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The Visual System: Retinal Mechanisms

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Presentation on theme: "The Visual System: Retinal Mechanisms"— Presentation transcript:

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 Organization
PR 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


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