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 visual processing 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 retina - Rod + cone receptor cells in retina stimulated > convert light into electrical/neural energy > sent to visual cortex 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 Poggendorff Illusion - The Thatcher Illusion = shows facial configuration is crucial for facial recognition but only when the face is upright - Illusion of spinning dancer = “accommodation/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 Visual System 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 3 - 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 = optic nerve 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 4 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) 5 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