The Structure of the Eye

Sight (Vision) - - - The Structure of the Eye: Sclera is the white part of your eye. It's a tough, protective covering and the muscles that control eye movement are connected to it Cornea is the outermost layer on the anterior surface of the eye, it is clear (serves to protect the front of the eye and to bend light just a little) Conjunctiva is a thing layer of epithelial cells that serves to protect the cornea Aqueous humor is found in the anterior chamber of the eyeball (basically made up of water and salt) Lens, which is biconvex, which serves as the main “bender of light” Ciliary body is what controls the shape of the lens (also secretes aqueous humor) Iris is the part of the eye that is colored and controls the size of the pupil (basically a hole which allows for light to pass to the lens) Vitreous humor is found in the posterior chamber of the eyeball (main component in here is albumin) Retina coats the entire back of the eyeball and is composed of a bunch of cells known as photoreceptors. The retina sends fibers from the back of the eye which for the optic nerve Choroid is a layer under the retina that supplies blood vessels in order to nourish the eye (pigmented black) Fovea (rich as fuck in cones) is the center of a broader region known as the macula Visual Sensory Information: The senses require a stimulus and a receptor in order to process the sensory information Two photoreceptors located in the retina: Rods (~120 million) are photoreceptors that are really sensitive to light and are great for night vision When no light is present, rod is turned on, but when a photon hits it, the rod is turned off When the rod turns off, it actually turns on a bipolar cell, which then turns on a retinal ganglion cell, and then enters the optic nerve for transmission to the brain Cones (~6-7 million) are photoreceptors that are responsible for color vision and are primarily centered in the area known as the fovea There are three kinds, red (60%), green (30%), and blue cones (10%) → each sensitive to their respective color Phototransduction cascade is a set of reactions that occur when a photon hits a photoreceptor and transmits a signal to the brain The Phototransduction Cascade: In the rod cell, there are hundreds of little optic discs stacked on top of one another which have proteins located on the outside of the optic disc Rhodopsin is a protein found in the optic discs of the rod cell which is made up of 7 subunits and at its center is a molecule known as 11-cis-retinal Light causes the molecule to change to all-trans-retinal, causing the rhodopsin protein to change shape, initiating a cascade of events Transducin is a GPCR which is attached to rhodopsin. Once rhodopsin changes shape, the alpha subunit leaves and binds to phosphodiesterase (PDE) Once activated, PDE will begin converting cGMP to GMP The increase in GMP concentration causes the Na+ channels to close due to no more cGMP bound to the channels The cell begins to hyperpolarize and turn off Then, the bipolar cell will get turned on in the presence of light, or when the rod cell is “turned off” Photoreceptors (Rods vs. Cones): Photopsin is the rhodopsin equivalent but in cones Rods vastly outnumber cones in each eye Also, cones are mostly found near the fovea while rods are found in the periphery of the eyeball Rods are 1000x more sensitive to light than cones Rods have a very slow recovery time while cones have a very fast recovery time Photoreceptor Distribution in the Fovea: The part of the retina where the optic nerve exits the back of the eye is known as the blind spot (because there are no photoreceptors found there) The fovea is indented so that light can get directly to the cones rather than go through a whole bunch of axons and other shit Visual Field Processing: Visual stimuli hitting the nasal side of the eye, will cross over at the optic chiasm to the other side of the brain Visual stimuli hitting the temporal side of the eye will not cross over at the optic chiasm THIS ALLOWS FOR ALL VISUAL INFORMATION COMING FROM EITHER SIDE OF THE BODY TO BE PROCESSED BY THE OTHER SIDE OF THE BRAIN! - - Feature Detection & Parallel Processing: Feature detection occurs when one looks at an object and breaks it down into its component features in order to make sense of what one is looking at. We can primarily look at three different important features: Color arises from the presence of cones Form is needed in order to determine the shape and boundaries of the object in question; a specific pathway used is the Parvo Pathway (which is responsible for figuring out the shape of an object and is therefore really good at spatial resolution → ability to find the boundaries of an object; has shitty temporal resolution → has to do with objects in motion, i.e. throwing a rose across the room) Motion allows us to encode motion; Magno pathways has high temporal resolution and poor spatial resolution and does not encode color Trichromatic theory of color vision is based on the premise that there are three classes of cone receptors subserving color vision Parallel processing is the idea that we get information from objects at the exact same time (we don’t think about them individually when looking at, say an apple, do we? Idk maybe you cause you’re fucking weird lmao) Extra Notes: Photopic vision occurs during high light levels Mesotopic vision occurs at dawn/dusk and involved both rods and cones Scotopic vision occurs at levels of very low light Selective attention is the ability to maintain attention while being presented with masking or interfering stimuli Joint attention is the focusing of attention on an object by two separate individuals Divided attention occurs when an individual must perform two tasks which require attention, simultaneously Directed attention allows attention to be focused substantially on a single task, in this case a single orientation of the Necker cube Review Marr’s 4 Stages of Vision