandel, Schwartz and Jessell (2000, Principles of Neural Science) Ch. 26

Retina

Types of cells: 1) photoreceptors

2) bipolar cells |

3) horizontal cells | interneurons

4) amacrine cells |

5) ganglion cells

- 2 types of photoreceptors: rods and cones

- rods: mediate night vision à dim light; more concentrated in periphery, not present in central fovea; achromatic

- cones: mediate color vision (+ day vision); chromatic (3 types of cones – blue + red-green); concentrated in the fovea; higher acuity

- pigment epithelium : lines the back off the eye.

In humans, cells in the pigment epithelium are filled with the black pigment melanin, which absorbs any light not captured by the retina. This prevents light from being reflected off the back of the eye to the retina again – which would degrade the visual image. Cats have reflective pigment epithelia, which leads to a scattering of photons and blurring of the image.

- light must travel through layers of other retinal neurons before striking the photoreceptors.

- To allow light to reach the photoreceptors without being absorbed or greatly scattered (which would distort the visual image), the axons of neurons in proximal layers of the retina are unmyelinated so that these layers of cells are relatively transparent.

- In the fovea, the cell bodies of the proximal retinal neurons are shifted to the side, enabling the photoreceptors there to receive the visual image in its least distorted form.

- -à hence the constant eye movements when humans examine visual scenes of interest – projecting to fovea

- optic disc: where the optic nerve fibers leave the retina = blind spot. No photoreceptors there.

- Ganglion cells: photoreceptors and bipolars do not fire action potentials (only graded changes in membrane potential) ; in contrast, ganglion cells DO fire APs

- axons of ganglion cells form the optic nerve

- ganglion cells are also called X and Y cells

- Ganglion cells are specialized for the detection of contrasts and rapid changes in the visual image.

2 classes: ON-CENTER ganglion cells and

OFF-CENTER ganglion cells

1) on-center ganglion cells: excited when light falls in the center of their receptive field (which is circular, and has a center-surround organization. They are inhibited when light falls on the surround.

2) Off-center ganglion cells: excited when light falls on surround, inhibited when light falls in center.

- receptive fields of ganglion cells are small in fovea, and large at the periphery of the retina

- à hence greater sensitivity to rapid changes in light in the periphery than in center of gaze

- Transient vs Sustained ganglion cells

X – sustained vs Y- transient

- M cells (magni) – movement

- P cells (parvi) - form and color

Visual Pathways

- optic chiasm

- from retina: to LGN, then striate cortex