Vision in Biological Psychology PDF

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Vision: The Eye:

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Lens – allows us to see things in focus Vitreous Humour – liquid, transparent and lets light through Retina – where most of the processing happens Optic Nerve – projects into the brain initially into the relay station, and then to the visual cortex

Function of Key Parts of Eye Anatomy:

1. Light comes in from the external world – light emits particles called Photons, which bounce of solid objects before reaches the eye 2. It then passes through transparent structures (in order to see well the tissue must be transparent) – the cornea, the crystalline lens, and the vitreous humour 3. The light then gets converted to nerve impulses once it hits the retina 4. Finally, it is sent up the optic nerve to the brain The Retina:

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It’s a key part of the eye for visual processing It aligns the inner surface of the eyeball

Optic Nerve: - It’s a key assembly/exit point from the retina - It’s through the optic nerve that information gets sent to the visual cortex, where it is processed further

Retinal Cells and Retinal Circuitry: Cells in the Retina:

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The retina consists of millions of cells, and they have a particular structure that enables their function

Types of Cells in the Retina – Layers:

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Bottom Layer of the Retina: 1. Photopigment cells (rods, cones) – sensitive to light Then two layers of neurons: 1. Middle Layer: Bipolar Cells – the first neurons of the visual system 2. Layer of Cells Closest to Inner Eyeball: Ganglion Cells

Photopigment Cells - Rods and Cones: - Rods:  They respond to light only in black and white  They function well in dim lighting conditions because of the mapping shown above – there are many rods mapped together so they pool together the light from multiple rods onto one bipolar cell - Cones:  They’re sensitive to light in specific colour – there are three types: blue, green and red – and they’re sensitive to light of that particular wavelength  They allow us to see in colour vision  They’re better for processing fine detail, as they’re in one-to-one mappings in bipolar cells (again shown above on the left-hand side) so they have a higher resolution

Rods

Cones

More in periphery of retina More in centre of retina Good for dim light (due to pooling Good for bright light Not good for detail Good for detail/colour Colour-blind - Rods and cones aren’t distributed evenly across the retina, as shown in the diagram below:

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There are a lot more cones in the centre of the retina, because they’re good for processing fine detail, and we tend to focus our eyes with the centre of our retina – this allows us to read The rods are more numerous in the periphery of the retina – they’re good for noting movement from the corner of your eye, and for seeing in dimmer lighting conditions

Transduction: Transduction- Photopigments: - The transduction process involves transforming the physical stimulus of light into a neural signal - The first step in this process is a reaction that occurs in the Photoreceptor Cells:  They contain Photopigments – chemicals that consist of Opsin (cones)/Rhodopsin (rods) and Retinal which is synthesised from Vitamin A – which highlights the need for micronutrients for maintaining vision health Transduction Step 1- Photopigment Release: - Light hits the photoreceptors at the back of the retina – to get there it has to travel through the other layers of cells - When light hits the photopigment cell, the photopigment contained in it is released - This then causes a complex chemical cascade Transduction Steps 2 and 3 – Neural Cell Activation: - The release of photopigment is a chemical event – at that stage there is not yet a neural event - The neural event starts when the bipolar cell, which is sensitive to the presence/absence to the photopigment, starts firing at a different rate from its resting rate in response to the release of the photopigment

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That in turn sends a signal to the nearby Ganglion Cells, and they then pass the signal into the brain (as they have very long axons), all the way through the optic nerve into a structure that’s part of the thalamus named the LGN Therefore, there is neural processing in the retina (as we have two layers of neurons – bipolar cells and ganglion cells)

Photopigment Reacting with Light: - When photopigments react with light, they undergo a process called Bleaching – they change structure and colour (e.g. in the case of Rhodopsin – it changes from pink to a pale yellow)

Visual Image and Retina: -

In the retina, each cell looks out for a small portion of its visual field and if it detects light of a particular level of interest in that part of the visual field, it starts changing its firing rate. Across the whole retina there are millions of cells, and they all do their own thing If you look at the whole pattern of activation, it’s clear to see a pattern of activation across the retina that resemble the object in the external world – this is called a Visual Map (it happens in the retina, but it also happens in the brain):  This is how we see the external world and how we represent it

Visual Field Map – Ganglion Cell Firing Patterns Across the Retina:

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The above image displays on the left hand side you can see a photograph of a person, on the right you can see how the retinal ganglion cells respond to that image (they pick out the edges and represent the shape of the external object through the pattern of firing that you see as the neural image)

Interim Summary – The Eye and the Transduction Process: -

Light stimulates photoreceptors in the retina to release photopigment This impacts the neuron (bipolar cell) in the eye, whose firing rate changes

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This is passed on to further nerve cells (ganglion cell), whose axons point to and form the optic nerve, sending visual information to the brain by way of firing rate patterns across millions of neurons The pattern of firing in the eye is like a moving “photocopy” of the real world in its spatial layout, but using dynamic firing in millions of cells instead of ink

The Visual Pathway:

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The above diagram displays an image of a brain cut between hemispheres, which outlines the major parts of the visual pathway – the Eye, Optic Nerve, LGN and Visual Cortex (visual information is relayed in this order through axons) The Lateral Geniculate Nucleus (LGN) is a small nucleus which is part of the thalamus – the thalamus is a major relay station for sensory information coming into the brain

On the above diagram, the brain has been sliced horizontally – it displays the visual pathway in a more detailed way

Visual Fields and the Optic Chiasm: - Everything you see in your left visual field is presented in your right visual cortex, and vice versa - This is displayed in the below diagram:  This happens because the long axons from the Ganglion cells in the eyes that are on the right of the retina of each eye point towards the LGM on the right, and from there to the visual cortex on the right, and vice versa

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To do this, the axons from the right of the retina of the left eye have to cross over to the right side of the brain, and the place where they cross over is called the Optic Chiasm

The Visual Cortex: -

The Visual Cortex is part of the occipital lobe It divides into two major parts: 1. The Primary Visual Cortex (a.k.a. V1; Brodmann area 17; Striate (Stripy) Cortex) – shown in the below image as a yellow colour: 2. The Extrastriate Visual Cortices (a.k.a. V2; V3; V3A; V4; V5) – shown in the below image as the remaining stripe colours:

Retinotopic Representation: - The ganglion cells in the retina preserve the spatial layout of a visual scene in the external world (the photograph analogy – it outlines an object in the visual world)– the same happens in the Primary Visual Cortex (V1) and the Extrastriate Visual Cortices:  Each of these areas make their own visual maps of the visual world outside – this kind of representation is called Retinotopic Representation (as it resembles the spatial layout at the retina) - This is represented in the image below:

Zooming in on the Visual Cells in V1: - The cortex is very neatly organised – all across the cortex there are 6 horizontal layers of cells (this is known as Laminar Organisation), as shown in the below diagram:

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This laminar organisation helps to keep the information streams organised

Segregation of Information Synapse Series: Eye – LGN – V1: - The cells from the LGN synapse mostly in V1

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In the eye the cones and the rods perform different functions – cones for colour and detail, rods for larger areas of space in black and white – this division of labour is maintained through the LGN and through the cell layering in V1 :  Cones (colour): the Koniocellular layer – information feeding up from the ganglion cells, through the LGN and into layers 2 and 3 of V1 – as they occur in clusters, they’ve been named “blobs”  Cones (detail): the parvocellular pathway (the pathway of small cells) – you get synapses into layer 4Cβ of V1  Rods: the magnocellular pathway (the pathway of large cells) - they project into layer 4Cα OF V1

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Magno – Spatial Information/Movement (Magnocellular) Parvo – Detail (Parvocellular) Konio – Colour (Koniocellular)

After V1 – Extrastriate Cortex: - The specialisation that started in the eye, through the LGN and into V1 (through the specialist cells – magnocellular, parvocellular and koniocellular) continues in the extrastriate cortex

Extrastriate Visual Cortex: -

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The cells from V1, having received their information from the eye via the LGN now synapse into the extrastriate cortex Specialist processing streams exist (e.g. the colour information from the cones ends up in extrastriate area V4), these are:  V2/V3 – further integration  V3a – dynamic form, colour-blind (from Magno)  V4 – space and colour (from Konio)  V5 (MT, middle-temporal) – motion (from Magno) The continued segregation of information is to optimise the efficiency of processing Just like the retina and V1, each of the extrastriate areas also makes its own visual map of the external world

Vision Interfacing with Other Functions: Vision Interfaces with Motor Control and Cognition: - The motor system allows us to perform visually guided actions (such as playing tennis, drawing, etc) - Vision allows us to recognise objects/people (object/person recognition) “What and “Where” Pathways:

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It has been proposed that we have different pathways from the visual system to help us process the visual information further – these have been called the “What” Pathway and the “Where Pathway:  The “What” Pathway: deals with recognising objects – projects from the visual cortex to the temporal lobe  The “Where” Pathway: deals with us interacting with the world and guiding our motor action – projects from the visual cortex to the parietal cortex The “What” and “Where” Pathways were proposed by Mishkin & Ungerleider (1982)

From Retinotopic to Abstract: - Within the temporal cortex, where object recognition might occur, the representations there become less retinotopic, and they get translated into more abstract concepts (e.g. words or names) – in this way the visual system interacts with our language system - However, there is still a need for visual maps in the parietal cortex to guide our visual actions Higher Level Visuo-Cognitive Area and Proposed Specialisation: - There are further areas that are thought to specialise in visual processing:  Fusiform Gyrus – an area in the temporal lobe (therefore outside the primary visual areas), it’s thought to be specialised for face processing (although some people disagree and think it’s specialised for object recognition)  It’s been found that people who have damage to this area suffer from Prosopagnosia (an inability to recognise faces)

Bottom-Up Processing and Top-Down Modulation: -

Bottom-Up Processing: processing information that comes from the external world Top-Down Modulation: information from our own brains can influence how we process visual information (affecting lower levels of processing) – sometimes this is under conscious control, and sometimes it isn’t

Top-Down Modulation: - E.g. Imagine you’ve lost your keys somewhere in your room and you know there’s a red key fob on your keys:  The frontal lobe will form the goal of finding these keys, and a defining characteristic is the red of the key fob  The frontal lobes will now configure the lower level visual areas that process colour to become more active, to help you in achieving these goals (THIS IS ONE FORM OF TOP-DOWN MODULATION) - Another form of top-down modulation can occur in the absence of external visual input:  This can be when there is visual imagery (e.g. when you try to imagine someone’s face or a location you’ve been to)  Some people experience visual hallucinations – they cause genuine activation in the visual areas, but in the absence of external visual input Blindsight: - Blindsight: This is where an individual cannot see consciously, but can use some visual information to navigate around objects:  This works as it’s thought there are some shortcuts directly via the eye and LGN to higher level visual areas...