Physics 1230: Light and Color Ivan I. Smalyukh, Instructor Office: Gamow Tower, F Phone: Lectures: Tuesdays & Thursdays, 3:30 PM - 4:45 PM Office hours: Mondays & Fridays, 3:30 PM – 4:30 PM TA: Jhih-An Yang Class # 18

The remaining lectures: 2 Ch. 7 (Retina and visual perception), Ch. 9 & 10 (color & color perception). We are here

Ch. 7 – Visual Perception Parts of the visual processing system Lightness and brightness Retinal processing: Lateral inhibition Hermann grid Receptive field Motion illusion Craik O’Brien illusion & simultaneous lightness contrast Other optical illusions 3 We are here

4 The Retina: Detecting the light and processing the images Has 10 8 nerve endings to detect image rods, for high sensitivity (night vision) cones, for color and detail, 7 million optic nerve = 10 6 transmission lines fovea, region of best vision (cones) The retina and optic nerves are recognized as actually parts of the brain (like your olifactory bulb in the nose). They start development IN the brain and migrate… More nerves in your retina than some creatures have in their entire brains. Processing Power.

6 Rods and cones Rhodopsin, a photochemical, responds to light It is destroyed and reformed. Signal goes to a synapse, a gap between nerve cells There are 3 kinds of cones for 3 colors red, green, blue (more later). A great deal is understood about how the individual cells of the retina receive light, respond to light, and transmit signals.

7 Rods and cones We will skip Most of cellular detail BECAUSE… Example: Rhodopsin and photosensitivity Photo-responsive membrane protein is known in atomic detail Light drives a change in molecular shape. Opens/closes membrane

8 Does our understanding of the individual rods, cones, and other cells of the retina do much to explain this?: (A) Creitanly (B) Myaby Not Mcuh We need to understand how NETWORKS of cells WORK TOGETHER to let us perceive.

Layers of the retina 9 Light

Layers of the retina 10

Layers of the retina are CROSS Connected 11 a, The rods (R) and cones (C) convey visual information to the ganglion cells (G) through the bipolar cells (B). Horizontal cells (H) allow lateral connections between rods and cones. Amacrine cells (A) allow lateral connections between bipolar and ganglion cells. The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects light directly; for this, they require the photopigment melanopsin, as now confirmed 1, 2, 3. b, Light, via melanopsin, causes changes in Ca 2+ levels in MG cells 9 (a fluorescent Ca 2+ indicator was used here). Counterintuitively, light passes through the transparent ganglion layer to reach the rods and cones. 1, 2, 3 9 From the following article: Neurobiology: Bright blue times Russell G. Foster Nature 433, (17 February 2005) doi: /433698a See text fig. 7.2

Connections and cross connections are MOST important. Photoreceptors: rods and cones connected to the bipolar cells connected to the ganglion cells, funnel “data” through axons into the optic nerve sideways connectors (these help with analysis) horizontal cells, next to the photoreceptors amacrine cells 12

13 Clicker question The arrow points to: A. Photoreceptors B. Horizontal cells C. Bipolar cells D. Amacrine cells E. Ganglion cells

14 Clicker question The arrow points to: A. Photoreceptors B. Horizontal cells C. Bipolar cells D. Amacrine cells E. Ganglion cells

Optic chiasma and brain structure 15 Brain damage on the left side hurts vision on the right side. See text fig. 7.3

Brain anatomy Optic chiasma Left field of view goes to right brain Right field of view goes to left brain from both eyes Visual cortex is where you “see” Brain damage at this location hurts vision. 16

17 Clicker question If the left side of your brain is injured, you might lose vision in your A. left eye B. right eye C. left field of view D. right field of view E. some loss in left and right field of view

All this ‘hardware’ allows us to perceive the world and function in it. Many complicated sub-systems have developed. Let’s study a few to get some insight into how vision works. 18

Interesting collective behavior 1: We detect RELATIVE Lightness, not total Brightness Brightness: amount of light Lightness: property of a surface newspaper = 0.65 (reflectance) printer paper = 0.84 photo quality paper = Total amount of light is far less important than the relative amount of light, particularly as compared with nearby objects. Demo with room lights.

Lightness and brightness Lightness constancy: brain and eye correct for amount of light so that white, gray, and black look the same independent of brightness. Weber’s law: we think lightness is equally spaced when the ratios are equally spaced Example: lightness 0.5, 0.25, look equally spaced. These numbers are ½, ¼, 1/8 etc. The spacing that looks equal is not 0.9, 0.8, 0.7, etc. 20 Demo: Lights on or lights off

Retinal processing that allows Relative Lightness sensitivity: Amacrine and horizontal cells “turn down” the signals from areas adjacent to bright areas. 21 See text fig. 7.5 “Lateral Inhibition”

“Receptive field” 22 Nerve cell fires rapidly See text fig The rods/cones and local cells are connected in a group: Center of group causes nerves to fire if illuminated. Surrounding group causes nerves to STOP firing if they are illuminated. Nerve cell doesn’t fire Nerve cell fires only a bit