PSYC236 Semester Study Notes

Introduction

Definitions - Sensation = when your sensory organs transform physical properties (e.g. light energy) from the environment into electrical signals which are sent to the brain - Perception = turning the sensory information into meaningful experience/information • PERCEPTION IS CONSCIOUS - Cognition = processes by which we elaborate/store/organise/recall sensory information • Involves things like memory, recall, storing, relating • Face perception = perception, Face recognition = cognition - Pre-conscious and conscious involved

Senses - Our senses turn physical properties in the world into psychological information (perceptions)

Visual perception - Light reflected off objects into eye onto back of - Rod + cone receptor cells in retina stimulated > convert light into electrical/neural energy > sent to in brain (V1) > V2, V3…

Visual perception and parallel processing - Parallel processing = the brain has separate systems that process information independently/separately before integration • Different areas specialised for specific tasks • Info about an image > each element (motion, colour, size) individually extracted by parallel/separate pathways

Visual perception - Visual perception is very automatic and usually very accurate - Visual perception OVERCOMES these problems so we get complete image • We have 2 eyes but see 1 world/complete image • Retina is 2D but we correctly see the 3D world • When shake head the world doesn’t “shake” • When get partial view of object can recognise the objects is still whole • Still continuous vision when blink 1

Our perception can get things wrong – shown by optical illusions - Muller-Lyer Illusion - The - The Thatcher Illusion = shows facial configuration is crucial for facial recognition but only when the face is upright - Illusion of = “/adaption” to one direction causes it to change - Brain interprets things but can be wrong (e.g. subjective contours)

Why visual illusions are important - They show us our systems aren’t always accurate - They show us perception is subjective (reality is what we perceive) - They show us we are always “guessing” based on visual cues - “Change blindness” good example that we pay attention to what we attend to • “The grand illusion” = thinking that we perceive everything around us

Two important questions - What visual info is available in the environment? - What mechanisms does the brain have for utilizing this info?

Ways we study visual perception - Physiological approaches = directly observe which areas of the brain are “lighting up” in response to a stimulus • PET, fMRI, lesion studies, single cell recordings, etc - Cons of physiological approaches: • A lot more is happening in brain not just responding to the selected stimulus • Lesions are coarse not fine • Restricted to studying simple visual stimuli - Psychophysical approaches = examine the relationship between the visual stimulus and perception - Examples of psychophysical approaches: • Thresholds = minimum amount of stimulus energy to detect a stimulus (e.g. dimmest light can see) • Sensitivity = 1/threshold

Useful measures of visual perception - Visual acuity = clarity of image • Measure of central vision only • The minimum resolvable separation between two lines • Example of visual acuity test is an eye test 2

The

Main areas of the visual system - The eye (retina) - The Lateral Geniculate Nucleus (LGN) in the thalamus - The visual/striate cortex in the occipital lobe - The extra striate cortex (areas in the temporal/parietal/frontal lobes)

Top-down and bottom-up processing - Bottom up = begin with simple sensory information first > then perception - Top down = perception driven by cognition first

Vision - Light is reflected off 3d objects in the environment and create 2d image on our retina - First stages of the visual system (bottom up) break up image into small components (lines, colours, etc)

2 main visual pathways for parallel processing - The “what” pathway = responsible for shape, motion, colour, brightness, depth (VENTRAL) - The “where” pathway = responsible for perceiving location of object in space (DORSAL)

Accommodation - Focusing vision by changing shape of the lens - Use accommodation to get a SINGLE CLEAR IMAGE - 70% of the focusing is done by the Cornea - the lens only does fine tuning - Closer focus = fat lens, further focus = narrow lens

The retina - Where the photoreceptors are - Light onto eye > onto retina (where image falls) - Focus light to put a clear image on the back of the retina - The retina is a network of neurons which cover the back of the eye - Light must pass a lot of neurons before it reaches the receptors (rods & cones)

Rod and cone receptor cells - Neurons that turn the light on the retina into electrical energy - This is achieved with light sensitive chemicals called visual pigments

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- When these pigments absorb light they trigger a chemical reaction, which generates an electrical signal in the receptors - They have differences in: • Shape • Retinal distribution • Number • Dark adaptation • Spectral sensitivity • Spatial summation

Rod/cones retinal distribution

- The fovea is a small region on the centre of the retina which has the highest visual acuity - The fovea has only cones - Peripheral regions of the retina have both rods and cones - No rod or cone cells in blind spot • Blind spot =

Rod/cones total number - More rods than cones = 20:1 - 120 million rods and 6 million cones

Rod/cones dark adaption

- Our sensitivity (dimmest light can detect) increases when illumination decreases - Rod vision has much higher sensitivity than cone vision (referred to as the rod-cone break) • At rod cone break the cones reach max sensitivity at 10 mins in dark but rods continue to maximise sensitivity longer • Rods increase to maximise sensitivity for up to 20 minutes • Rod cone break = point where only rods continue to become more sensitive

Rod/cones spectral sensitivity - Rods more sensitive to short wavelengths of light (BLUE) - Cones responsible for “colour vision” - Different colours for wavelengths: • Short = blue • Medium = yellow/green • Long = red

Rod/cones spatial summation - Rods more sensitive than cones - Rods high summation - Many rods add up responses by connecting to the one ganglion cell = high sensitivity

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Other retinal neuron (A, B, H) - Between the receptors and ganglion cells - Bipolar cells = transmit info from receptors to ganglion cells - Horizontal + amacrine cells = transmit signals from one receptor to another and from one bipolar cell to another (INHIBITORY SIGNALS) • INHIBITORY SIGNALS WHICH CAUSE LATERAL INHIBITION

Ganglion cells - Ganglion cells make up the optic nerve fibre - Ganglion cells don’t respond to overall changes in light but to differences in light (DETECT EDGES)

Ganglion cell receptive fields - Receptive field = part which receives signals - Ganglion cells have circular receptive fields - On and off surround centres = THEY ARE ANTAGONISTIC - If no light on receptive fields it will have a baseline firing rate - If falls on centre of receptive field firing will increase/decrease - If light falls on both the centre and surround of the ganglion cells receptive field, there will be no increase in firing rate (excitation cancelled by inhibition)

Different types of ganglion cells - Parvocellular • Sustained response • Centre-surround antagonism • Most near the fovea • Important for detail/image/pattern perception - Magnocellular • Transient response • 4% of all ganglion cells • Spread across the whole retina • Important for motion perception - Koniocellular

Eyes to the brain - Electrical signals leave the eye through the optic nerve and then through several structures before they reach the brain - Retina to the LGN in the thalamus • BUT 10% also goes to superior colliculis (controls eye movements)

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Lateral Geniculate Nucleus (LGN) - There are 2 LGNs in each hemisphere - 2 right halves of visual field = go to left hemisphere - 2 left halves of visual field = go to right hemisphere

Retinotopical mapping in the LGN - The LGN is a Retinotopic map = each location in the LGN corresponds to a SAME location on the retina - Cells in the LGN have adjacent receptive fields on the retina - Parts of the LGN are specialised for motion (magno layers) and other parts are specialised for colour, form, and depth (parvo layers)

Occipital lobe – visual cortex – striate cortex - Many axons go from the LGN to the occipital lobe of the cortex - Striate Cortex = visual receiving area of the occipital lobe - The LGN AND the striate cortex are organised into layers and retinotopically

3 types of feature detectors cells in the striate cortex (Hubel & Wiesel) - Simple cells: • Responds to bars of a particular orientation - Complex cells: • Respond best to movement of a correctly oriented bar across the receptive field - Hypercomplex cells: • Respond to corners, angles, or bars of a particular length moving in a particular direction

Parallel pathways - All info in a visual scene extracted from parallel and independent pathways (the colour, shape, etc) then integrated together

Organisation of the visual cortex - Location columns = neurons responding to the same retinal location are found in columns - Orientation columns = neurons responding to same orientation are found in columns, so that all possible orientations are included within 1mm of the cortex - Ocular dominance columns = alternating columns which respond best to each eye - Hyper columns = - location/orientation/ocular dominance columns are combined to form hyper columns • The Cortex is made up of 1000s of hyper columns - each doing the processing for one small area of the retina

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Cortical magnification factor - Fovea = accounts for 8% of the neurons in the striate cortex - Fovea receptors tightly packed, peripheral cells widely spaced

Extrastriate visual areas - Extrastriate areas = areas outside the striate cortex also responsible for vision - 2 pathways: • The “what” pathway = goes to the temporal lobe and is crucial for identifying objects

Tilt after-effect - Caused by adaptation - Example of distributed coding firing - After staring at a pattern of tilted lines or gratings, subsequent lines appear to have a slight tilt in the opposite direction - A visual illusion in which prolonged adaptation to an oriented stimulus causes shifts in subsequent perceived orientations - This is thought to be due to an imbalance in the distribution of activity of orientation cells in the cortex (in V1) brought about by adaptation - Interocular transfer (eye change) of the tilt after-effect = if you adapt with just the left eye, the illusion can be seen with just the right eye but it is not as strong

Some definitions - Convergence = occurs when a number of neurons synapse onto a single neuron • Rods = high convergence ganglion cell > greater spatial summation > greater sensitivity in the dark - Excitation/inhibition = input from a neuron increases/decreases the firing rates of neighbouring neurons - Lateral inhibition = sending inhibition signals across the retina (explains , Herman grid, etc) - Sensory code = info contained in the firing of neurons - Specificity coding = specific stimuli trigger responses from neurons which are specilised to respond only to them - Distributed coding = representation of a stimuli by the overall pattern of firing of many neurons • Distributed coding is more efficient than specificity coding – since the activity of a small number of neurons can represent a large number of different objects

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Clinical aspects of vision

Blindness - Legally blind people can still have vision • Poor foveal vision most common cause - Legal blindness = visual acuity of 20/200 or less - Tunnel vision = when eye disease affects peripheral retina receptors NOT foveal • Can get psychological tunnel vision (hysteria)

20/20 vision = normal vision - A person with 20/20 vision is able to see letters 1/10th as large as someone with 20/200 vision - 20/15 vision is better than 20/20

Levels of vision - 20/20 = normal vision (minimum for pilot) - 20/40 =can pass driver’s licence in US - 20/200 = legal blindness

5 types of vision damage - Light not focused clearly on retina - Light is blurred as it enters the eye - Damage to the retina - Damage to the optic nerve - Damage to the brain visual areas

Damage 1 = Light not focused clearly on retina - Causes: • Eyeball too short or too long • Damage to the cornea • Lens doesn’t work properly - Conditions: • Myopia = near-sightedness • Hyperopia = far-sightedness • Presbyopia = old eye • Astigmatism = misshapen cornea

Myopia - Myopia = can’t see distant objects clearly - 2 types: • The cornea/lens bend light too much 8

• Eyeball is too long - Can be fixed with corrective lenses or laser eye surgery to give cornea LESS power

Hyperopia - Hyperopia = can’t see close objects clearly - Cause = the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short - Can be fixed with corrective lenses or laser eye surgery to give cornea MORE power

Presbyopia - Presbyopia = can’t accommodate eyes due to old age - This decrease in accommodation affects the location of the near point (the closest distance at which a person can still see an object in focus) • Near point becomes further away as get older

Astigmatism - Astigmatism = has misshapen cornea which distorts light reaching the retina - Normal cornea = spherical, astigmatic cornea = elliptical

Measurement of refractive error - Diopter = unit of measurement of the refractive error - Myopia = negative, hyperopia = positive - EG The degree of myopia can vary from low (-1 to -3 diopters) to high (greater than 6 diopters) (A -1.00 diopter myope is able to see objects at 1 meter clearly)

Problems with optical correction - Correction of myopia with lenses results in image minification - Correction of hyperopia with lenses results in image magnification - surgery DOES NOT PREVENT the age-related loss of the eye's ability to vary its focusing power (accommodation) - If you are over 40 and have both your eyes fully corrected for distance vision, you will need reading glasses for near work - As an alternative, you may elect to leave one eye slightly nearsighted

Damage 2 – Light is blurred as it enters the eye - Causes: • Scarring of the cornea OR clouding of the lens blurs entering light - Conditions: • Cornea injury/disease • Cataract

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Cornea disease/injury - Cornea must be transparent for clear vision - Damage/disease = scar tissue which decreases visual acuity (halo of light) - 2 types of treatment: • Drugs which make cornea clear • Cornea transplant

Clouding of the lens = CATARACT - Lens needs to be transparent for clear vision - Cataract surgery = remove lens but keep capsule and replace with plastic lens but CAN’T focus this) - Clouding of the lens (cataract): • = present at birth • Secondary cataract = caused by eye disease • Traumatic cataract = caused by injury • Senile cataract = caused by old age (Most common cause is old age – 75% of people over 65 and 95% of people over 85 have cataracts)

Damage 3 – Damage to the retina - Causes: • Disruption the blood vessels that supply blood • Separating it from its blood supply • Disease that attack the receptors - Conditions: • Macular degeneration • Detached retina • Diabetic retinopathy (hereditary degeneration) - Damage to retina = SCOTOMA • Partial loss of vision or blind spot

Macular degeneration - Symptoms = you can see everywhere except where you are looking (centre) - The macula (surrounds the fovea) makes centre vision blindness (CAN’T READ)

Age-related macular degeneration - Most common type - Mild form = thinning of cone receptors + retina lumps - Worse form = new blood vessels grow rapidly in macula and leak fluid > kills cone receptors

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Detached retina - When the retina becomes separated from the pigment epithelium - Usually caused by dramatic sport injury (e.g. boxing) - Warning signs: • Flashing lights • Floaters in vision • Shadows in periphery • Grey curtain moves across field of vision - Why it affects vision: • For clear image = retina must lie smoothly on pigment epithelium • The visual pigments in the detached area are separated from enzymes needed for pigment regeneration > pigments can’t regenerate and go blind - Treatment for detached retina: • Operation = freeze the retina back onto the eye (NEEDS TO BE DONE QUICKLY)

Damage 4 – Damage to the optic nerve (GLAUCOMA) - Glaucoma = optic nerve degeneration produced by a build-up of pressure in the eyeball • Because the fluid in the eye (aqueous humour) does not drain properly from the eye > pressure increases • This increased pressure presses on the optic nerve at the back of the eye and cuts off its circulation > degeneration of the nerve - Glaucoma treatment: • Medication (if early) • Iridectomy (hole made in iris with laser to decrease pressure) - Check for eye pressure over 40 = intraocular pressure - Other causes of optic nerve degeneration = bad circulation, toxic substances, tumours

Parallel vision pathways - WHAT pathway = ventral stream • From P-ganglion cells on the retina to Players of the LGN & Striate Cortex, then goes to the Inferotemporal cortex - WHERE pathway = dorsal stream • From M-ganglion cells on retina to M-layers of the LGN and striate cortex, then goes to Posterior Parietal Cortex

Damage 5 – Damage to the visual cortex - Brain damage can cause many different vision issues - To find area of damage look for DISSOCIATIONS (where one function is absent while another is present) 11