Vision: From Light to Neural Signals ch 2 notes - PDF

Vision: From Light to Neural Signals Sensation and Perception The Lighter Side of Physics š Light: A narrow band in the electromagnetic spectrum that can be expressed as either: š A wave š A stream of photons š Wave: a disturbance that travels through a medium from one location to anoth...

Vision: From Light to Neural Signals Sensation and Perception The Lighter Side of Physics š Light: A narrow band in the electromagnetic spectrum that can be expressed as either: š A wave š A stream of photons š Wave: a disturbance that travels through a medium from one location to another location without causing any permanent displacement of the medium š Photon: a quantum of visible light (or other form of electromagnetic radiation) demonstrating both particle and wave properties. Try this at home… š You can do a simple experiment at home that shows that light behaves like a wave š The double slit experiment: https://www.youtube.com/w atch?v=kKdaRJ3vAmA  Shedding a little light on the visible spectrum… This spectrum is shown in logarithmic scale, meaning the length has been shortened so we can see how the visible spectrum fits into the full electromagnetic spectrum Why? How Light Moves š Light can be absorbed, scattered, reflected, transmitted, or refracted š Absorbed: Energy that is taken up and is not transmitted at all. š Objects absorb some wavelengths of light more than others, which is why each appears to be a certain color š Scattered: Energy that is dispersed in an irregular fashion š When light enters the atmosphere, much of it is absorbed or scattered and never makes it to the perceiver How Light Moves š Reflected: Energy that is redirected when it strikes a surface, usually back to its point of origin š This is where most of our sensation of light comes from š Transmitted: Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface) š Windows transmit light š Refracted: Energy that is altered as it passes into another medium š This is how our eyes focus light on the retina From Physics to Biology… š In order to see the electromagnetic energy of light, we need specialty sensors that can detect the different wavelengths of light and convert it into signals the brain can interpret š Let’s take a tour of the visual pathway in the eye… A Tour of the Eye š Cornea: Protective, transparent layer over the surface of the eye š Made up of fine fibers and transparent sensory receptors that sense pressure causing the eyes to close and produce tears A Tour of the Eye š Aqueous humor: watery fluid filling the anterior chamber of the eye š Provides oxygen and nutrients to the cornea and lens while removing waste material A Tour of the Eye š Crystalline lens (Lens): ovoid structure of clear crystalline proteins that bends to focus light from an image onto the retina Under Pressure š Angle-closure glaucoma: a disease that results in damage to the optic nerve caused by an excess buildup of aqueous humor in the from of the eye š Normally new aqueous humor is produced, and the old aqueous humor drains through channels near the iris š If the iris swells, these channels can be blocked causing aqueous humor to build up rather than drain š The excess fluid increases the pressure in the eye chamber putting pressure on the optic nerve š Affects approximately 0.6% of the population A Tour of the Eye š Iris: muscular diaphragm that regulates light entering the eye through expansion and contraction of the pupil š Pupil: a circular opening in the center of the iris š In rare cases, the iris can grow over the pupil, partially or completely obscuring the lens and preventing light from entering the eye š Affects less than 2% of the population A Tour of the Eye š Vitreous humor: The transparent fluid that fills the large chamber in the posterior part of the eye. š 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. Try this at home! š If you look up at the blue sky on a sunny day, you may see stringlike shapes or flecks that appear to move across the sky š These are floaters created by bits of biodebris floating in the aqueous humor of the eye Persistent Hyperplastic Primary Vitreous š When the eye is developing before birth, the eye chamber (where the vitreous humor forms) contains extra blood vessels called the embryonic hyaloid vasculature or primary vitreous š This fetal vasculature allows the eye to receive the nutrients to support the development of the lens š Once eye development is complete, these blood vessels regress or disappear Persistent Hyperplastic Primary Vitreous š In rare cases, the embryonic hyaloid vasculature does not regress, causing a disorder called persistent hyperplastic primary vitreous š Increases risk of glaucoma, cataracts, retinal detachment, and interocular hemorrhage š Severe cases results in blindness š Affects approximately 0.064% of the population Refraction of Light in the Eye š As light enters the eye the waves of energy are bent, or refracted, by the tissues they encounter š Cornea – about 80% of light refraction š Aqueous humor – minimal light refraction š Lens – about 20% of light refraction š Vitreous humor – minimal light refraction Refraction of Light in the Eye š Accommodation: The process in which the lens changes its shape, thus altering its refractive power š Ideally the lens focuses the image on the retina, but sometimes errors occur and the lens focuses images in front of or behind the retina Conditions of Refraction š Emmetropia: the condition of experiencing no refractive error – normal vision š Presbyopia: Literally “old sight”; the age-related loss of accommodation, which makes it difficult to focus on near objects š Myopia: When light is focused in front of the retina and distant objects cannot be seen sharply; nearsightedness. š Hyperopia: When light is focused behind the retina and near objects cannot be seen sharply; farsightedness. š Astigmatism: Unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. If the cornea was a ball…. If the cornea was a ball…. The Eye as a Camera – what’s right š A camera aperture is an opening that allows more or less light into the camera š Aperture š The iris functions like a camera aperture, adjusting pupil dilation to allow more or less light into the eye The Eye as a Camera – what’s wrong š The retina also adjusts to the light that enters the eye, changing the cell structures (rods and cones) to make š Aperture them more or less sensitive depending on the amount of light available š The camera cannot do that The Eye as a Camera – what’s wrong š A camera lens moves back and forth to adjust the focus angle, making the camera longer or shorter š Focus š The lens in the eye actually changes shape to adjust the focal length, as the length of the eye cannot change The Eye as a Camera – what’s wrong š Film in a camera captures an image in response to light, passively recording the image š Film š The retina is not passive – the cells in the retina sense and respond to patterns of light and send active signals about changing light to the brain Retinal Imagining š Ophthalmoscope – tool similar to a flashlight that contains a small lens that allows the user to view the back of the eye (fundus) š Retinal cameras can take more detailed images of the back surface of the eye Retinal Imagining Optic disc fovea š Optic disc – blind spot š Fovea – point of highest visual acuity located near the center of the macula š Macula – pigmented region near the center of macula the retina vasculature Feeling a little photosensitive… š Photoreceptors: Cells in the retina that initially transduce light energy into neural energy; named for their shapes. š Rods: Photoreceptors specialized for night vision. š Respond well in low luminance conditions š Do not process color š Cones: Photoreceptors specialized for daytime vision, fine visual acuity, and color. š Respond best in high luminance conditions  Retinal Topography š Light passes through several layers of cells before reaching rods and cones š Light activates a photoreceptor, which signals the horizontal and bipolar cells š Bipolar cells are connected to amacrine cells and ganglion cells š Ganglion cells have axons that leave the retina through the optic disc Wait a second… Why are the light sensing cells at the back of the retina behind all the other cells??? Let’s get transparent… š Ganglion, amacrine, bipolar, and horizontal cells are all transparent š Photoreceptors rely on nutrients and energy from the retinal pigmentation epithelium š The retinal pigment epithelium are opaque Retinal Geography š Humans have duplex retinas – meaning they have both rods and cones š The distribution of rods and cones is not constant over the retina š Cones are specialized for colour vision, and fine visual acuity; they are not sensitive in low light š Rods are specialized for low-light vision and peripheral vision ; they do not process colour š This means that you have very poor color vision in your periphery š It may seem as if your entire field of view has full-resolution color, but it does not  Visual Angle š Vision scientists measure the size of visual stimuli by how large an image appears on the retina, not by how large the object is š The standard way to measure retinal size is in terms of “degrees of visual angle” š Rule of thumb: If you hold your thumb out at arms length, the width of your thumbnail is about 2 degrees of visual angle Bright of Dim? š One of the most remarkable things about the human visual system is the incredible range of luminance levels we can adjust to. š Two mechanisms for dark and light adaptation: š Pupil dilation š Photoreceptors and their replacement Photoreceptors and Replacement š Each time a photoreceptor senses a quantum of light, the cell becomes bleached š Bleaching: when a photoreceptor cell senses light, it creates a reaction within the cell causing the photopigments to drain š After bleaching, the cell undergoes a period of desensitization where it cannot fire or sense light again until the pigment regenerates š In bright lighting conditions, this regeneration is is slower than the rate of light entering the eye, so we process less of the light, leading to a reduction in sensitivity as brightness increases š In dim light. photopigments are used up more slowly, and the more photopigments there are to process what little light is there. How do we sense light? š Photoreceptors have three main segments š Inner segment: pigment producing factory š Outer segment: pigment storage facility š Synaptic terminal: the signal communication point where the chemical signal is released when the photoreceptor fires Six stages of light detection 1. Light absorption: A photon of light is absorbed by the chromophore, a small molecule attached to the opsin protein. 2. Isomerization: The chromophore's configuration changes, isomerizing it from cis-11 retinal to all- trans retinal. 3. Deactivation: The pigment is briefly activated before dissociating (also called bleaching). 4. Phototransduction: The light absorption triggers a series of enzymatic reactions that change the electrical potential across the photoreceptor's plasma membrane. Results in hyperpolarization – closure of calcium channels at the synaptic terminal resulting in a negative charge across the cell membrane 5. Electrical transmission: The electrical activity is transmitted to other retinal neurons, and eventually to the brain via the optic nerve. 6. Visual perception: The brain interprets the electrical activity as a visual signal. Cell types and connections š Photoreceptors send signals to horizontal cells š Lateral inhibition: a process that occurs when excited neurons in the retina reduce the activity of nearby neurons allowing neural signals sent via the optic nerve to reflect differences in activation between retinal areas Cell types and connections š Amacrine cells – receive signals from bipolar cells and other amacrine cells, and send signals to bipolar, amacrine, and ganglion cells š the gossip cells of the retina – they get all the info from all the other cells and send the story along to anyone they meet š Together amacrine and horizontal cells send signals horizontally across the retina Cell types and connections š Bipolar cells – pools signals from several photoreceptors and passes it along to ganglion or amacrine cells š Pooling of signals allows for greater sensitivity – ability to detect and respond to transmitted signals from photoreceptor cells (important in peripheral vision) š Bipolar cells are specialized for rods or cones – they do not connect to both š Bipolar cells connect information in vertical pathways between photoreceptors and ganglion cells Cell types and connections š Ganglion cells – receive signals that have been processed and pooled by travelling through horizontal and vertical pathways through the retina š Divided into P cells, M cells, and koniocellular cells š Named for the regions they send signals to in the LGN – the first stop in the brain Receptive Fields š Receptive field: The region on the retina in which stimuli influence a neuron’s firing rate š ON-center ganglion cells: Excited by a spot of light that falls on their center and inhibited by light that falls in their surround. š OFF-center ganglion: Inhibited when a spot of light falls in their center and excited when light falls in their surround. P vs M ganglion cells P cells M cells š Small receptive fields š Large receptive fields š Connect to fewer photoreceptors š Connect to more photoreceptors š Finer resolution š More sensitive to dim light conditions š Responsible for visual acuity š Responsible for night vision š Sustained firing when excited by light š Initial burst of firing followed by š Less sensitive to changes in stimuli spontaneous firing rate when excited by light š More sensitive to changes over time Why Center-Surround Receptive Fields? š Each ganglion cell will respond best to spots of a particular size (and respond less to spots that are too big or too small). š Retinal ganglion cells act like a filter for information coming to the brain. š Retinal ganglion cells are most sensitive to differences in intensity of light between center and surround and are relatively unaffected by average intensity. š Luminance variations tend to be smooth within objects and sharp between objects. š Thus, center-surround receptive fields help to emphasize object boundaries. The retina is a camera? š Vertical and horizontal pathways in the retina pool, inhibit, enhance, and processing signals š Some signals are emphasized (contrast), others are discarded (ambient intensity) š The retina acts more like a set of filters extracting key information at each stage for the brain to assemble into a visual experience How sensitive is our vision? š Tinsley and colleagues built a single- photon quantum source š Delivering one photon of light at a time, participants were asked to indicate whether or not they perceived a flash of light š Participants performed significantly above chance in detecting single photons, especially in trials were they rated their responses as highly confident Tinsley, J., Molodtsov, M., Prevedel,. et al. Direct detection of a single photon by humans. Nat ComRmun 7, 12172 (2016). https://doi.org/10.1038/ncomms12172 Next Week š From optic nerve to primary visual cortex – how the brain turns light into interpretations of the world