Lecture 5 - Vision - Lancaster University PDF

25/11/2024 PSYC112/132: Introduction to Neuroscience Whilst we wait to get started… A riddle! Week 7: Thursday 21st November 2024 Dr Abigail Fiske...

25/11/2024 PSYC112/132: Introduction to Neuroscience Whilst we wait to get started… A riddle! Week 7: Thursday 21st November 2024 Dr Abigail Fiske “I speak without a mouth, I have no ears, but I can be [email protected] heard. What am I?” Remember to check in! 1 1 An echo! (link to our Hearing lecture) An echo is a repetition of a sound due to reflection Sound waves reflect off of different surfaces (e.g., bottom of a well, in an empty room, inside a cave) There is usually a delay between the source of the sound and the sound arriving at the listener’s ears Some animals (dolphins, whales, bats) use echo for location and navigation! Echoes are the basis of sonar technology (submarine navigation) and also echocardiograms (a high frequency sound wave creates echoes when they bounce around different parts of the body e.g., the heart) 2 2 1  25/11/2024 Questions, Comments, Concerns? Please do get in touch – I’m very happy to help [email protected] Office: Fylde C42 Book a meeting with me here Message me on Microsoft Teams Post in the Discussion Forum 3 3 Lecture 4: Vision 4 4 2  25/11/2024 Learning Objectives Describe the anatomy and function of the different structures of the eye Understand the role of the retina, rods and cones in vision Understand how visual information (light) is converted into electrochemical signals in the brain By the end of the lecture, you will have a basic understanding of the visual system and the process by which the brain processes visual information. 5 5 The “Why” As psychologists, it is important to understand how the brain processes visual information Insights into: Visual perception Visual attention Visual illusions Object recognition (development) Face perception And many more 6 6 3  25/11/2024 Neuroscience in the Real World 7 7 Part I: Light and the Eye 8 8 4  25/11/2024 Light Light = waves of electromagnetic energy between 380 and 760 nm Two properties of light: Wavelength –important for perception of colour Intensity – important for perception of brightness Light waves also have different frequencies – the frequency of visible light ranges from 430 trillion Hz to 750 trillion Hz (1 Hz = 1 wave per second) Light travels very quickly: 299, 792, 458 m/s (~300,000,000 m/s) Light reflects off objects and surfaces which gives us information about location, shape and colour 9 9 Electromagnetic Spectrum Electromagnetic radiation is a form of energy that is all around us Many types: Radio waves Microwaves X-Rays Gamma Visible light is only a very small part of the spectrum 10 10 5  25/11/2024 Electromagnetic Spectrum 11 11 Visible Light The human eye can detect “visible light” – that is – wavelengths of electromagnetic radiation between 380 – 700nm Cone-shaped cells in our eyes (“cones”) act as receivers tuned to the wavelengths in this visible light spectrum Viewing light through a prism 12 12 6  25/11/2024 The Human Eye Cornea – transparent part of the eye that covers the iris and the pupil Vital protective layer of the eye Refraction of light that enters the eye Filters some UV rays 13 13 The Human Eye Cornea – transparent part of the eye that covers the iris and the pupil Vital protective layer of the eye Refraction of light that enters the eye Filters some UV rays Keratoconus – thinning of the cornea that forms a cone-shape and distorts vision Myopia (near-sightedness) occurs when the cornea is too curved. Distant objects appear blurry! 14 14 7  25/11/2024 Keratoconus An example of how the visual system interferes with our perceptual reality – the brain processes the input appropriately, but vision is distorted due to the physical shape of the cornea 15 15 The Human Eye Iris – donut-shaped bands of tissue (coloured part) – regulates the amount of light that reaches the eye Light enters the eye through the pupil (the black hole in the iris) Pupil size changes in response to brightness Bright light = small pupil because sensitivity is not important, sharper image Dim light = large pupil to let in more light but sacrificing acuity and depth of focus 16 16 8  25/11/2024 The Human Eye Lens (behind each pupil) focuses light onto the retina Ciliary muscles support the lens to change shape to focus on objects Near objects = lens assumes its natural cylindrical shape Far objects = lens flattens Process of adjusting shape of lens to focus is called accommodation 17 17 The Human Eye Retina – converts light to neural signals Fovea – provides the clearest vision – structured to let light fall directly onto cones Blind spot – small portion of the visual field where the optic nerve meets the retina. There are no photoreceptors (rods or cones) here 18 18 9  25/11/2024 The Human Eye Retina – converts light to neural signals The image on the retina is inverted (back-to-front and upside-down) but your brain interprets the image the right way up The image is inverted because the curved cornea bends the light entering the eye 19 19 The Position of the Eye Humans have their eyes mounted side-by-side on the front of their heads Sacrifice the ability to see behind But what is in front can be seen with both eyes simultaneously (depth perception) Important for predator to know how far away their prey is Some animals have eyes on the side of their heads Can see behind them Important for prey to know where their predator is 20 20 10  25/11/2024 Eye Movement Movement of the eyes are coordinated so that each point in the visual field is projected to corresponding points on the two retinas Convergence – turn slightly inward, greatest for close-up vision Binocular disparity – difference in position of same image on two retinas 21 21 Eye Movement We constantly move our eyes to scan a visual scene Saccades – quick, small eye movements Snapshots of the visual scene are then integrated by the brain Static retinal image fades quickly, visual system responds best to change / movement We can measure eye movements using an eye-tracker 22 22 11  25/11/2024 Eye Tracking in the Real World! 23 23 Where else could we use eye-tracking to collect information about looking behaviour in the real world? 24 24 12  25/11/2024 Part II: The Retina, Rods and Cones 25 25 The Retina Retina is a layer of photoreceptor cells and glial cells at the back of your eye Captures incoming photons Transmits photons as neural signals (electric and chemical) to the brain to perceive a visual image 26 26 13  25/11/2024 27 27 The Retina Retina is comprised of five types of neuron Receptors (rods & cones) Horizontal cells Bipolar cells Amacrine cells Retinal ganglion cells Retinal neurons communicate chemically via synapses (NTs) and electrically via gap junctions (ionic movement through channels in neurons that are connected by cytoplasm) 28 28 14  25/11/2024 The Retina Light reaches the receptor layer after it passes through the other layers Receptors are activated and transmit the signal back to the retinal ganglion cells, which are connected to the optic nerve The blind spot is a gap in the layer of retinal ganglion cells where the optic nerve meets 29 29 The Blind Spot Close your left eye and stare at the cross You should notice the black circle but don’t look at it Move closer to the image, and eventually you’ll see the black spot disappear! 30 30 15  25/11/2024 Cone and Rod Vision Cones = photopic vision Good lighting Less sensitive receptors High acuity (fine-detailed) Colour ~7 million per retina Rods = scotopic vision Dim light More sensitive receptors Lack of detail Lack of colour ~100 million per retina 31 31 Cone and Rod Vision - Convergence Photopic system = lower convergence Only a few cones converge onto each retinal ganglion cell (good acuity, but poor sensitivity) Scotopic system = greater convergence Several hundred rods converge onto a single retinal ganglion cell (more sensitivity but poorer acuity) 32 32 16  25/11/2024 Cone and Rod Vision - Convergence Dim light stimulates many rods simultaneously, but the outputs summate onto the retinal ganglion cell (sensitive but poor acuity) The output of dim light on the cones cannot summate so the retinal ganglion cells may not respond at all to light 33 33 Cones and Colour Vision Three different types of cones (to give us trichromatic vision!) 10%: Short-wave cones (blue) – ~445nm 30%: Medium-wave cones (green) – ~535nm 60%: Long-wave cones (red) – ~575nm Our perception of colour depends on the relative activities of these three cone types 34 34 17  25/11/2024 Colour Blindness Colour blindness is due to damage or differences in the cones of the eyes Red-green colour blindness (most common) is usually present at birth / inherited Shades of red and green are hard to distinguish and may appear brown Blue cone monochromacy is also inherited and mainly affects males Red and green cones don’t work properly Blue cones function as normal 35 35 Part III: Converting Light to Neural Signals 36 36 18  25/11/2024 Visual Transduction Visual transduction = conversion of light to neural signals by the visual receptors (rods and cones) Light reaches the receptor layer after it passes through the other layers Receptors are activated and transmit the signal back to the retinal ganglion cells, which are connected to the optic nerve 37 37 Visual Transduction Rods and cones (photoreceptors) contain photo-sensitive pigments These pigments are coupled with proteins that respond to light When light reaches these proteins, a chemical reaction occurs (neurotransmitter release across synapses) that generates electrical signals (action potentials) that are carried to the optic nerve The retina and the optic nerve meet at the optic disc 38 38 19  25/11/2024 From Retina to Primary Visual Cortex The retina-geniculate-striate pathway conducts signals FROM each retina TO the primary visual cortex (also known as the striate cortex or V1) VIA the lateral geniculate nuclei of the thalamus 39 39 Retina-Geniculate-Striate Pathway All signals from the left visual field reach the right primary visual cortex (and vice versa) Lateral geniculate nucleus (thalamus) has six layers – three layers receive input from one eye, three layers receive input from the other Neurons in the lateral geniculate nucleus project to the primary visual cortex where visual information is analysed and processed Retinotopic – each level of the primary visual cortex is organised like a map on the retina 40 40 20  25/11/2024 Visual Cortex Hierarchical flow of visual information Primary visual cortex (striate) Secondary visual cortex (pre-striate) Visual association cortex As information flows through the hierarchy, receptive fields of neurons become larger and respond to more complex stimuli 41 41 Dorsal and Ventral Streams Dorsal = back Ventral = belly Dorsal stream “where” flows from the primary visual cortex  dorsal pre-striate  posterior parietal cortex Ventral stream “what” flows from the primary visual cortex  ventral pre-striate cortex  inferotemporal cortex 42 42 21  25/11/2024 Part IV: Visual Processing 43 43 Centre Surround Retinal Ganglion Cells 44 44 22  25/11/2024 Receptive Fields of Retinal Ganglion Cells Receptive field of a visual neuron = the area of the visual field that can influence the firing of that neuron On-centre cells = respond to light in the central region of the receptive field by neuronal firing Off-centre cells = respond to light in the central region of receptive field with inhibition, and respond with neuronal firing to light in the periphery Responds to degree of brightness contrast between two areas of receptive field 45 45 Linearity of Ganglion Cell Receptive Fields The retinal ganglion cell firing response is a weighted sum of stimulus intensities, with positive weights in ON subregions, and negative weights in OFF subregions. 46 46 23  25/11/2024 Centre-surround Receptive Fields: Edges! White = large response by on- centre ganglion cells Black = large response by off- centre ganglion cells Grey = no response Centre-surround receptive fields of retinal ganglion cells emphasise edges Primary visual cortex will analyse the edges and “fill in” the image 47 47 Seeing Edges Perception of an “edge” is really just perception of a contrast between two adjacent areas of the visual field Contrast enhancement – enhancing the contrast at each edge that make the edges easier to see Mach band illusion – non-existent stripes of brightness and darkness running adjacent to the edges that appear as soon as the bars touch by triggering edge detection in the visual system 48 48 24  25/11/2024 Selective Sensitivities Receptive fields of retinal ganglion cells can be selective to: Contrast (on-centre / off-centre) Uniform illumination Orientation Motion Direction of Motion Receptive fields of lateral geniculate cells can be selective to: Contrast (on-centre / off-centre) Orientation Motion Direction of motion 49 49 Part V: Visual Impairments 50 50 25  25/11/2024 Would you expect there to be links between visual impairment and mental health? Why / why not? 51 51 Vision and Mental Health 52 52 26  25/11/2024 HOMEWORK 1. READ CHAPTER 6 OF THE TEXTBOOK 2. Check out the short YouTube videos on this topic in the Optional Reading table for Lecture 5 3. Look at the prep work for Lecture 6 ahead of tomorrow’s lecture 53 53 Thank you for your attention, engagement and contributions! 54 54 27  25/11/2024 How important is vision compared to other senses in shaping our perception of the world? Could we adapt fully if we lost it? 55 55 28