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Biopsych - The Eye (Y1) - Coggle Diagram
Biopsych - The Eye (Y1)
The eye
Input to the retina is light - electromagnetic energy emitted in the form of waves; wavelength and amplitude of visible light - retinal ganglion cells generate potentials, photoreceptors convert through transudction Gross anatomy of the eye:
- Pupil - opening where light enters, containing light absorbing pigments
- Sclera - white of the eye
- Iris - gives colour to the eye
- Cornea - glassy transparent external surface that protects the eye - refraction
- Optic nerve - bundle of axons from the retina - transports information
Eye collects light rays emitted by or reflect off objects in the environment and focuses them in the retina to form images
- Ciliary muscles control the lens - contraction and accommodation
- Fovea and blind spot behind image - high acuity
Visual transduction - transduction; conversion of one energy to another
- Visual - conversion of light to neural signals through visual receptors
- Discovered through the extraction of the red pigment from the rods (pigment - substance that absorbs light)
- When the pigment was exposed to bright lights, it was bleached - rhodopsin - and lost its ability to absorb light
- But when returned to the dark, it regained its redness and its light absorbing capacity
- Rod-mediated vision - our sensitivity to various wavelengths is a direct consequence of rhodopsin's ability to absorb them, which matches almost identically the scotopic sensitivity curve, explaining why rhodopsin absorption levels can predict how much we can see
- Rhodopsin - G-protein coupled receptor (metabotropic) that responds to light, not neurotransmitter molecules - causes intracellular chemical events when activated
Phototransduction - conversion of light to neural signals - retinal output of ganglion cells
- Rods and cones contain photosensitive pigment
- Absorption of photos of light by pigment generates electrical signal
- G protein linked receptor that responds to light not neurotransmitters
- In the dark, Na+ channels remain partially open, allowing depolarisation, releasing glutamate
- In the light, Na+ channels close, rods hyperpolarise, inhibiting glutamate release - three different pigments in cones; different spectral sensitivity
- Ganglion cells fire action potentials - other cells in the retina respond to stimulation with graded changes in membrane potential (EPSPs and IPSPs)
- Direct pathway of a photoreceptor - bipolar cell and ganglion cell
- When light hyperpolarise photoreceptor cells, they stop releasing glutamate to bipolar cells, and these then release NTs to the ganglion cells (action potential)
- Responses can be modified by the lateral connections of horizontal cells and amacrine cells
- The visual field is mapped point to point on the retina (100 million, each cell with receptive field that combines input from a number of photoreceptors)
- Information about light and dark - on centre and off centre ganglion cells
- Centre surround receptive fields - cells respond best when there is contrast
- On surround, off centre - light in periphery, off in centre (increased AP)
- Off surround, on centre - light in centre, off in periphery (increased AP)
Retinotopic organisation - retina-geniculate striate system is organised like a map of the retina
- Two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system
- Retinotopic layout of primary visual cortex has a disproportionately large representation of the fovea - small part of the retina, but large proportion of the primary visual cortex
- Dobelle, Mladejovsky and Girvin (1974) - implantation of electrodes in the primary visual cortex of blind patients and triggered currents in the electrodes to make a shape, the individuals could see a glowing image of it - prosthetics for blindness
The M and P channels - two parallel channels of communication that flow through each lateral geniculate nucleus
- Parvocellular layers - run through top 4 layers - cells with small bodies
-> responsive to colour, fine pattern details and stationary or slow moving objects
-> Cones provide majority of input
- Magnocellular layers - bottom two layers, large cell bodies
-> Particularly responsive to movements - rods provide majority of input
These neurons project to different areas in the low part of layer IV of the striate cortex, and so these M and P areas of lower layer IV project to different visual cortex areas.
Convergence - cones and rods - ganglion cells
- only cones are found at the fovea - high acuity and low sensitivity
- More convergence in rod system - increasing sensitivity whilst decreasing acuity
Duplexity theory of colour vision - cones and rods mediate different kinds of vision
- Cones.- photopic (daytime) vision; mainly in fovea, 3 pigments for colour vision, high acuity (1:1 ganglion cell), function of high acuity and colour information in good lighting
- Rods - scotopic (nighttime vision) - mainly in periphery, non colour vision, low acuity and function - low acuity vision in dim light, movement perception
Image formation - light reflected onto back of eyes and image is inverted
- Light focused on retina to form image
- Focus - refractive powers of cornea and the lens mean image is inverted when placed on the fovea
- Fovea region - place in the retina where we see best (surrounded by macula)
- As light enters the eyes after being reflected off other objects, it is refracted by the cornea first which then is refracted and focused onto the retina by the lens
- Accommodation by the lens - lens involved in forming crisp images of objects located closer than 9 m
-> Shape of the lens is changed - far objects have a flat lens, but for near objects the lens is concave due to the contraction of ciliary muscles to allow focus
-> Glasses correct lack of focus when ciliary muscles stop contracting
- Convergence of lens when things are near - never fully correspond as the two eyes do not view the world from the exact same position - binocular disparity - difference in position of the same image on the two retinas, which is greater for close objects than distant objects
- Visual system cause use degree of binocular disparity to construct one 3D perception from two 2D retinal images
Eye movement - temporal integration - means the world does not disappear every-time we blink - eyes continually scan the visual field and so our visual perception is a summation of recent visual information
- Eyes move continuously even when we try to keep them still
- Involuntary fixed eye movements -
-> Tremor, drifts and saccades (small jerky movements or flicks)
- When eye movements or their main effect (movement of images on the retina) are blocked, visual objects being to fade and disappear - most visual neurons only respond to changing images, and so if retinal images are artificially stabilised the images start to disappear and reappear, and so eye movements enable us to see during fixation by keeping images moving on the retina
Pupillary light reflex - pupil adjusted for different ambient light levels - expands when dark, constricts when small
- This reflex involves connections between retina and neurons in the brainstem that control the muscles that constrict pupils
- Consensual - shining a light into one eye causes the constriction of both pupils, when one receives light, both have the same reaction
- The visual field - amount of space viewed by the retina when the eye is fixated straight ahead
- Visual acuity - ability to distinguish two nearby points (high spatial resolution)
- Retinal disparity - a binocular cue for perceiving depth by comparing images from the retinas in two eyes, the brain computes distance - greater disparity (difference) between the two images, the closer the object
- Difference between images on two retinas
- Greater when objects are close - provides brain with a 3D image and distance information
- Floating sausage illusion
The blind spot - towards the nose is the optic disk - the place where all nerve fibres leave the eye, forming the optic nerve
- No photoreceptors, consequently creating a blind spot in the visual field
- Filling in - fills in background pattern, or it fills in an object passing through the blind spot
-Blind spot - no receptors where information exits the eye and the visual system uses information from cells around the blind spot for completing, filling in the blind spot
- Fovea - high acuity area at centre of retina, thinning of ganglion cell layer reduces distortion due to cells between the pupil and retina
Completion phenomenon - visual system fills in the blindspot
- Completion also has a role in normal vision, as the visual system does not conduct an image of that object from your retina to the cortex, it instead extracts key information about the object, primarily information edges and location and conducts information to the cortex
- Colour and brightness of large unpatterned surfaces are not directly perceived but are filled in by a completion process known as surface interpolation (we perceive surfaces where appearance is inferred by edge information)
The retina - thickness of a postage stamp but contains over 100 million cells
- light has to pass through overlying tissue to reach the photosensitive rods and cones (photoactive cells)
- Advantage of the inside-out arrangement - pigmented epithelium below the photoreceptors absorb light that passes entirely through retina - minimise light scattering (nocturnal animals)
Laminar organisation -
- Cellular structure - photoreceptors
- photoreceptors - rods and cones (based on outer segment)
- Main regions - outer segment, inner segment and synaptic terminal
- Rods and cones have different regional distributions
- Fovea - many cones
- Retinal periphery - more rods
- Peripheral retina - higher ratio of photoreceptors to ganglion cells, making them more sensitive to light
Spectral sensitivity - the relative brightness of lights at the same intensity presented at different wavelengths
- Animals and humans with cones and rods have a photopic spectal sensitivity curve and a scotopic sensitivity curve
- Human photopic spectral sensitivity curve can be determined by having subjects judge the relative brightness of different wavelengths shone on the fovea
- Scotopic sensitivty can be determined by shining light on the periphery of the retina at an intensity too low to activate the few peripheral cones there (mainly rods)
- Purkinje Effect - as the light levels change at dusk, the switch from photopic to scotopic makes some colours appear brighter, and everything else became a shade of grey as the rods in scotopic wavelengths only pigment for blue-green, making everything else devoid of colour
- Lights of the same intensity but different wavelengths may not all look as bright
- A special sensitivity curve shows the relationship between wavelength and brightness
- Spectral sensitivity curves for cones are white lines, for rods they are black lines - cones for blue-green, red-yellow, and blue, rods are blue green and useful for night vision
- Colour is a construct - it does not exist in the physical world, and are specified by the photo pigments we have in our eyes
- For example, dogs only have blue and green cones, butterflies can see ultraviolet and mantis shrimp can see many colours
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From the retina to the cortex - the Retinofugal projection (eye to brain)
- Optic nerve
- Optic chiasm
- Optic tract - hypothalamus (biological rhythms, sleep and wakefulness), superior colliculus (orients the eye in response to new stimuli)
- Lateral geniculate nuclei (LGN)
- Optic radiation
- Primary visual cortex
- Non-thalamic targets of the optic tract
Right and left visual hemifields - left hemifield projects to the right side of the brain, right hemifield foes to left side of the brain, do not confuse with the eye - both receive both visual fields, but only RVF goes to the LH and the LVF goes to the RH; information only goes to one side, but eye receives both
- Rays go to the nasal and the temporal retina - LVF goes to left nasal and right temporal, RVF goes to the right nasal and the left temporal
- Temporal - ear side, nasal - nose side
- Nasal cross over the optic chiasm, resulting in all information from the left visual field being mapped to the right and vice versa - temporal sides are relevant to the side of the brain in which the information ends up
- Ganglion cell axons from nasal retina cross (contralateral) and temporal retinal axons stay ipsilateral
Visual deficits from lesions in the retinofugal projection -
- In optic tract, lose information on the contralateral side - left optic tract would lose RVF
- Transection of left optic nerve reduces left visual field
- Transection of optic chiasm reduces periphery
Lateral geniculate nucleus - thalamic structure, first synaptic relay in the primary visual pathway
- Monocular input - the input from the two eyes is kept separate
- Axons arising from the M and P type cells synapse on cells in different LGN layers
- Receptive fields similar to the ganglion cells
- Retinotopic organisation - neighbouring region of the visual field are processed by neighbouring regions of the LGN
Striate cortex - parts of the visual cortex -
- Subdivided into many areas - Striate cortex; primary visual cortex (V1)
- Extrastriate cortex - V2; VP; V3; V4; MT or V5
Retinotopic mapping - retina, LGN and striate cortex - retina 1, LGN 5, striate 9 for example (2,6,10 / 3,7,11 / 4,8,12
- Visual space is not sampled uniformly by the cells in the retina central visual field, overrepresented, magnified - more cortex is devoted to areas of high acuity (25% input from fovea)
- When the retina is stimulated by a point of light, the activity in the striate cortex is a broad distribution with a peak of corresponding retinotopic location
- No such thing as a picture of the world in the primary visual cortex - action potentials that respond to different properties of the stimulus
Ocular dominance columns - cells with the same preference are grouped together (blue is right, yellow is left)
- Cells with the same column have same ocular dominance and receptive fields in similar areas of the visual field, with cells in neighboring columns possessing alternating ocular dominance
Receptive fields of cortical visual neurons - the area of the visual field within which it is possible for a visual stimulus to influence the firing of a given neuron
- Hubel and Wiesel looked at receptive fields in cat retinal ganglion, LGN and striate cortex
- Two different types of cells in the V1 - simple and complex
- Complex cells - larger and their receptive field cannot be divided into on and off areas, V1 has cells with monocular and binocular receptive fields
- Binocular receptive fields in V1 means that neurons respond to receptive fields in the ipsilateral (same side) and contralateral (opposite) eye - input from both eyes forms a single image of the world
Orientation and direction selectivity - in V1, most neurons have a rectangular receptive field, possibly composed of input from three LGN cell axons with centre surround receptive fields
- Configured to detect lines
- Orientation selectivity - respond to a particular orientation
- Direction selectivity - respond to direction of movement
- Orientation columns - orientation selectivity of nearby neurons is related
columnar organisation of visual cortex -
- Cortical module of each module is capable of analysing every aspect of a portion of a visual field
- Orientation selection columns and ocular dominance columns
- Seeing edges - as we develop, we can distinguish more shapes
Simple striate cells - divided into antagnoistic on and off regions and are unresponsive to diffuse light (monocular)
- Straight lines not circles
- Respond maximally only when it is preferred straight edge stimulus is in a particular position and orientation
Complex striate cells - more numerous, rectangular receptive fields, respond best to straight line stimuli in specific orientation and do not respond to diffuse light
- Larger receptive fields, not divided into static on and odd
- Respond to straight edged stimulus of a particular orientation regardless of its position in the field - continuous response to movement
- Respond more robustly to the movement of the straight line across receptive fields - binocular
- ocular dominance - respond to the stimulation of one eye more than the stimulation of the other
- Some binocular cells fire best when the preferred stimulus is presented to both eyes at the same time but in different positions on the retinas; retinal disparity involved in depth perception
4 commonalities of receptive fields of each level - at each level, receptive fields in the foveal area of the retina were smaller than those on the periphery - this is consistent with the fact that the fovea mediates high acuity vision
- All neurons have circular receptive fields
- All neurons are monocular - each neuron has a receptive field in one eye, not the other
- Many neurons at each of the three levels had receptive fields comprised of an excitatory area, and an inhibitory area separated by a circular boundary - on and off
- Visual system neurons are continually active, even without visual input - spontaneous activity is characteristic of most cerebral neurons, and responses to external stimuli consume only a small portion of the energy required for ongoing brain activity
Seeing edges - edges are the most informative features of any visual disparity as they define the extent and position of the various objects in it
- Contrast enhancement - Mach band illusion
- Every edge we seen is highlighted by the contrast-enhancing mechanisms of our own nervous systems - our perception of edges is better than the real thing
- Hubel and Wiesel - receptive fields of visual neurons
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Seeing colour - component theory - trichromatic theory of colour vision, three different kinds of colour receptors, or cones, with spectral sensistivity and the colour of the particular stimulus is encoded by the ratio of activity in each of the three
- Young and Helmholtz - all three combine to produce other colours
Opponent process theory - Hering - two different classes of cells in the visual system - one for colour, one for brightness encoding
- Complementary colour perception - signalled red by hyperpolarising in one direction, and the complementary colour green is depolarised
Wald (1964) - both systems exist in colour processing
- Other species are dichromats
Colour constancy and Retinex theory - perceived colour of an object is not a simple function of wavelengths reflected on to it - objects stay the same colour despite major changes
- Can tell objects apart despite light level changes and respond appropriately
- Land (1977) - demonstrated this with projectors of different colours
- Colour is determined by reflectance
- Retinex theory - visual system calculates the reflectance of surfaces and so perceives their colours
Illusions - Fortification illusions -
- Individuals with migraines often see an illusion before the headaches in which a grey area of blindness begins to resemble and expand into a horseshoe shape, with a zigzag pattern of flickering lines at its advancing edge - when it reaches the periphery, headaches begin
- Able to trace the details of the illusion onto the blank sheet of paper
McGurk Effect - You hear different sounds when you look at different mouth movements for the same sound - eyes open makes sound match mouth movement, but if you close your eyes then you hear the sound correctly
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