PSYC 212 Perception PDF

PSYC 212 Perception Chapter 2: The First Steps in Vision: From Light to Neural Signals Outline A Little Light Physics Eyes That Capture Light Dark and Light Adaptation Retinal Information Processing Page 2 The First Steps in Vision Chapter 2 A Little L...

PSYC 212 Perception Chapter 2: The First Steps in Vision: From Light to Neural Signals Outline A Little Light Physics Eyes That Capture Light Dark and Light Adaptation Retinal Information Processing Page 2 The First Steps in Vision Chapter 2 A Little Light Physics Background Light: – A narrow band of electromagnetic radiation that can be conceptualized as a wave or a stream of photons Photon: – A quantum of visible light (or other form of electromagnetic radiation) demonstrating both particle and wave properties Page 3 The First Steps in Vision Chapter 2 A Little Light Physics Background Light can be absorbed, scattered, reflected, transmitted, or refracted – Absorbed: Energy (e.g., light) that is taken up and is not transmitted at all – Scattered: Energy that is dispersed in an irregular fashion When light enters the atmosphere, much of it is absorbed or scattered and never makes it to the perceiver Page 4 The First Steps in Vision Chapter 2 A Little Light Physics Background Light can be absorbed, scattered, reflected, transmitted, or refracted – Reflected: Energy that is redirected when it strikes a surface, usually back to its point of origin – Transmitted: Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface) – Refracted: Energy that is altered as it passes into another medium Page 5 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Cornea: The transparent “window” into the eyeball Page 6 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Aqueous humor: The watery fluid in the anterior chamber Page 7 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Crystalline lens: The lens inside the eye, which focuses light onto the back of the eye Page 8 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Pupil: The dark circular opening at the center of the iris in the eye, where light enters the eye Page 9 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Iris: The coloured part of the eye, a muscular diaphragm, that regulates light entering the eye by expanding and contracting the pupil Page 10 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Vitreous humor: The transparent fluid that fills the large chamber in the posterior part of the eye Page 11 The First Steps in Vision Chapter 2 Eyes That Capture Light The Human Eye Made up of various parts Retina: A light-sensitive membrane in the back of the eye that contains rods and cones. The lens focuses an image on the retina, which then sends signals to the brain through the optic nerve Page 12 The First Steps in Vision Chapter 2 Eyes That Capture Light Refraction Refraction is necessary to focus light rays onto the retina and this is accomplished by the lens – Accommodation: The process in which the lens changes its shape, thus altering its refractive power – Controlled by the ciliary muscles – Presbyopia: Literally “old sight” The age-related loss of accommodation, which makes it difficult to focus on near objects Page 13 The First Steps in Vision Chapter 2 Eyes That Capture Light Problems of Refraction The lens may focus the image either in front of or behind the retina – In these cases, corrective lenses are needed for normal vision Emmetropia: – The happy condition of no refractive error Myopia: – When light is focused in front of the retina and distant objects cannot be seen sharply – Nearsightedness Page 14 The First Steps in Vision Chapter 2 Eyes That Capture Light Problems of Refraction The lens may focus the image either in front of or behind the retina – In these cases, corrective lenses are needed for normal vision Hyperopia: – When light is focused behind the retina and near objects cannot be seen sharply – Farsightedness Astigmatism: – Unequal curving of one or more of the refractive surfaces of the eye, usually the cornea Page 15 The First Steps in Vision Chapter 2 Eyes That Capture Light Camera Analogy for the Eye F-stop = Iris/pupil – Regulates the amount of light coming into the eye Focus = Lens – Changes shape to change focus Film = Retina – Records the image Page 16 The First Steps in Vision Chapter 2 Eyes That Capture Light Fundus Using the ophthalmoscope, doctors can view the back surface of patients’ eyes, called the fundus Page 17 The First Steps in Vision Chapter 2 Eyes That Capture Light The Optic Disc The location where the axons of the retinal ganglion cells leave the eye via the optic nerve And where blood vessels enter the eye Responsible for the blind spot Page 18 The First Steps in Vision Chapter 2 Eyes That Capture Light Photoreceptors Cells in the retina that initially transduce light energy into neural energy – named for their shapes Rods: – Specialized for night vision – Respond well in low luminance conditions – Do not process colour Cones: – Specialized for daytime vision, fine visual acuity, and colour – Respond best in high luminance conditions Page 19 The First Steps in Vision Chapter 2 Eyes That Capture Light Photoreceptors Light passes through several layers of cells before reaching rods and cones – Light activates a photoreceptor Which signals the horizontal and bipolar cells that synapse with it – Bipolar cells connect to amacrine cells and ganglion cells Page 20 The First Steps in Vision Chapter 2 Eyes That Capture Light Photoreceptors The distribution of rods and cones is not constant over the retina Cones process colour; rods do not – This means that you have very poor colour vision in your periphery – It may seem as if your entire field of view has full-resolution colour, but it does not Page 21 The First Steps in Vision Chapter 2 Eyes That Capture Light Photoreceptors Page 22 The First Steps in Vision Chapter 2 Eyes That Capture Light Retinal Size Vision scientists measure the size of visual stimuli by how large an image appears on the retina, not by how large the object is The standard way to measure retinal size is in terms of “degrees of visual angle” – Your useful field of view is ~180–190 degrees of visual angle – Rule of thumb: If you hold your thumb out at arms length, the width of your thumbnail is about 2 degrees of visual angle Page 23 The First Steps in Vision Chapter 2 Eyes That Capture Light Retinal Size In summary: The visual angle of an object is a function of both its actual size and distance from the observer – The farther away something is, the smaller the visual angle – The larger something is, the larger the visual angle – These two factors play off of each other Page 24 The First Steps in Vision Chapter 2 Eyes That Capture Light Fovea and Periphery Properties Page 25 The First Steps in Vision Chapter 2 Dark and Light Adaptation One of the most remarkable things about the human visual system is the incredible range of luminance levels we can adjust to Two mechanisms for dark and light adaptation: 1. Pupil dilation 2. Photoreceptors and their replacement Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels Page 26 The First Steps in Vision Chapter 2 Dark and Light Adaptation Pupil Dilation When we are in bright light, our pupils constrict to limit the amount that gets into the retina In dark environments, our pupils dilate to let in more light and improve our vision Page 27 The First Steps in Vision Chapter 2 Dark and Light Adaptation Photoreceptors & Their Replacement Rods are more sensitive to light than cones – And we have more rods in the periphery of our vision Therefore, our peripheral vision is better at detecting images in low light conditions than our foveal vision – e.g., when trying to see a dimly lit star, it helps to look just to the side of it rather than right at it By looking to the side, we use rods rather than cones Page 28 The First Steps in Vision Chapter 2 Dark and Light Adaptation Photoreceptors & Their Replacement The amount of photopigment available in photoreceptors changes over time – The more light entering the retina, the faster the photopigments are used up, and the fewer photopigments there are to process more light – The less light entering the retina, the more slowly photopigments are used up, and the more photopigments there are to process what little light is there Page 29 The First Steps in Vision Chapter 2 Dark and Light Adaptation Putting it Together The visual system regulates the amount of light entering the eye and ignores whatever variation in overall light level is left over – In bright light, the pupil dilates, letting in less light – Next, the number of photopigments in the photoreceptors decreases over a few minutes Page 30 The First Steps in Vision Chapter 2 Dark and Light Adaptation Putting it Together Being light-adapted means that even though there are more photons coming into the eye, there are fewer photopigments available to process them, so some of the light is “thrown away” In this sense, the remaining variations in light are ignored, beyond whatever level of luminance the eye is adapted to Page 31 The First Steps in Vision Chapter 2 Dark and Light Adaptation When Good Retina Goes Bad Age-related macular degeneration (AMD): – A disease associated with aging that affects the macula The central part of the retina containing the fovea – AMD gradually destroys sharp central vision Resulting in a blind spot in the visual field called a scotoma Page 32 The First Steps in Vision Chapter 2 Dark and Light Adaptation When Good Retina Goes Bad Retinitis pigmentosa (RP): – A family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium – Many people may not notice the onset of retinitis pigmentosa at first because it primarily affects peripheral vision The person might notice problems driving at night or in low-light conditions Page 33 The First Steps in Vision Chapter 2 Dark and Light Adaptation When Good Retina Goes Bad New technologies may help people with visual field loss: – Prosthetic retinas May replace damaged photoreceptors with an implanted device – Gene therapies Can improve functioning of surviving photoreceptors – Chemical therapies Convert retinal ganglion cells into photoreceptors Page 34 The First Steps in Vision Chapter 2 Dark and Light Adaptation When Good Retina Goes Bad Retinal prostheses Page 35 The First Steps in Vision Chapter 2 Retinal Information Processing Photoreceptors Photoreceptors contain: – An outer segment Adjacent to the pigment epithelium Stores visual pigments – An inner segment Manufactures visual pigments – A synaptic terminal Page 36 The First Steps in Vision Chapter 2 Retinal Information Processing Photoreceptors Photoreceptors also contain: – An opsin: a chromophore that captures photons, A type of protein whose structure determines the wavelength of light to which the photoreceptor responds Rods have rhodopsin Cones have three different opsins – Which respond to long, medium or short wavelengths Some photoreceptors contain melanopsin – Monitor ambient light levels and influence our sleep/wake cycle Page 37 The First Steps in Vision Chapter 2 Retinal Information Processing Photoreceptors Cones work best in photopic (high-illumination) situations Rods work best in scotopic (low-illumination) situations Page 38 The First Steps in Vision Chapter 2 Retinal Information Processing Photoactivation When light hits a photoreceptor, the process of photoactivation begins – Photoreceptors become hyperpolarized Changes in photoreceptor activation are communicated to the bipolar cells in the form of graded potentials – These vary continuously in their amplitudes Bipolar cells synapse with retinal ganglion cells (RGCs) – RGCs fire in an all-or-none fashion rather than in graded potentials, since the APs travel farther Page 39 The First Steps in Vision Chapter 2 Retinal Information Processing The Retina’s Horizontal Pathway Horizontal cells: – Specialized retinal cells that run perpendicular to the photoreceptors and contact both photoreceptors and bipolar cells – Are responsible for lateral inhibition Creates the center-surround receptive field structure of retinal ganglion cells Amacrine cells: – Synapse horizontally between bipolar cells and retinal ganglion cells – Remain a mystery Have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time) Page 40 The First Steps in Vision Chapter 2 Retinal Information Processing The Retina’s Vertical Pathway Consists of photoreceptors, bipolar cells, and ganglion cells Bipolar cell: – Synapses with one or more rods or cones and with horizontal cells, then passes the signals to ganglion cells – Diffuse bipolar cell: Receives input from multiple photoreceptors – Midget bipolar cell: Receives input from a single cone Page 41 The First Steps in Vision Chapter 2 Retinal Information Processing The Retina’s Vertical Pathway Consists of photoreceptors, bipolar cells, and ganglion cells P ganglion cells: – Connect to the parvocellular pathway Receive input from midget bipolar cells Parvocellular pathway is involved in fine visual acuity, colour, and shape processing – Poor temporal resolution but good spatial resolution Page 42 The First Steps in Vision Chapter 2 Retinal Information Processing The Retina’s Vertical Pathway Consists of photoreceptors, bipolar cells, and ganglion cells M ganglion cells: – Connect to the magnocellular pathway Receive input from diffuse bipolar cells Magnocellular pathway is involved in motion processing – Excellent temporal resolution but poor spatial resolution Page 43 The First Steps in Vision Chapter 2 Retinal Information Processing Receptive Fields The region on the retina in which stimuli influence a neuron’s firing rate Kuffler mapped out the receptive fields of individual retinal ganglion cells in the cat – Found that the spatial layout is concentric Page 44 The First Steps in Vision Chapter 2 Retinal Information Processing Receptive Fields ON-center ganglion cells: – Excited by light that falls on their center and inhibited by light that falls in their surround OFF-center ganglion: – Inhibited when light falls in their center and excited when light falls in their surround Page 45 The First Steps in Vision Chapter 2 Retinal Information Processing Retinal Ganglion Cells Why center-surround receptive fields? – Each RGC will respond best to spots of a particular size And respond less to spots that are too big or too small – RGCs act like a filter for information coming to the brain – Are most sensitive to differences in intensity of light between center and surround, and are relatively unaffected by average intensity Luminance variations tend to be smooth within objects and sharp between objects Thus, center-surround receptive fields help to emphasize object boundaries Page 46 The First Steps in Vision Chapter 2 Retinal Information Processing Retinal Ganglion Cells P ganglion cells: – Small receptive fields, high acuity, work best in high luminance situations, sustained firing – Provide information mainly about the contrast in the retinal image M ganglion cells: – Large receptive fields, low acuity, work best in low luminance situations, burst firing – Provide information about how an image changes over time Page 47 The First Steps in Vision Chapter 2 Retinal Information Processing Retinal Ganglion Cells Intrinsically photo sensitive retinal ganglion cells (ipRGCs) in the developing retina – ipRGCs respond to light, but they receive no input from rods or cones – First photoreceptors to mature in the retina Send light signals to the developing brain, as early as in the second trimester Babies in the womb can detect light long before they can see images Page 48 The First Steps in Vision Chapter 2 Retinal Information Processing Is One Photon Enough To See? Question: – Previous research suggests people can detect as few as five photons. Can they detect a single photon? Hypothesis: – People can detect a single photon Test: – Researchers built a single-photon quantum light source Results: – People could detect single photons at above chance levels Conclusion: – Evolution has optimized the visual system to detect a single photon Page 49 The First Steps in Vision Chapter 2  NEXT TIME: CHAPTER 3 SPATIAL VISION: FROM SPOTS TO STRIPES Page 50 The First Steps in Vision Chapter 2