2071 - Early Visual System

What is visual angle ?

• measure of distance. It measures how large an image appears on your retina compared to its actual physical size. A 1 cm object 57 cm away is 1 visual angle.

What is spatial frequency ?

Rate of change over space, representing how often a pattern of light and dark appears in an image.

What is temporal frequency ?

Rate of change over time, representing how often an image changes in brightness.

What are the three subtypes of retinal cells, and their quantity ?

• Photoreceptors → 110 million

• Bipolar cells → 10 million 

• Retinal ganglion cells → 1.2 million

What are the sensitivity/acuity/location/colour characteristics of the two photoreceptor types ?

1. Rods (90-100 mil)

• High sensitivity (get bleached by most intensities of light), low acuity 

• Peripheral only, no colour

2. Cones (4-5 mil)

• Low sensitivity, high acuity

• Highest in fovea, colour

What is acuity ?

Highest spatial frequency that can be seen

What does the central retinal pathway consist of ?

1 foveal cone that diverges into an ON-center bipolar cell, and an OFF-center bipolar cell, which diverge into ON and OFF ganglion cells respectively.

What does the peripheral retinal pathway consist of ?

1) Multiple rods that converge into a single ON-center Rod Bipolar cell.

2) Multiple peripheral cones that diverge into a Diffuse Bipolar cell (both ON/OFF types)

What are the acuity and efficiency characteristics of the two retinal pathways ?

Central - Acuity ↑ Efficiency ↓

Peripheral - Acuity ↓ Efficiency ↑ Sensitivity ↑

What is the principle of univariance ?

• A single class of photoreceptor cannot tell the difference between changes in the wavelength and intensity of light.