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Joe Kim (1,023)
Lecture

Visual Processing

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Department
Psychology
Course
PSYCH 1XX3
Professor
Joe Kim
Semester
Winter

Description
Visual Processing Introduction • 1/3 of brain devoted to processing visual information • Important role reflected in visual references we use in language • When visual information is in conflict with information from another sense – bias trust towards sense of vision (seeing is believing) • Visual cues more important than source of sound Visual Sense • See with brain • Eye is instrument to collect, focus and sense light stimulus • Heavy duty processing occurs in brain Case Study • John rendered blind by an occipital lobe stroke at age 30 • Unable to perceive visual world, struggles to maintain independence • John has little difficulty navigating way through home • Has no perception of vision The Stimulus Light • Eye o Instrument that detects light stimulus o Takes care of initial stages of processing o Sends result to brain for further processing Amplitude • Light travels as wave o Amplitude – height of each wave  Variations affect perception of brightness  Greater amplitude of light wave, the more light is being reflected or emitted by that object  Object appears brighter/more intense to us o Wavelength – distance between the peaks of successive waves  Variations affect perception of colour  Measured in nanometers/millionths of millimetre  Smaller wavelengths refer to light waves with a higher frequency – less distance between successive peaks  Larger wavelengths – light waves with a lower frequency  Humans only sensitive to tiny portion of total range of wavelengths of electromagnetic radiation • Tiny portion that we’re sensitive to – visible spectrum • Shortest wavelength – 360 nanometers (violet) • Longest wavelength – 750 nanometers (red) • Only from human perspective  Other species see light outside visible spectrum • Insects (bees) – shorter than 360 nm (ultraviolet spectrum) o Perceive differences in colours of flowers that all look the same colour to us • Snakes – see light made up of wavelengths longer than 750 nm in the infrared spectrum o Allows them to find prey in the dark by being able to see the body heat that is emitted by the prey  Infrared spectrum – lower in frequency and longer in wavelength than light making up the visible spectrum Purity • Two physical characteristics of light (amplitude and wavelength) translate into our perceptions of brightness and colour respectively • Final physical characteristic of light – purity Purity – affects the perception of saturation/richness of colours • Light made up of single wavelength is a pure light • Perceived colour described as completely saturated • Natural light – experience is combination of many wavelengths o Light described as desaturated • Most of the colours we see in everyday life are not pure but a mixture of wavelengths o Less intense than pure colours The Eye • Light first passes through curved cornea o Begins focusing process o Cornea is transparent window at the front of the eye • Sclera – white part that covers rest of the eye o Tougher membrane • After the cornea, light passes through the pupil o Round window that you see as a black dot in the middle of your eye • Iris/coloured part of your eye – controls the size of the pupil o Contains a band of muscles that is controlled by the brain  if not enough light is reaching the retina, muscles cause pupil to dilate into a larger opening  if too much light is entering the eye, then muscles cause pupil to constrict into a tiny opening • after passing through pupil, light passes through the lens – transparent structure that does the final focusing of light onto the retina at the back of the eye Lens • curvature of the lens causes images to land on the retina upside-down and reversed from left to right • final perceived image is a product of brain activity • rather than seeing everything upside-down and reversed o there is a correction that allows us to see a properly oriented image • lens is a flexible piece of tissue o shape can be altered by surrounding muscles – allowing it to focus on objects that are close or far away o if object is close – lens of eye gets fatter/rounder to produce a clear image o if object is far away – lens of eye gets elongated to focus image on the back of your eye  Accommodation – change in the shape of the lens to focus on objects that vary in distance The Retina • After travelling through lens • Light passes through vitreous humour - clear, jelly-like substance that comprises the main chamber inside the eyeball • Light finally lands on the retina – neural tissue that lines the back of the eye • Nasal side of the retina cross over, temporal side of the retina does not cross over The Retina • Retina – where physical stimulus/light is first translated into neural impulses • Start with retina when discussing neural processing of visual information • Retina is paper-thin sheet that covers back of the eye • Made up of complex network of neural cells arranged in three different layers • Organization of layers – counter-intuitive Retinal Layer 1: Photoreceptors  Layer at very back of the eye; farthest away from the light  Photoreceptors are located • cells in the retina that are responsible for translating physical stimulus of light into a neural signal that the brain can understand • to reach the photoreceptors, light must pass through the two other layers of retinal tissue which are transparent • the inside-out arrangement has to do with where photoreceptors get their nutrients from – a layer of cells at the very back of the eye (retinal pigment epithelium, RPE) • photoreceptors would die without access to RPE cells • if photoreceptors were located at the front of the retina, racing the light, they would not have access to the RPE that they need to survive Photoreceptors: Rods and Cones • two types of photoreceptors – rods and cones o each named for their respective shapes • human eyes have about 125 million rods but only 6 million cones • cones designed to operate at high light intensities and are primarily used for day vision • cones provide us with the sensation of colour and provide good visual acuity/sharpness of detail • cones become more concentrated towards the fovea – tiny spot in the middle of the retina that contains exclusively cones o want to see something in detail – move eyes so that image falls directly into fovea • rods are designed to operate at low light intensities • primarily used for night vision • provide no colour information and offer poor visual acuity • no rods in the fovea itself – with increasing concentration in the region just surrounding the fovea • arrangement make rods very useful for peripheral vision o explains why when you’re trying to see an object in an environment that is dimly lit, you’re better off looking slightly to one side of the object as opposed to trying to stare right at it o when you stare right at it, the image is focused on the cone-rich fovea which don’t work well in a dimly lit environment o by staring to one side of the object, you’ll be using your rods and increasing the change that you’ll see it o ex. star gazing and trying to focus on a dim star Bipolar and Ganglion Cells • photoreceptors send information to the next layer of cells in the retina – bipolar cells (layer 2) • bipolar cells send their information on to the next layer of cells in the retina – ganglion cells (layer 1) Retinal Layers 2&3 • ganglion cells collect information from a larger segment of the retina • axons of these cells all converge on one point in the eye – optic disc • leave eye to join the optic nerve – travels to the brain • optic disc is an exit hole in the eye for ganglion axons – small area contains no photoreceptors at all (constitutes our blind spot) Overview of Light through Retina • light enters the eye and must pass through the ganglion cells, bipolar cells and strike the photoreceptors on the retina at the very back of the eye • light is converted into a neural signal that is sent from the photoreceptors to the bipolar cells • then on to the ganglion cells, axon makes up the optic nerve Processing in the Retina • cells in the retina that allow areas within a retinal layer to communicate with each other – horizontal cells and amacrine cells o cells allow information from adjacent photoreceptors to combine their information o information from 130 million rods and cones in the retina converge to travel along 1 million axons in the optic nerve  some amount of visual processing is done in the retina, before the signal is sent on the brain Receptive Field in the Retina • photoreceptors in the retina divided up into specific groups, information from each group getting assimilated into one signal that affects the ganglion cell down the line • in the fovea, the photoreceptor “group” for a particular ganglion cell may only contain one cone – ganglion cell is representing a very small area of the image • each cone in the fovea has a direct link with the brain o a lot of detail is preserved and more visual acuity occurs in the fovea • more often, input from many rods and cones is combined into one neural signal for one retinal ganglion cell • groups get larger as we move toward the periphery of the eye – reason why visual acuity is lower for peripheral vision • collection of rods and cones in the retina that, when stimulated, affects the firing of a particular ganglion is called the receptive field of that retinal ganglion cell Conclusion • cells just a few synapses away from our visual receptors are already beginning to process the incoming information by accentuating certain features of a stimulus, like its edges Visual Pathways Introduction • brain – place where visual perception all comes together • visual system comprised of a set of assembly lines • areas along visual pathways process parts of the visual input before sending those partially-processed bits of information on to the next set of areas down the line for further processing Visual Fields & Hemispheres • right and left halves of our visual field are processed by the contralateral side of our brain • visual input from our right visual field travels along the optic nerve to the left hemisphere • visual input from the left visual field travels along the optic nerve to the right hemisphere • visual field sends information to both eyes, each hemisphere receives information from both eyes • before reaching respective hemispheres, axons from inner region of each retina (region of retina closest to the nose) have to cross over to the opposite hemispheres • point at which the optic nerves from inside half of each eye cross over to the opposite hemisphere – optic chiasm Main Pathway: Occipital Lobe • after optic chiasm, information from each visual field arrives in the opposite hemisphere, at which point the optic nerve fibres split and travel along two pathways • most of the retinal/ganglion cell axons travel along the main pathway and synapse in the lateral geniculate nucleus (LGN) o part of the thalamus that receives visual information o after being processed here, visual signals are sent to the areas in the occipital lobe that make up the primary visual cortex • over 20 cortical areas that process visual information • most of the research done on visual processing has concentrated on area V1 of the occipital lobe, known as the primary visual cortex • collectively, the visual processing areas in the occipital lobe outside of the striate cortex are know
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