Connectivity Patterns

Question 1

Connectivity Patterns

Answer 1

play a critical role in determining information-transmission functions…

Question 2

Convergence

Answer 2

• Receptors converge (via their Bipolars) onto Ganglion cells;
• Rods: High Convergence, avg.
• 120:1 Ganglion
• Cones: Low Convergence, avg.
• 6:1 Ganglion;
• In Fovea: Very Low, Cones often only
• 1:1 Midget Ganglion

Question 3

Acuity

Answer 3

Convergence helps to account for Acuity (detail resolution) differences between Rods & Cones

Question 4

Acuity - Rods

Answer 4

light from 3 environmental points falls on 3 Rods, but if all 3 Rods converge (via Bipolar) on 1 Ganglion
==> that 1 Ganglion can only send the message: “Something out there”

Question 5

Acuity - Cones

Answer 5

: light from 3 environmental points falls on 3 Cones, each communicates (via Bipolars) with just 1 Ganglion
==> those 3 Ganglions send message: “3 things out there” = High Acuity

Question 6

Sensitivity

Answer 6

Convergence also helps account for Sensitivity differences between Rods and Cones

Question 7

Sensitivity - Cones

Consider what happens in dim light (i.e. very little light reaching Photopigments in Receptors)

Answer 7

=> little change in inhibitory NT released from each Cone => little excitatory NT released from each Bipolar

=> may be insufficient to trigger AP from each Ganglion - So, no info sent to brain (i.e. no light detected)

Question 8

Sensitivity - Rods

Consider what happens in dim light (i.e. very little light reaching Photopigments in Receptors)

Answer 8

little change in inhibitory NT released from each Rod => little excitatory NT released from each Bipolar
=> BUT since many converge on one ganglion, sum of NT is sufficient to trigger AP from Ganglion
- So, info sent to brain (i.e. even dim light detected - So see dim star best by NOT looking directly at it!)

Question 9

Receptive Field (RF)

Answer 9

Set of Receptors whose activity influences the activity of a “Target” cell (i.e. any downstream cell)

Question 10

Size and type of a Target’s RF is determined by…

e.g. ?

Answer 10

….patterns of Convergence and Lateral influences

• e.g. Ganglion (target) along path from converging Rods has large RF, while Ganglion along path from Cones has small RF

Question 11

cells with small receptive fields are often involved in…

Answer 11

…high acuity perception

Question 12

Center-Surround RFs

Answer 12

Some Ganglions, LGN, and V1 cells have them per pattern of Excitation, Inhibition and Convergence

Question 13

example, if a Ganglion Cell has an Excitatory Center-Inhibitory Surround RF…

Answer 13

• Light > the Center of its RF results in that Ganglion being excited (by activity of Bipolars from that area)
• Light > the Surround of its RF results in that Ganglion being inhibited (by activity of Amacrines from that area)
• Light > more Center than Surround results in Ganglion being more likely to reach threshold or to fire at higher rate
• Light > more Surround than Center results in Ganglion being less likely to reach threshold or to fire at lower rate

Question 14

Lateral Inhibition

Answer 14

Another important connectivity pattern, seen throughout brain (e.g. Help create Center-Surround RFs)

• Functions mainly to exaggerate differences (e.g. between areas of dark/light, serving to highlight edges of objects)
• Responsible, in part, for “illusions” like Simultaneous Contrast

Question 15

e.g. of lateral inhibition with Amarcrine

Answer 15

Amacrine, excited by its Bipolar, sends Lateral Inhibition to Ganglions nearby

• Amacrine’s response is graded, so the more excitation from the Bipolar the more inhibition sent to the neighbors

Question 16

Simultaneous Contrast

Answer 16

Given a stimulus in which a wide, light gray border surrounds a medium gray patch (“Medium 1”)
vs. one in which a different wide, dark gray border surrounds a patch of the SAME gray (“Medium 2”)

• Even tho Medium 1 is physically the same as Medium 2, Medium 1 appears darker than Medium 2.
• This is because Bipolars are more excited by the light gray border than by the dark gray border, so their Amacrines,
  near the light border, send more Lateral Inhibition to the center region, than Amacrines from the dark border do

Question 17

Direction Sensitive Cells

Answer 17

• Motion Detectors

- involve Uni-Directional Lateral Inhibition