PSY280 Midterm 2 Study Sheet

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Matthias Niemeier

Lecture 6: First Steps in Vision (CH 2) Light: Energy produced by vibrations of electrically-charged material Also: A wave or stream of photons - Takes up a very small portion of the electromagnetic spectrum Wave: When light moves around the world Oscillation traveling through medium through energy transfer of particles (no permanent displacement of the medium occurs) Photons: When light is absorbed A quantum of visible light demonstrating both particle and wave properties How Light is lost in the atmosphere Absorption: To take up light or energy and not transmit it at all e.g. When light travels from a star, it encounters dust, vaporized water etc. Scattering: To disperse light in an irregular fashion e.g. When light travels from a star, it is sometimes scattered by various particles Light will eventually hit an object If the object is light in colour, the light will be reflected (redirection) If the object is dark in colour, the light will be absorbed (transformed to other forms of energy e.g. heat) If light is neither reflected nor absorbed, it is transmitted (convey light from one thing to another through a transparent medium) through the object and the light is refracted (alter the course of a wave of energy – for example: light energy into water) Diffracted: Light is bent or has spread out waves (e.g. waves of light encountering a narrow aperture/obstacle) Parts of the Eye Cornea: Transparent window where light photons are transmitted through - Contains a rich supply of transparent sensory nerve endings - Most powerful refractive surface of the eye Aqueous Humor: Watery fluid (derived from blood) in the anterior chamber of the eye that supplies oxygen & nutrients to the cornea and removes waste from the cornea and the crystalline lens Crystalline Lens: The lens inside the eye enabling visual focus – has no blood supply and is transparent like the cornea; controlled by the ciliary muscle Pupil: Dark circular opening in the center of the eye (iris) where light initially enters Iris: Muscular diaphragm surrounding and pupil and regulating light entrance (expand and contracts pupil) (the coloured part of an eye) Direction of Light Source -> Cornea -> Pupil -> Crystalline Lens -> Vitreous Humor (light is refracted) -> Retina Vitreous Humor: Transparent fluid filling the vitreous chamber (80% of internal volume of the eye), refraction here is the longest part of the journey through the eyeball. It is viscous, gel-like, and transparent (like egg white) Retina: Light-sensitive membrane in the back of the eye containing rods & cones which receive an image from the lens and send it to the brain via optic nerve. Only some of the light will actually reach the retina – most of light energy is lost in space/atmosphere or lost in the eyeball. Roles: Detect light and tell the brain about aspects of light related to objects of the world The Retina - Refraction is necessary to focus light rays - The cornea, aqueous, and vitrous humours work together to refract light (refraction is fixed and cannot be used to ring close objects into greater focus) - The backmost layer of the retina contains cells called photoreceptors (light- sensitive receptors) which then stimulate neurons in other retina layers up to the front layer, which contains ganglion cells Accommodation: The process by which the eye changes its focus (the lens alters refraction / refractive power by changing shape) - Refraction is done by: crystalline lens, ciliary muscle, iris, aqueous humour - Done through the contraction of the ciliary muscle - Lens is attached to ciliary muscle through fibers called the zonules of Zinn - For far away objects, the lens lay flat – for closer objects, the ciliary muscles contracts which causes the lens to bulge and get thicker Fundus: The back layer of the retina Vision Conditions Presbyopia: “Old sight” – the loss of near vision because of insufficient accommodation Cataract: An opacity of the crystalline lens - Causes problems because it will absorb & scatter more light than a true transparent lens Emmetropia: No refractive error – refractive power of the eye is perfectly matched to the length of the eyeball Myopia: Near-sightedness (distant objects cannot be seen sharply) - Eyeball is too long - Focus occurs in front of retina - Corrected with (-) lenses which will diverge light before it hits eye Hyperopia: Far-sightedness (close objects cannot be seen sharply) - Eyeball is too short - Focus occurs behind the retina - Corrected with + lenses which will converge light before it hits eye Astigmatism: Condition where there is unequal curving of the eye’s refractive surfaces (usually the cornea) Retinal Information Processing Optic Disc: The point where arteries and veins that feed the retina enter the eye - Where axons of ganglion cells leave the eye via optic nerve - Contains no photoreceptors (is blind) Opsin: Light-sensitive protein Photoreceptors: The cells that make up the retina and responsible for transducing light energy into neural energy Two Kinds of Photoreceptors: 1. Rods: Specialized for night/scotopic vision - Located outside of fovea, high sensitivity and low acuity 2. Cones: Specialized for day/photopic vision, fine visual acuity and colour - Located throughout fovea, low sensitivity and high acuity 3. Photoreceptor for circadian rhythm: Adjusts biological rhythm to match day and night of the external world – sensitive to ambient light containing melanopsin (a photopigment) – the signals are then sent to the suprachiasmatic nucleus (SCN) Synaptic terminal -> Inner segment -> Outer segment -> [Pigment Epitheleum] Inner Segment: Where visual pigments are made and incorporated into the membrane Visual pigment components: - Opsin: Light-sensitive protein which determines which wavelengths of light is absorbed - Chromophore: captures the light photons - These are both located in the outer segment Types of visual pigments - Rhodopsin: The visual pigment found in rods - S-cones (very few of these and missing from foveal center) - M-cones - L-cones (more of these than M-cones) - Each visual pigment for cones vary in wavelength which will provide the basis for colour vision - Note: Each photoreceptor contains only one of the four (??) visual pigments Photoactivation: Activation by light. When photon reaches the outer segment of a photoreceptor (e.g. a rod), it is absorbed by rhodopsin (visual pigment) and transfers its energy to the chromophore (light capturer) portion of the visual pigment. Capturing a photon (rods and cones) - Light goes in through to the retina - Photoactivation: photon reaches outer segment of the photoreceptor, which is located in the retina. It is then absorbed by the visual pigment, specifically the chromophore - This photoactivation closes channels (a membrane) that allow ions to escape in the outer segment - This closure then causes the photoreceptor to hyperpolarize (become negative) - The hyperpolarization causes the calcium channels in the synaptic terminal to close - This closure reduces the number of neurotransmitters (glutamate) which corresponds to the number of photons absorbed – this signals that photons have been captured - The information is then passed via graded potential (varies in amplitude) Parts of the Retina: Fovea: A small depression in retina where visual acuity is at its highest (the center of field in vision) - There’s significantly more cones than rods in the foveal center Used to: identify objects, read, and inspect fine detail. Macula: Central part of the retina with high concentration of cones Periphery: Used to detect and localize stimuli that aren’t being looked at directly (i.e. “the corner of your eye”) Location of rods and cones on the retina - Contains more rods than cones (90 vs 5 million) - Rods are absent in the center of the fovea - Cones are concentrated in center of fovea and density drops with retinal eccentricity (distance between retinal image and fovea) Aging-related macula degeneration (AMD): Disease that affects the macula by gradually destroying sharp central vision making it difficult to read, drive etc. It causes a scotoma (blind spot in visual field – caused by loss of cones in macula) How light travels through the retina - Light travels to the back layer of the retina which contains photoreceptors - Photoactivation occurs which results in the photoreceptors sending graded potential to horizontal cells - Horizontal cells communicate with the bipolar cells (either diffuse or midget) - There is then communication with the amacrine cells - And then finally, communication with the ganglion cell which is the final layer of the retina and sends information to the brain/mid-brain Lateral pathway: Various regions of the retina interacting Horizontal cell: Retinal cell that contacts both photoreceptor and bipolar cells Amacrine cells: Also makes synaptic contacts with bipolar and ganglion cells Lateral pathway: Horizontal cells, amacrine cells Lateral inhibition: Neural interaction between adjacent regions of the retina – involved: horizontal cells and amacrine cells - Enables signals reaching retinal ganglion cells to be based on differences in activation between nearby photoreceptors Vertical pathway: Photoreceptors, bipolar cells, ganglion cells Bipolar cells: Synapse with either rods or cones and horizontal cells and passes signals to ganglion cells Diffuse bipolar cells: Processes is spread out and receives input from multiple cones – convergence of information from many photoreceptors to one bipolar cell (used in peripheral vision). Important mechanism for increasing visual sensitivity, but wreaks havoc in visual acuity because ganglion cell unable to tell which pattern of light is present (from info passed to it by the bipolar cell). INFO GOES TO M-GANGLION CELLS -> magnocellular layer of LGN Midget bipolar cells: Receive input from single cones and pass information to single ganglion cells (one-to-one pathway between cones and ganglions) – exist in the fovea which accounts for visual acuity (but low sensitivity due to low convergence) INFO GOES TO P-GANGLION CELLS -> parvocellular layer of LGN Two Kinds of Bipolar Cells contacted by foveal cone ON Bipolar Cell: Responds to increase in light captured by cones OFF Bipolar Cell: Responds to decrease in light captured by cones Ganglion cells: The final layer of the retina that receives visual information form photoreceptors via bipolar cells/amacrine cells and transmits information to the brain and midbrain - Receptive field is concentric - Filters tuned to spots of a particular size - Sensitive to difference in light intensity (contrast) Types of ganglion cells M ganglion cell: Receive input from diffuse bipolar cells - Feed the magnocellular (“large cell”) layer of the LGN - Dendrites spread out moreso than P g-cells - Comprises 10% of ganglion cells - Listen to more photoreceptors than P-cells - Signal information about how image changes over time (brief burst of impulses and return to spontaneous rate even while spot remains lit) - Focuses on: Visual sensitivity, CHANGES IN IMAGE - Larger receptive field, coarser spatial resolution, transient responses, “insensitive” to colour P ganglion cell: Receive input from midget bipolar cells - Feed the parvocellular (“small cell”) layer of the lateral geniculate nucleus (LGN) - Comprise 70% of ganglion cells - Have smaller receptive fields (provides finer resolution) - Provide information about the contrast in retinal image (respond with more sustained firing) - Focuses on: Visual acuity, CONTRAST IN IMAGE - Smaller receptive field, finer spatial resolution, sustained responses, sensitive to colour Koniocellular cells: Project to koniocellular layers in the LGN Concentric center-surround organization of ganglion cells: ON-center cell: Increases fire rate when a light is turned on in the center of the receptive field ONLY (decreases rate with light in the surround) OFF-center cell: Decreases fire rate when light is turned on in the center of the receptive field ONLY (increases rate with light in the surround) Functional results of concentric: - Each ganglion cell responds best to spots of a particular size thus, acting as a filter - Ganglion cells are sensitive to the differences in intensity of the light in the center versus the surround – thus, it detects light contrast (which remain similar regardless of the lighting conditions) TLDR: Detects light contrast, acts as filter Mechanisms for dark/light adaptation - Pupil dilation - Photo receptors - Photopigment replacement - Doesn’t adapt but is a mechanism: neural circuitry of retina Coping with differences in light - Alter sensitivity (close pupil for bright lights; open pupil for dark lights) - Photoreceptors: - Photopigment replacement: More time spent in the dark; less threshold for rods - Neural Circuitry: Visual system regulates amount of light entering eye – ignore excess variation Diseases! Retinis pigmentosa: Hereditary diseases involving progressive death of phororeceptors and the pigment epithelium Lecture 7: Spatial Vision (CH 3) Sample Midterm Question: Which neurons in which pathways play a role in retinal processes? Describe some of the functions of the pathways In which important way does retinal information processing differ from a computer? - Lateral (horizontal, amacrine) and vertical (photoreceptors, bipolar cells, ganglion cells) pathway - There is a regulation on the amount of light processed Types of gratings - Rectangular - Sine wave - Gabor (sine viewed through circular aperture) Characteristics of gratings - Frequency (cycles per degree) - Amplitude - Phase Importance of gratings - Important cues for visual recognition (edge of any object is frequently seen in the world) - Efficient coding - Identifies basic components of natural images Acuity: The smallest spatial detail that can be resolved Cycle: For a grating, a pair consisting of a dark and light bar Visual angle: The angle subtended by an object at the retina (e.g. vertical or horizontal) Gratings can be distinguished from a uniform gray field as long as the adjacent pairs of light and dark are separated by at least 1 arc minute of visual angle Rods and cones in the periphery are packed together less tightly (recall that in the periphery, rods are more tightly packed than cones), but many receptors converge on each ganglion cell causing visual acuity to be poorer in the periphery compared to the fovea Measuring visual acuity: For opthalmologists: Distance Distance at which you can just identify the letters / Distance at which a normal person can just identify the letters For vision scientists: Smallest visual angle of cycle of a grating Spatial frequency: The number of grating cycles in a given unit of space (e.g. wider stripes would have a lower spatial frequency – number of times you see alternation in a given space) – measured in cpd Cycles per degree:The number of pairs of dark and bright bars per degree of visual angle Resolution acuity is limited by - Spacing of photoreceptors in retina - Convergence (e.g. multiple photoreceptors projecting to single bipolar cell) Aliasing: Misperception of grating due to undersampling (e.g. when an entire cycle falls on a single cone) Contrast Sensitivity Function (CSF): Describes how the sensitivity to contrast depends on the spatial frequency of the stimulus Basically: For something to be visible, it must have a specific spatial frequency and sufficient contrast sensitivity – the less the SF, the less CS – if more CS, it will be invisible & little SF Primary Visual Pathway - Left eyeball sends left foveal image to left LGN and left striate cortex; right foveal image to right LGN and right striate cortex - Right eyeball sends left foveal image to left LGN and left striate cortex; right foveal image to right LGN and right striate cortex - This is done via OPTIC CHASM Visual defects Heteronymous hemianopia: Left side left eyeball + right side right eyeball blind (something wrong with optic chiasm) Homonymous hemianopia: Left side left eye + left side right eye OR RIGHT SIDE FOR BOTH– something wrong with travel from LGN Quadrantanopia/Scotoma: Lesion AFTER optic chiasm Fourier Analysis: Although “pure” sine wave gratings may be rare in the real world, the visual system appears to break down real-world images into a vast number of components, each of which is a sine wave grating with a particular spatial frequency. This method of processing is the fourier analysis. ON Ganglion cells response to gratings (opposite for OFF ganglion cells) - Low frequency yields weak response (part of the light lands on the cell’s surround which dampens the response) - Medium frequency yields strong response (here, the spatial frequency is just right) - High frequency yields weak response (both dark and light fall within receptive center of the cell) Thus: ganglion cells are tuned to: - specific spatial frequency that matches its receptive-field size. The response - also depends on the phase (relative position) of the grating (i.e. it depends on the difference in light intensity of the center and the surround parts of the cell based on the cell’s location on the grating) - not orientation The Lateral Geniculate Nucleus: A structure in the thalamus that receives input from the retinal ganglion cells and has input and output connections to the visual cortex (acts a relay station between the retina and the cortex) • Six-layered structure (geniculate means “bent”) • Receive important input about attention/arousal • There is one in each cerebral hemisphere • Respond to same pattern as ganglion cells • Respond to either the left or right eye, not both • Location where various parts of he brain can modulate input from the eyes (since there are many connections from visual cortex to LGN and it connects to other parts of the brain) • Layers 1&2 Magnocellular layers: Bottom two layers with larger cells - Receive input from M ganglion cells - Responds to large, fast-moving objects - High luminance sensitivity, motion/flicker • Layers 3-6 Parvocellular layers: Top four layers with smaller cells - Receive input from P ganglion cells - Processes details of stationary targets - High spatial frequency, colour* • Koniocellular cells: Processed between the magno and parvo layers – involved in different aspect of processing (e.g. relay signals from S-cones and part of blue-yellow pathway) • Left LGN receives projection from left side of retina of both eyes Right LGN receives projection from right side of retina of both eyes • Layers 1, 4, 6 receive input from the contralateral (opposite side) eye • Layers 2, 3, 5 receive input from the ipsilateral (same side) eye Important features of striate cortex: Topographical mapping: The orderly mapping of the world in the lateral geniculate nucleus and the visual cortex (details above) such that neighbouring points are projected into neighbours The layer on which the image is mapped is directly correlated to the image’s position on the visual field Cortical magnification: The amount of cortical area (mm) devoted to a specific region (e.g. 1 degree – e.g. if you stick your arm and index finger out, roughly the size of your fingernail) in the visual field – important for scaling of info • There is a trade-off: our visual system provides high resolution in the center and lower resolution in the periphery • The real problem with periphery is visual crowding: the effect of clutter on peripheral object recognition – sets limits on perception and impairs object discrimination Striate Cortex/Primary Visual Cortex/ V1/ Area 17: Area of the cerebral cortex that receives direct inputs from the LDN as well as feedback from other brain areas – responsible for processing visual information • Fibers from the LGN project mainly to layer 4 of the striate cortex • Stimulated by bars and gratings of different orientations (interested in the BASIC features of the visual image – response to edges, lines, motions, size etc.) – small and precise receptive fields – cells will only respond to its PREFERRED stimulus based on location and relative to person’s glance • From V1 to parietal lobe (dorsal pathway): Concerned with where things are • From V1 to temporal lobe (ventral pathway): Concerned with what things are • Magnocellular axons project to 4Ca and parvocellular axons to 4Cb • Major and complex transformation of visual information takes place here • Images in edge of periphery are allocated only a small portion on the striate cortex (cortical magnification) • Receptive fields are elongated instead of circular and thus, neurons respond more vigorously to bars, lines, edges, and gratings • Orientation tuning: Neurons in the striate cortex respond optimally to certain orientations less than others (will not respond if tilted more than 30 degrees from this optimal orientation) - Humans respond better to horizontal/vertical targets than oblique • Each striate cortex cell is tuned to a particular spatial frequency (more narrowly tuned than retinal ganglion cells) – forms a filter for the image • Responds to input from both eyes • Ocular dominance: Property of striate cortex neurons (their receptive fields) by which they demonstrate a preference to stimulus presented in one eye rather than the other – unlike the LGN which can ONLY respond to one eye and not the other • Simple cells: Cortical neuron with clearly defined excitatory inhibitory regions - Phase-sensitive: Positioning on receptive field affects response • Complex cells: Neuron whose receptive characteristics cannot be easily predicted – preference to moving stimuli - Phase-insensitive: Gives robust response with little to no modulation Responds more generally than simple cells with large fields • End-stopping: (Each simple/complex cell is either e-stopped or not) Process by which by which a cell in the cortex first increases its firing rate as bar length increases to fill receptive field and then decreases as bar expands further through the field - Plays a role in how we detect luminance boundaries and discontinuities • Column arrangement of neurons: Line orientation, ocular preference Hypercolumn: A 1mm block of striate cortex containing two sets of columns covering all possible orientations (0-180 degree) which one column preferring left eye input and the other, right eye input - Contains everything necessary to look after everything striate cortex is responsible for • Concisely: Striate cortex responsible for analyzing orientation, shape, speed, direction of objects in the world and it does so using modular grouping of neurons (e.g. hypercolumns) • The left striate cortex gets left info from both eyes; vice versa with right striate cortex Methods of learning brain function: Adaptation • Supports the idea that human visual system contains individual neurons selective for different orientations • Provides evidence that the human visual system contains neurons selective for visual frequency • Adaptation: A reduction in response due to constant stimulation • Tilt aftereffect: The perceptual illusion of tilt, produced by adaptation to a pattern of given orientation - Shows that human visual system contains neurons selective for different or
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