Sensation & Perception• Ch. 3: Vision© Takashi Yamauchi (Dept. of Psychology, Texas A&M University)•Main topicsMain topics– convergence–Inhibition, lateral inhibition and lightness perception,gpp– Interactions between neurons–Feature detectorsch 3 1Question 1• What do these devices have in common?ch 3 2These devices make use of lielectromagnetic wavesCapture electromagnetic waves and transform them into various forms.ch 3 3What does the eye do? Transducing light energy into electrical energelectrical energych 3 4Transduction•  Light enters the eye•  A photon hits a receptor pp•  changes the shape of pigment molecules•  triggers massive chemical reactions•  generate electrical signalsch 3 5ch 3 6Rods and cones• Morphology•Distribution on the retinaDistribution on the retina• Dark adaptationSliii•Spectral sensitivitych 3 7Photo receptors: Rods and conesPhoto receptors: Rods and cones• Rods have bigger outer segments than cones.• Why?ch 3 8Outer segments capture photonsOuter segments capture photonsBi t t t li ht•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 mainly for day vision.– Cones are for color perception.ch 3 9How can we see objects?• How can we see a book?• How can we see a desk?• Why don’t we see light?ch 3 10Reflection of lightReflection of light• What we see is a reflection of light.• Different objects reflect different wavelengths, g,–  different objects show different colorsch 3 11• Photo receptors in the eye are geared to pygcapture different wavelengthsch 3 12Lens: focuses light rays.Iris: control the size of the pupil  regulating the amount of light reaching the retinaRetina: a layer of receptor cellsRtllddch 3 13Receptor cells rods and cones• Photo receptors are facing away from the light source.pgyg• The optic nerve carries neural information to this spot. •What happens?ch 3 14What happens?– No receptors, no vision  blind spotSome messages: how to improve yourSome messages: how to improve your vision• Massage your eye muscles• Eat carrots• Massage the back of your head.ch 3 15Rods and cones• Morphology•Distribution on the retinaDistribution on the retina• Dark adaptationSliii•Spectral sensitivitych 3 16The distribution of cones and rodsThe distribution of cones and rods on the retina• Cones are concentrated mainly yon the fovea.• There are no rods on the fovea.• We move eyes to capture images on the foveach 3 17on the fovea.Dark adaptation rates of rods andDark adaptation rates of rods and cones• When you enter a dark room from outside, you can’t see well at first. But gradually, ygy,your eyes are adjusted to the dark, and see better.ch 3 18Reviewch 3 19Visual perception• What is the difference between (a) & (b)?(a)• What is going on in your head when you see (a)head when you see (a) versus when you see (b)?(b)()ch 3 20What’s going on?• When you see the square, what’s going on?• How do you find out?ch 3 21• In terms of the activity of neurons, what is the difference betweenA and B ?Any guess?A.B.ch 3 22Measuring the electrical activity of a neuron ydirectly by inserting a thin needle into animal brains.ch 3 23The frequency of action potentialThe number of paction potential emitted by a neuron isTime0tneuron is correlated with the intensity of hilthe stimulus.Time0tch 3 24Time0tNeural Processing byNeural Processing by Convergence• Why are rods more sensitive to light than cones?ch 3 25Activities of neurons can beActivities of neurons can be schematically shown asa1 a2 a3 a4The firing rate of neuron B is determined by the activation sent by neurons a1-a4.Bch 3 26Ganglion cellGli ll•Ganglion cellch 3 27• Convergence:•The ratio of connections with two groups ofThe ratio of connections with two groups of neurons.•Rods vs Ganglion cells•Rods vs. Ganglion cells– 120:1li ll• Cones vs. Ganglion cells–6:1ch 3 28ch 3 29Why does this matter?Why does this matter?•How is this related to the higher sensitivityHow is this related to the higher sensitivity of rods?ch 3 30The cones result in better detailThe cones result in better detail vision than the rods• Visual acuity–How far apart are two dots?How far apart are two dots?ch 3 31The frequency of action potentialThe number of paction potential emitted by a neuron isTime0tneuron is correlated with the intensity of hilthe stimulus.Time0tch 3 32Time0tch 3 33ch 3 34ch 3 35Fig. 2.11, p.53ch 3 36ch 3 37ch 3 38•Demonstration:Demonstration:• On a scratch paper, draw two vertical lines of about 2 inches (1/2 inch apart)lines of about 2 inches (1/2 inch apart).• Close your left eye, and focus your ih id fi dright eye on your index figure, and move the figure.• At some point, you can’t distinguish the two vertical lines.ch 3 39The distribution of cones andThe distribution of cones and rods on the retinaCdilhf•Cones are concentrated mainly on the fovea.• There are no rods on the fovea.ch 3 40• We move eyes to capture images on the fovea.Visual Cortexch 3 41ch 3 42ch 3 43The cones result in better detailThe cones result in better detail vision than the rods• Visual acuity–How far apart are two dots?How far apart are two dots?ch 3 44Neurons• How do you detect there are two separate dots (lights)?ch 3 45ch 3 46• How do you detect there are two d(lih)?separate dots (lights)?ch 3 47• Rods are bigger than cones• Convergence: gch 3 48Lateral Inhibition & Mach bandsLateral Inhibition & Mach bandsch 3 49ch 3 50Herman gridch 3 51ch 3 52The frequency of action potentialThe number of paction potential emitted by a neuron isTime0tneuron is correlated with the intensity of hilthe stimulus.Time0tch 3 53Time0tQuestions: What happens to B?Qpp0tch 3 54Questions: What happens to B?EiIhibich 3 55ExcitatoryInhibitoryReceptive field• The receptive field of a neuron in the visual system is the area on the retina thatsystem is the area on the retina that influences the firing rate (action potential) of the neuron.the neuron.• Measuring the receptive field of a ganglion cellch 3 56Receptive field of a ganglion cellMeasuring the frequency of actionfrequency of action potentials elicited by this ganglion cell.ConesGanglion ConescellBch 3 57BReceptive field of a ganglion cellConesGanglion cell12 3 4 5B5 6 7Firing rate of Bch 3 584 3-5 2-6