Psy393: Cognitive Neuroscience. Prof. Anderson Department of Psychology Week 3

Transcription

1 Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Week 3

2 The Eye: Proof for the existence of God?

3 And then there was light Optics Perception Absorption Eye is receiver not sender Plato Euclid Send & receive Echolocation Sonar

4 Perception is relative Perception not of environment (realism) but of our interpretation (relativism) Electromagnetic spectrum Butterflies see ultraviolet markings Snakes see infrared waves The first stage of transformation Retinal sensitivity to visible light nanometers (nm is 1/100 millionth of a meter)

5 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

6 Retina: Rods & Cones Not just morphological differences 2 types of vision Photopic Diurnal Colour Bright light Scotopic Nocturnal Monochromatic Dim light Predator vs prey

7 Distribution of rods & cones 5 million cones per retina 1% are in the fovea, 99% in periphery 120 million rods per retina None in fovea Rods:cones 20:1 in periphery Fovea Periphery

8 Retina: Its all backwards Pigment epithelium at back of eye Nourishment of photoreceptors Light must pass through neural machinery Back Front Epithelial layer

9 Why don t t we see the cells/blood vessels in our eyes? Images stabilized on retina disappear How s s that for perception!

10 The retinal black hole The blind spot Ganglion cells >optic nerve exit eye From fovea

11 Filling in the blind spot filling-in the blind spot? Ramachandran example Inference or perception

12 Transduction: Light to energy Visual pigment molecules Opsin & retinal Retinal Light reactive chemical Absorbs a single photon! Isomerization: Morphing Change in shape of ion channels Change in membrane potential Electricity! Amplification: 1 pigment molecule > > cascade of million others Perception of light Can perceive a single rod activation Photoreceptor

13 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 Dar Light Time

14 Dark adaptation: Switching visual systems Dark adaptation curve Switch from photopic to scotopic vision Max adapt Cones Test fovea 3-55 min Rods Rod monochromat min Racoon vision?

15 In living colour: Spectral sensitivity Monochromatic light 1 wavelength Method of adjustment Fovea (cones) Periphery (rods) After dark adaptation Sensitivity = 1/threshold Overall cone sensitivity Need less photons Need more photons Cones Threshold Spectral sensitivity curve

16 1 rod, 3 types of cones 3 cone pigments types Short (S) 419 nm Medium (M) 531 nm Long (L) rod pigment Btwn S & M (green-blue) Not color specific E.g., blue, green, red Maximally responsive to these colours Spectral sensitivity associated with absorption spectra Weighted towards long wavelength cones Most prominent Absorption spectra S M L

17 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

18 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 L A D N X

19 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

20 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

21 Neural circuit designs: Excitation & inhibition More complex response properties Preferred response (cell likes medium sized lines!

22 Transformation of information Between photoreceptors and ganglion cells Horizontal Bipolar Amacrine Pattern of convergence btwn these cells in ganglion cells

23 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

24 Receptive fields A neuron s 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

25 Journey through the visual system RF properties tell us about the development of perception Like the development of complex behaviour Early versus later stages

26 Receptive field: Ganglion Restricted portion of space cells Small receptive fields (RF) Convergence from photoreceptors RF properties Center-surround antagonism On-cells (on center, off surround) Off-cells (off center, on surround) On-cell What is it for? Enhancing contrast Goal: Detection of change

27 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

28 Ghosts in the machine

29 Lateral inhibition and perception Experience of light is diminished by summation of inhibitory influences Maximal inhibition Result: Illusory Dark spots Reduced inhibition

30 Lateral inhibition and perception Mach bands Dark and light bands at contrast borders Hyper-realism realism Perceptual contrast effects in renaissance drawings Raphael s Madonna

31 Lateral inhibition and perception Mach bands Objective Physical reality Subjective perception Transformation from physical to perceptual energies Transformation takes place in ganglion cells

32 Lateral inhibition and perception Less inhibition from lighter side More inhibition from darker side

33 Simultaneous contrast Perception of lightness is influenced by more than just lateral inhibition (LI) Lightness perception is achieved by ganglion cells alone

34 Other higher-order contrast effects LI at ganglion cell insufficient to explain illusory perceptions of lightness White s s illusion Opposite of what would be predicted from L belongingness Your visual system reasons Perceptual rationalization Hypothesis testing Dichoptic viewing

35 Does understanding the retina explain vision? World projected on retina = vision? No Why? Illusory lightness Can t t be accounted for by retina alone Retinal representations of world is local Bits of lightness and darkness Need sharing of information

36 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