2B: Early Vision

Question 1

What is a Ganglion Cell?

Answer 1

• Innermost layer of the retina
• long axons exit the eye as the optic nerve

Question 2

P ganglion cells

Answer 2

• Small size
• small axons: slow conduction rate
• located in the fovea
• Receive input from several midget BCs
• low convergence

Question 3

M ganglion cells

Answer 3

• Large size
• Large axons: fast conduction rate
• Located in the periphery
• Receive input from many diffuse BCs
• High convergence

Question 4

Activity in Ganglion Cells

Answer 4

• Ganglion cells fire APs spontaneously
• Firing rate increases or decreases when stimulated by input from bipolar cells

Question 5

What is a receptive field for a ganglion cell?

Answer 5

• Circular region of space on the retina where light alters the firing rate of a particular ganglion cell
• Defined by a group of photoreceptors
• i.e., the region of space on the retina for which a ganglion cell is “responsible”
• Corresponds to fixation point
• RFs for M cells (periphery) tend to be larger than those for P cells (fovea), due to higher convergence
• RFs can overlap a: photoreceptor can provide input to more than one ganglion cell

Question 6

What is meant by the center-surround organization of ganglion cell receptive fields?

Answer 6

• Each RF has two regions: a center and a surround

Question 7

What is the distinction between an ON-center and an OFF-center cell? And its primary funcion?

Answer 7

• ON-center cells are excited when light hits the center, and are inhibited when light hits the surround
• OFF-center cells are the opposite
• Useful for detecting light/ dark edges : i.e., variations in brightness over space

Question 9

What is lateral inhibition and what types of cells in the retina are responsible?

Answer 9

• Inhibition of neighboring bipolar cells which influences relative activity of ganglion cells
• Enhances (exaggerates) light/dark boundaries
• Leads to brightness contrast illusions

Question 11

What is Acuity?

Answer 11

• The ability to distinguish differences in spatial distribution of light in an image
• High acuity = ability to see fine detail

Contributing factors

• Better for cells with small RFs (i.e., P cells)
• Better with low degree of convergence
• Better in the fovea

Question 12

Fovea Function

Answer 12

• Important for object identification
• Requires fine detail (high acuity) & color analysis, limited to daylight conditions

Question 13

Dark & Light Adaptation; How do we adapt?

Answer 13

• Changes in pupil size
• “duplex” perception; rely on rods in dim light and cones in bright light
• “throw away” extra photons; in very bright conditions photopigments get used up, such that some light never gets absorbed
• Ganglion cells emphasize contrast rather than overall brightness
• Explains light consistency

Question 14

What is Sensitivity?

Answer 14

• The ability to detect light
• High sensitivity = ability to detect dim light

Contributing factors

• Better for cells with large RFs (i.e, M

cells)

• Better with high degree of convergence
• Better in periphery

Question 15

Fovea vs. Periphery Chart

Answer 15

Question 16

Periphery Function

Answer 16

• Important for navigation (movement through space) & object localization
• Requires fast analysis of coarse info

Question 17

Lightness Constancy

Answer 17

• Perception of brightness of an object tends to remain constant despite variations in illumination
• Occurs because ganglion cells respond to relative differences in brightness