1. The Visual System: Retinal Mechanisms
Lesson 15

2. The light stimulus Electromagnetic energy visible light
350 – 700 nm (violet – red)
Photon
Wave
Color (hue) - wavelength
Brightness (intensity) – amplitude ~

3. Color
Wavelength
Amplitude
Brightness

4. Visual Pathway LGN LGN V1 V1 LVF RVF Retina Optic Nerve Optic Chiasm
Optic Tract
LGN LGN
Lateral Geniculate
Nucleus (Thalamus)
Optic Radiations
V V1
Primary Visual Cortex

5. Retinal Organization Photoreceptors (PR) transduction
Bipolar cells (BP)
no APs
Retinal ganglion cells (RGC)
1st APs in pathway
Amacrine & Horizontal cells
communication b/n retinal cells
parallel to retina ~

6. Retinal Organization Which direction PR does the light enter? BP RGC
Optic Nerve

7. Photoreceptors (PR)
Transduction
Rods
achromatic
Cones
color ~

8. Photoreceptors - PR Dark current Depolarized in dark Na+ influx
NT is released BP
inhibited ~

9. Photoreceptors - PR Light strikes PR Receptor potential
hyperpolarization
 NT release
BP depolarizes
 NT release
Excites RGC
 APs ~

10. Rods Around edges of retina 120 million Pigment = rhodopsin
Convergence
high sensitivity
low acuity ~

11. Cones Color vision - 3 types red = long wavelength green = medium
blue = short
6 million
Concentrated in fovea
Little convergence
low sensitivity
high acuity ~

12. Receptive Field (RF) Region of the retina where...
changes in illumination...
will change the activity of a particular neuron
Center-Surround Organization BP
RGC
LGN neurons
convergence ~

13. Center-Surround OrganizationPR BP
RGC

14. Center-Surround Organization
2 arrangements
OFF ON ON
OFF

15. Center-Surround Organization
Light strikes  On area of RF
RGC excited   APs
Light strikes  Off
RGC inhibited   APs
Contrast  maximal effects on RGC
center & surround lighted cancels out ~

16. Lateral Inhibition - - - Center-surround RF Horizontal cells no APs
Dark
Light PR - PR -
Center-surround RF
Horizontal cells
no APs
PR stimulates HC
inhibit adjacent BPs
Enhances contrast ~ + H - BP + BP + G G
Surround
Center