Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Week 3 The Eye: Proof for the existence of God? And then there was light Optics Perception Eye is receiver not sender Absorption Plato Euclid Send & receive Echolocation Sonar Perception is relative Perception not of environment (realism) but of our interpretation (relativism) • Butterflies see Electromagnetic spectrum ultraviolet markings • Snakes see infrared waves The first stage of transformation Retinal sensitivity to “visible light” 400-700 nanometers (nm is 1/100 millionth of a meter) Accomodation Start with getting things in focus on the retina Do it all the time unconsciously Accomodation Changes is lens curvature though muscles Far-sightedness in the elderly More convex Retina: Rods & Cones Not just morphological differences 2 types of vision Photopic Scotopic Diurnal Colour Bright light Nocturnal Monochromatic Dim light Predator vs prey Distribution of rods & cones 5 million cones per retina 1% are in the fovea, 99% in periphery 120 million rods per retina Fovea None in fovea Rods:cones 20:1 in periphery Periphery Retina: Its all backwards Pigment epithelium at back of eye Epithelial layer Back Nourishment of photoreceptors Light must pass through neural machinery Front Why don’t we see the cells/blood vessels in our eyes? Images stabilized on retina disappear How’s that for perception! The retinal “black hole” The blind spot Ganglion cells—>optic nerve exit eye From fovea Filling in the blind spot “filling-in” the blind spot? Ramachandran example Inference or perception Transduction: Light to energy Visual pigment molecules Retinal Light reactive chemical Absorbs a single photon! Isomerization: Morphing Change in shape of ion channels Change in membrane potential Electricity! Amplification: Opsin & retinal 1 pigment molecule —> cascade of million others Perception of light Can perceive a single rod activation Photoreceptor Visual pigments Not all pigments are created equal Rods vs Cones Dark adaptation Changes in sensitivity to light related to difference in time to pigment regeneration Timecourse parallels light sensitivity in dark adaptation curve Light Time Dar Dark adaptation: Switching visual systems Dark adaptation curve Switch from photopic to scotopic vision Max adapt Cones Test fovea 3-5 min Rods Rod monochromat 25-30 min “Racoon” vision? In living colour: Spectral sensitivity “Monochromatic” light 1 wavelength Method of adjustment Fovea (cones) Periphery (rods) Threshold Cones After dark adaptation Sensitivity = 1/threshold Overall cone sensitivity Need less photons Need more photons Spectral sensitivity curve 1 rod, 3 types of cones 3 cone pigments types Short (S) Btwn S & M (green-blue) S M L Not color specific 558 1 rod pigment 531 nm Long (L) 419 nm Medium (M) Absorption spectra E.g., blue, green, red Maximally responsive to these colours Spectral sensitivity associated with absorption spectra Weighted towards long wavelength cones Most prominent Convergence: Acuity vs Sensitivity Tradeoffs: Power vs grace Less light needed for rod receptors Also, differential convergence on to neurons Rod:ganglion cell, 120:1 Cone:ganglion cell, 6:1 Decreases threshold for ganglion response Foveal and peripheral vision Differential convergence Why periphery is blurry relative to fovea? Fovea: All cones Most acute But least sensitive What the use of sensitivity if you cant tell what it is? Foveation Z C H S K E T D K F L F G LAD N X Neural transformation Convergence is allows transformation of information Different forms of convergence allow diversity in response Up in the CNS circuits get more complex Thousands of interconnected neurons Electrical engineering Neural circuit designs: Excitation “feature” detectors Output of red neuron Preferred response No convergence Convergence Responsive to line length But not unique to line length Neural circuit designs: Excitation & inhibition More complex response properties Preferred response (cell likes medium sized lines! Transformation of information in ganglion cells Between photoreceptors and ganglion cells Horizontal Bipolar Amacrine Pattern of convergence btwn these cells Receptive fields Area of space (retina for vision) that when stimulated influences a neurons firing rate Receptive field properties The features of a stimulus that increase a neurons firing rate Receptive fields A neuron’s window onto the world Classical definition: Region of sensory surface (retina for vision) that when stimulated influences a neurons firing rate Receptive field properties The features of a stimulus that increase a neurons firing rate Simple: spot of light Complex: A friends face Journey through the visual system RF properties tell us about the development of perception Like the development of complex behaviour Early versus later stages Receptive field: Ganglion cells Restricted portion of space Small receptive fields (RF) RF properties Convergence from photoreceptors Center-surround antagonism On-cells (on center, off surround) Off-cells (off center, on surround) What is it for? Enhancing contrast Goal: Detection of change On-cell Lateral inhibition Center-surround Amacrine & horizontal cells Lateral network that allows cross-talk Transformation of information Spots of light at photoreceptors Center-surround at ganglion cells Ghosts in the machine Lateral inhibition and perception Experience of light is diminished by summation of inhibitory influences Result: Illusory Dark spots Maximal inhibition Reduced inhibition Lateral inhibition and perception Mach bands Dark and light bands at contrast borders Hyper-realism Raphael’s Madonna Perceptual contrast effects in renaissance drawings Lateral inhibition and perception Mach bands Transformation from physical to perceptual energies Transformation takes place in ganglion cells Objective Physical reality Subjective perception Lateral inhibition and perception Less inhibition from lighter side More inhibition from darker side Simultaneous contrast Perception of lightness is influenced by more than just lateral inhibition (LI) Lightness perception is achieved by ganglion cells alone Other higher-order contrast effects LI at ganglion cell insufficient to explain illusory perceptions of lightness Your visual system “reasons” Perceptual rationalization Hypothesis testing Dichoptic viewing White’s illusion Opposite of what would be predicted from LI “belongingness” Does understanding the retina explain vision? World projected on retina = vision? No Why? Illusory lightness Can’t be accounted for by retina alone Retinal representations of world is local Bits of lightness and darkness Need sharing of information It takes a village … Its all about sharing of information Retina has no global “representation” of Brad Pitt Photoreceptor A doesn’t talk with photoreceptor B Respond to small spots of light A B