The Visual System: Retinal Mechanisms

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The Visual System: Retinal Mechanisms Lesson 15

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

Color Wavelength Amplitude Brightness

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

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 ~

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

Photoreceptors (PR) Transduction Rods achromatic Cones color ~

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

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

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

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

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 ~

Center-Surround Organization PR BP RGC

Center-Surround Organization 2 arrangements OFF ON ON OFF

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 ~

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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