PSYC304 Vision Lecture 9 PDF

PSYC304 Vision Jay Hosking, PhD Visual system: great model for the CNS Introduction to vision The retina Receptive fields Pathways to the cortex Visual cortex Other visual processing Overview 2  Lea...

PSYC304 Vision Jay Hosking, PhD Visual system: great model for the CNS Introduction to vision The retina Receptive fields Pathways to the cortex Visual cortex Other visual processing Overview 2  Learning objectives 1. What does it mean to say, “The brain is not a hard drive, and the eye is not a camera”? i.e. What does it mean to say that perception is active? 2. Describe numerous examples of active perceptual processing, both in this lecture and the previous. 3. Describe parallel processing and its relation to the visual system. 4. Describe separate systems for processing visual information in the retina. 5. Describe a receptive field, and how it varies across the visual system. 6. Describe separate pathways for visual information from retina to thalamus to cortex. 7. What two theories describe colour vision? Are they mutually exclusive or complementary? Do they completely describe colour perception? Describe the putative neural mechanisms for each theory, and what else might account for our perception of colour. 8. How does the visual cortex turn basic light stimulus into perceptions? 9. Describe separate pathways for processing visual information in the cortex. What do these suggest about discrete damage to visual areas? 3  DARPA Grand Challenge 2004 Intro 4  Where are we today? First self-driving car fatality: 2018 2024 Intro 5  A selective, active process Intro 6  A selective, active process Intro 7 Reminder: vision has a restricted range Based on evolutionary usefulness Varies by species As ever, has a biological basis Intro  Adaptation – A consequence of constant stimulation – Influences how the world is perceived in a personally useful way – Explains why we (usually) don’t see the blood vessels in our eyes – Thus, we need constant eye movement (saccades) to prevent the world from fading Intro 9  A messy path to travel The retina 10  Reminder: a less messy path to travel The retina 11  The cornea and lens focus images on retina The retina 12  A well organized structure Many translucent layers ~130 million photoreceptors Light Photoreceptors convert light into neural activity The retina 13  A well organized structure Light The retina Note 1: graded potentials 14  Specialized function Rod , one The retina 15  Specialized function Rods Cones (scotopic) (photopic) Long, thin, Short, thick, cylindrical tapered Highly Less sensitive to sensitive to light light Note 2: signal causes The retina hyperpolarization via GPCRs 16  Function influences localization O blindspot The retina 17  Rods ~120 million cells Very sensitive to light Not sensitive to wavelength of light Located everywhere except fovea Critical for vision at night (convergence) Explains why colour vision is poor in low light Easily “bleached” during the day Important for perceiving movement The retina 18  Cones ~7 million cells Less sensitive to light, but Sensitive to wavelength of light Three types of cones Located mostly in the fovea (explains why fovea less useful at night) Critical for perceiving colour Important for acuity (not cones per se…) The retina 19 Rods and cones have different sensitivities of wavelength The retina 20  Most mammals have dichromatic colour vision The retina 21  Colour blindness in humans Trichromatic colour vision Protanopia Deuteranopia Tritanopia Achromatopsia The retina 22  Acquired trichromacy Brain can make sense of new information Therapeutic (and science fiction!) implications The retina 23  Parallel processing, convergence The retina 24  Pro’s and con’s + High sensitivity – Low acuity The retina 25  Pro’s and con’s + High acuity – Low sensitivity The retina WHY HAVE BOTH? 26  Lateral inhibition and Mach bands We are built to detect contrast, not to sense absolute values Receptive fields 27  Relative, not absolute values Centre-surround organization Contrast enhancement Receptive fields 28  How to measure receptive fields - ActionPotential Receptive fields 29  ON and OFF channels On center surround off ⑳ intent offenth a Receptive fields 30  ON and OFF channels via lateral inhibition Receptive fields 31 Lateral Inhibition in the Retina Receptive fields 32  A messy path to travel 2: the revenge Pathways 33  Optic nerve, chiasm, tract Pathways 34  Cutting the optic chiasm Pathways 35  The lateral geniculate nucleus (LGN) Pathways 36  Specialized processing continues Handy mnemonic: P is for perception M is for motion Pathways 37  Welcome to V1, the primary visual cortex aka Brodmann Area 17 Striate cortex Visual Cortex 38  Retinotopic map Cortical magnification, again Visual Cortex 39  Simple cells https://www.youtube.com/watch?v=IOHayh06LJ4 Visual Cortex https://www.youtube.com/watch?v=8VdFf3egwfg 40  Ocular Dominance Columns Visual Cortex 41  Orientation Columns Visual Cortex 42 Colour perception: not simply wavelength Colour constancy Visual Cortex 43  Theories of colour Colour processing occurs in two stages. – 1. Young-Helmholtz trichromatic theory Visual Cortex (well, this one is in the retina) 44  Theories of colour (part 1) Colour processing occurs in two stages. – 2. Hering’s opponent process theory Visual Cortex (this one’s really cortex) 45  Theories of colour Colour processing occurs in two stages. – 2. Hering’s opponent process theory These two theories alone are still insufficient (i.e. a lot of colour perception is top-down) Visual Cortex (this one’s really cortex) 46 Reminder: Perception is heavily influenced by top-down processes Perceptual constancy Visual Cortex 47 Reminder: Perception is heavily influenced by top-down processes Visual Cortex 48  Specialized processing continues! Visual Cortex 49 Parallel processing Visual Cortex 50  Specialized processing continues! Visual Cortex 51  Cortical damage may not affect all vision Monkeys Patient D.F. Visual Cortex 52  Is the Halle Berry neuron a visual neuron? Other visual processing 53  Non-conscious vision? Loss of vision ≠ loss of circadian rhythm Can’t consciously see (V1 damage) but can guess stimuli with higher-than-chance accuracy and navigate a room with obstacles — “Blindsight” Other visual processing 54  Visual system: great model for the CNS Active, not passive Relative, not absolute Parallel processing Convergence and divergence Contralateral, -topic organization Summary 55