Lecture 3 - The First Steps in Vision: From Light to Neural Signals PDF

Lecture 3: The First Steps in Vision: From Light to Neural Signals © Oxford University Press © 2024 2022 Oxford University...

Lecture 3: The First Steps in Vision: From Light to Neural Signals © Oxford University Press © 2024 2022 Oxford University Press  2 Chapter 2 The First Steps in Vision: From Light to Neural Signals 2 2.1 A Little Light Physics 2.2 Eyes That Capture Light 2.3 Dark and Light Adaptation 2.4 Retinal Information Processing © Oxford University Press © 2024 2022 Oxford University Press  3 2.1 A Little Light Physics 3 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. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.1 The electromagnetic energy spectrum © 2024 Oxford University Press  5 2.1 A Little Light Physics 5 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. © Oxford University Press © 2024 2022 Oxford University Press  6 2.1 A Little Light Physics 6 – 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. © Oxford University Press © 2024 2022 Oxford University Press  7 2.2 Eyes That Capture Light 7 The human eye is made up of various parts – Cornea: The transparent “window” into the eyeball. – Aqueous humor: The watery fluid in the anterior chamber. – Crystalline lens: The lens inside the eye, which focuses light onto the back of the eye. © Oxford University Press © 2024 2022 Oxford University Press  8 2.2 Eyes That Capture Light 8 The human eye is made up of various parts (continued) – Pupil: The dark circular opening at the center of the iris in the eye, where light enters the eye. – Iris: The colored part of the eye, a muscular diaphragm, that regulates light entering the eye by expanding and contracting the pupil. © Oxford University Press © 2024 2022 Oxford University Press  9 2.2 Eyes That Capture Light 9 The human eye is made up of various parts (continued) – Vitreous humor: The transparent fluid that fills the large chamber in the posterior part of the eye. – 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. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.2 The human right eye in cross section (viewed from above) © 2024 Oxford University Press  11 2.2 Eyes That Capture Light 11 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. – Presbyopia: Literally “old sight”; the age-related loss of accommodation, which makes it difficult to focus on near objects © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.4 The precipitous drop in amplitude of accommodation with age © 2024 Oxford University Press  13 2.2 Eyes That Capture Light 13 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. © Oxford University Press © 2024 2022 Oxford University Press  14 2.2 Eyes That Capture Light 14 Problems of refraction (continued) – 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. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.5 Optics of the human eye © 2024 Oxford University Press  16 2.2 Eyes That Capture Light 18 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. Using the ophthalmoscope, doctors can view the back surface of patients’ eyes, called the fundus. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.8 Fundus of the right eye of a human © 2024 Oxford University Press FIGURE 2.9 Your blind spot © 2024 Oxford University Press  19 2.2 Eyes That Capture Light 21 Photoreceptors: Cells in the retina that initially transduce light energy into neural energy; named for their shapes. – Rods: Photoreceptors specialized for night vision. Respond well in low luminance conditions Do not process color – Cones: Photoreceptors specialized for daytime vision, fine visual acuity, and color. Respond best in high luminance conditions © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.11 Photoreceptors Rod and cone. © 2024 Oxford University Press  21 2.2 Eyes That Capture Light 23 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 are connected to amacrine cells and ganglion cells. – Ganglion cells have axons that leave the retina through the optic disc (blind spot). © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.2 The human right eye in cross section (viewed from above) © 2024 Oxford University Press FIGURE 2.10 Optical coherence tomography of the retina © 2024 Oxford University Press  24 2.2 Eyes That Capture Light 26 The distribution of rods and cones is not constant over the retina. Cones process color; rods do not. This means that you have very poor color vision in your periphery. It may seem as if your entire field of view has full- resolution color, but it does not. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.12 Photoreceptor density across the retina © 2024 Oxford University Press Cone and rod ratios vary by species depending on ecological niche © 2024 Oxford University Press  27 2.2 Eyes That Capture Light 28 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.” Rule of thumb: If you hold your thumb out at arms length, the width of your thumbnail is about 2 degrees of visual angle. In summary: The visual angle of an object is a function of both its actual size and distance from the observer. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.13 The “rule of thumb” © 2024 Oxford University Press TABLE 2.1 Properties of the fovea and periphery in human vision Property Fovea Periphery Photoreceptor type Mostly cones Mostly rods Bipolar cell type Midget Diffuse Convergence Low High Receptive-field size Small Large Acuity (detail) High Low Light sensitivity Low High © 2024 Oxford University Press  30 2.3 Dark and Light Adaptation 31 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: – Pupil dilation – Photoreceptors and their replacement © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.14 Luminance levels © 2024 Oxford University Press FIGURE 2.16 Two pupils © 2024 Oxford University Press  33 2.3 Dark and Light Adaptation 34 Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels. © Oxford University Press © 2024 2022 Oxford University Press  34 2.3 Dark and Light Adaptation 35 – 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. © Oxford University Press © 2024 2022 Oxford University Press  35 2.3 Dark and Light Adaptation 36 – The amount of photopigment available in photoreceptors changes over time. 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. © Oxford University Press © 2024 2022 Oxford University Press  36 2.3 Dark and Light Adaptation 37 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. © Oxford University Press © 2024 2022 Oxford University Press  37 2.3 Dark and Light Adaptation 38 – 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. © Oxford University Press © 2024 2022 Oxford University Press  38 2.3 Dark and Light Adaptation 39 Age-related macular degeneration (AMD): A disease associated with aging that affects the macula. AMD gradually destroys sharp central vision. – Macula: The central part of the retina containing the fovea. AMD causes central vision loss, resulting in a blind spot in the visual field called a scotoma. © Oxford University Press © 2024 2022 Oxford University Press  39 2.3 Dark and Light Adaptation 40 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. © Oxford University Press © 2024 2022 Oxford University Press  40 2.3 Dark and Light Adaptation 41 New technologies may help people with visual field loss: – Prosthetic retinas o May replace damaged photoreceptors with an implanted device – Gene therapies Can improve functioning of surviving photoreceptors – Chemical therapies Convert retinal ganglion cells into photoreceptors © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.17 Retinal prostheses © 2024 Oxford University Press New retinal prostheses Science Inc. (2024) © 2024 Oxford University Press New retinal prostheses Science Inc. (2024) © 2024 Oxford University Press  44 2.4 Retinal Information Processing 47 Once photoactivation starts, photoreceptors become hyperpolarized (negatively charged). Changes in photoreceptor activation are communicated to the bipolar cells in the form of graded potentials. – Graded potentials vary continuously in their amplitudes. Bipolar cells synapse with retinal ganglion cells, which fire in an all-or-none fashion rather than in graded potentials. © Oxford University Press © 2024 2022 Oxford University Press  45 2.4 Retinal Information Processing 50 The retina’s horizontal pathway: horizontal and amacrine cells – Horizontal cells: Specialized retinal cells that run perpendicular to the photoreceptors and contact both photoreceptors and bipolar cells. These cells are responsible for lateral inhibition, which creates the center-surround receptive field structure of retinal ganglion cells. © Oxford University Press © 2024 2022 Oxford University Press  46 2.4 Retinal Information Processing 51 The retina’s horizontal pathway: horizontal and amacrine cells (continued) – Amacrine cells: These cells synapse horizontally between bipolar cells and retinal ganglion cells. These cells have been implicated in contrast enhancement and temporal sensitivity (detecting light patterns that change over time). © Oxford University Press © 2024 2022 Oxford University Press  47 2.4 Retinal Information Processing 52 The retina’s vertical pathway: 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. © Oxford University Press © 2024 2022 Oxford University Press  48 2.4 Retinal Information Processing 53 The retina’s vertical pathway: photoreceptors, bipolar cells, and ganglion cells (continued) – P ganglion cells: Connect to the parvocellular pathway. Receive input from midget bipolar cells. Parvocellular pathway is involved in fine visual acuity, color, and shape processing; poor temporal resolution but good spatial resolution. © Oxford University Press © 2024 2022 Oxford University Press  49 2.4 Retinal Information Processing 54 The retina’s vertical pathway: photoreceptors, bipolar cells, and ganglion cells (continued) – M ganglion cells: Connect to the magnocellular pathway. Receive input from diffuse bipolar cells. Magnocellular (“large cell”) pathway is involved in motion processing; excellent temporal resolution but poor spatial resolution. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.19 Retinal ganglion cells © 2024 Oxford University Press  51 2.4 Retinal Information Processing 56 Receptive field: 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. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.20 Retinal ganglion cell receptive fields © 2024 Oxford University Press  53 2.4 Retinal Information Processing 58 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. © Oxford University Press © 2024 2022 Oxford University Press  54 2.4 Retinal Information Processing 60 Why center-surround receptive fields? – Each ganglion cell will respond best to spots of a particular size (and respond less to spots that are too big or too small). Retinal ganglion cells act like a filter for information coming to the brain. © Oxford University Press © 2024 2022 Oxford University Press  55 2.4 Retinal Information Processing 61 Why center-surround receptive fields? (continued) – Retinal ganglion cells 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. © Oxford University Press © 2024 2022 Oxford University Press FIGURE 2.21 Mach bands © 2024 Oxford University Press  57 2.4 Retinal Information Processing 63 P and M ganglion cells, revisited – 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 © Oxford University Press © 2024 2022 Oxford University Press  58 2.4 Retinal Information Processing 64 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 Establish circadian rhythm linked to vision? © Oxford University Press © 2024 2022 Oxford University Press  2 Next time – Spatial vision © Oxford University Press © 2024 2022 Oxford University Press