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Sensation & Perception • Ch. 3: Vision © Takashi Yamauchi (Dept. of Psychology, Texas A&M University) • Main topics – convergence – Inhibition, lateral inhibition and lightness perception – Interactions between neurons – Feature detectors ch 3 1 Question 1 • What do these devices have in common? ch 3 2 These devices make use of electromagnetic waves Capture electromagnetic waves and transform them into various forms. ch 3 3 What does the eye do? Transducing light energy into electrical energy ch 3 4 Transduction • • • • • Light enters the eye A photon hits a receptor changes the shape of pigment molecules triggers massive chemical reactions generate electrical signals ch 3 5 ch 3 6 • Solar cells (photovoltaics) produce electricity in a similar way as our eyes do. ch 3 7 Rods and cones • • • • Morphology Distribution on the retina Dark adaptation Spectral sensitivity ch 3 8 Photo receptors: Rods and cones • Rods have bigger outer segments than cones. • Why? ch 3 9 Outer segments capture photons • Bigger outer segments can capture more light. • Rods have bigger outer segments. – Rods allow us to see in the dark. – Cones are mainly for day vision. – Cones are for color perception. ch 3 10 How can we see objects? • How can we see a book? • How can we see a desk? • Why don’t we see light? ch 3 11 Reflection of light • What we see is a reflection of light. • Different objects reflect different wavelengths, – different objects show different colors ch 3 12 • Photo receptors in the eye are geared to capture different wavelengths ch 3 13 Lens: focuses light rays. Iris: control the size of the pupil regulating the amount of light reaching the retina Retina: a layer of receptor cells Receptor cells rods and cones ch 3 14 Retina: ch 3 15 • Photo receptors are facing away from the light source. • The optic nerve carries neural information to this spot. • What happens? – No receptors, no vision ch 3 blind spot 16 Some messages: how to improve your vision • Massage your eye muscles • Eat carrots • Massage the back of your head. ch 3 17 Rods and cones • • • • Morphology Distribution on the retina Dark adaptation Spectral sensitivity ch 3 18 The distribution of cones and rods on the retina • Cones are concentrated mainly on the fovea. • There are no rods on the fovea. • We move eyes to capture images on the fovea. ch 3 19 Demonstration • Blind spot ch 3 20 Rods and cones are different • In their dark adaptation rates ch 3 21 Dark adaptation rates of rods and cones • When you enter a dark room from outside, you can’t see well at first. But gradually, your eyes are adjusted to the dark, and see better. ch 3 22 • In terms of the activity of neurons, what is the difference between A and B ? Any guess? B. A. ch 3 23 Measuring the electrical activity of a neuron directly by inserting a thin needle into animal brains. ch 3 24 The frequency of action potential 0 t 0 t The number of action potential emitted by a neuron is correlated with Time the intensity of the stimulus. Time ch 3 0 25 t Time Neural Processing by Convergence • Why are rods more sensitive to light than cones? • Because rods are bigger than cones. • Because rods and cones are connected to ganglion cells in different manners. ch 3 26 Activities of neurons can be schematically shown as a1 a2 a3 a4 The firing rate of neuron B is determined by the activation sent by neurons a1-a4. B ch 3 27 Ganglion cell • Ganglion cell ch 3 28 • Convergence: • The ratio of connections with two groups of neurons. • Rods vs. Ganglion cells – 120:1 • Cones vs. Ganglion cells – 6:1 ch 3 29 ch 3 30 Why does this matter? • How is this related to the higher sensitivity of rods? ch 3 31 The cones result in better detail vision than the rods • Visual acuity – How far apart are two dots? ch 3 32 The frequency of action potential 0 t 0 t The number of action potential emitted by a neuron is correlated with Time the intensity of the stimulus. Time ch 3 0 33 t Time ch 3 34 ch 3 35 ch 3 36 Fig. 2.11, p.53 ch 3 37 ch 3 38 ch 3 39 • Demonstration: • On a scratch paper, draw two vertical lines of about 2 inches (1/2 inch apart). • Close your left eye, and focus your right eye on your index figure, and move the figure. • At some point, you can’t distinguish the two vertical lines. ch 3 40 The distribution of cones and rods on the retina • Cones are concentrated mainly on the fovea. • There are no rods on the fovea. • We move eyes to capturech 3images on the fovea. 41 Visual Cortex ch 3 42 ch 3 43 ch 3 44 The cones result in better detail vision than the rods • Visual acuity – How far apart are two dots? ch 3 45 Neurons • How do you detect there are two separate dots (lights)? ch 3 46 ch 3 47 • How do you detect there are two separate dots (lights)? ch 3 48 • Rods are bigger than cones • Convergence: ch 3 49 Lateral Inhibition & Mach bands ch 3 50 ch 3 51 Herman grid ch 3 52 ch 3 53 The frequency of action potential 0 t 0 t The number of action potential emitted by a neuron is correlated with Time the intensity of the stimulus. Time ch 3 0 54 t Time Questions: What happens to B? 0 ch 3 t 55 Questions: What happens to B? Excitatory Inhibitory ch 3 56 Receptive field • The receptive field of a neuron in the visual system is the area on the retina that influences the firing rate (action potential) of the neuron. • Measuring the receptive field of a ganglion cell ch 3 57 Receptive field of a ganglion cell Measuring the frequency of action potentials elicited by this ganglion cell. Cones Ganglion cell B ch 3 58 Receptive field of a ganglion cell Ganglion cell Cones 12 3 4 5 6 7 B Firing rate of B ch 3 4 3-5 2-6 2-7 59 Questions: What happens to B? ch 3 60 Measuring the receptive field of a ganglion cell Change the size of the stimulus and61 see ch 3 the way a ganglion cell respond Cones Ganglion cell 12 3 4 5 6 7 B ch 3 62 Excitatorycenterinhibitorysurround receptive field Excitatory Inhibitory ch 3 63 Questions: What happens to B? Excitatory Inhibitory ch 3 64 Excitatory and inhibitory connections • What neurons transmit is electricity. • Some neurons send positive (excitatory) signals (+) increase the firing rate of the target neuron. • some neurons send negative (inhibitory) signals (-) depress the firing rate of the target neuron. ch 3 65 Spatial Summation a1 + a2 + a3 a4 + + B =4 a1 a2 The firing rate + + of neuron B can be expressed by the overall summation of the signals that B receives. ch 3 c1 c2 c3 c4 a3 a4 + + - - - - B =0 66 ch 3 67 ch 3 68 How does this happen? ch 3 69 =sum(B =sum(B) ch 3 70 ch 3 71 Fig. 3-6, p. 50 ch 3 72 Fig. 3-7, p. 51 Why is this important? • help you to detect the edge of a figure ch 3 73 ch 3 74 Light intensity Perceived Light intensity location location ch 3 75 Physical stimuli ch 3 Your perception 76 Lateral inhibition ch 3 77 100 20 -- + + ---- + + --- + + ---- + + -- ch 3 78 ch 3 79 ch 3 80 White’s illusion • Can you explain this by lateral inhibition? ch 3 81 ch 3 82 ch 3 83 ch 3 84 ch 3 85 ch 3 86 ch 3 87 ch 3 88 Application: Machine vision • Implementing the mechanism of lateral inhibition to a computer program. ch 3 89 Image ch 3 90 ch 3 91 ch 3 92 ch 3 93 • Edge detection algorithm – Zero-crossing ch 3 94