Connectivity Patterns Flashcards
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Connectivity Patterns
play a critical role in determining information-transmission functions…
Convergence
- 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
Acuity
Convergence helps to account for Acuity (detail resolution) differences between Rods & Cones
Acuity - Rods
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”
Acuity - Cones
: 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
Sensitivity
Convergence also helps account for Sensitivity differences between Rods and Cones
Sensitivity - Cones
Consider what happens in dim light (i.e. very little light reaching Photopigments in Receptors)
=> 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)
Sensitivity - Rods
Consider what happens in dim light (i.e. very little light reaching Photopigments in Receptors)
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!)
Receptive Field (RF)
Set of Receptors whose activity influences the activity of a “Target” cell (i.e. any downstream cell)
Size and type of a Target’s RF is determined by…
e.g. ?
….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
cells with small receptive fields are often involved in…
…high acuity perception
Center-Surround RFs
Some Ganglions, LGN, and V1 cells have them per pattern of Excitation, Inhibition and Convergence
example, if a Ganglion Cell has an Excitatory Center-Inhibitory Surround RF…
- 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
Lateral Inhibition
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
e.g. of lateral inhibition with Amarcrine
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