Intro, Eye & Retina, Receptive Field, Visual Cortex.pdf

Lecture 4: Visual Cortex PSYC11312: Sensation and Perception Dr. Becky Champion [email protected] Last week: Receptive fields • How retinal ganglion cells respond to changes in patterns of light within their receptive field? • How receptive fields of these cells can expl...

Lecture 4: Visual Cortex PSYC11312: Sensation and Perception Dr. Becky Champion [email protected] Last week: Receptive fields • How retinal ganglion cells respond to changes in patterns of light within their receptive field? • How receptive fields of these cells can explain a number of visual illusions? + + + - - - - - - - - - Questions from last week… 1. If light falling on a region of the retina causes a decrease in the response of a ganglion cell – what type of region is this? 2. If a spot of light falls outside the receptive field of a ganglion cell what firing rate will be produced? 3. Why do receptive fields increase in size with eccentricity (distance from fovea)? 4. What kind of stimuli are ganglion cells good at detecting? Today: Visual cortex • How does the brain process complex visual stimuli? • Which visual areas are responsible for processing different types of visual stimuli? Primary visual cortex The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Lecture Overview • Recap – Ganglion cell receptive fields • Optic nerve, optic tract and LGN • V1 (Primary visual cortex) • Receptive fields in V1 • Visual processing beyond V1 Photo receptor cells Bipolar cells Retinal ganglion cells light Recap… Retinal Ganglion Cells Horizontal cells Amacrine cells Recap… Retinal Ganglion Cells • Receptive field = region of the retina which, when stimulated, affects the firing rate of the cell • Ganglion cell receptive fields have centre - surround organisation • They detect spots of light and edges + + + - - - - - - - - - + + + - - - + + + + + ON -centre OFF -surround OFF -centre ON -surround Lecture Overview • Recap – Ganglion cell receptive fields • Optic nerve, optic tract and LGN • V1 (Primary visual cortex) • Receptive fields in V1 • Visual processing beyond V1 Optic Nerve & Optic Tract Retinal ganglion cells • Ganglion cell fibres leave retina along optic nerve Optic Nerve & Optic Tract • O ptic chiasm = cross over point • Some fibres cross over and some don’t • Beyond optic chiasm the optic nerve becomes the optic tract Optic tract Optic Nerve & Optic Tract Optic Tract • Information now separated by visual field rather than by eye. • Information from right visual field represented by left hemisphere and vice versa. Optic tract • The optic tract feeds into LGN LGN - Lateral Geniculate Nucleus LGN • LGN = bilateral structure (one in left hemisphere and one in right) • Each LGN receives input from left and right eyes – but keeps these inputs separate LGN Receptive F ields • LGN cells have the same receptive field organisation as retinal ganglion cells: centre - surround antagonism. • Ideal for detecting spots of light & edges • B ut NOT able to detect orientation of bars/ edges + + + - - - - - - - - - + + + - - - + + + + + ON -centre OFF -surround OFF -centre ON -surround Lecture Overview • Recap – Ganglion cell receptive fields • Optic nerve, optic tract and LGN • V1 (Primary visual cortex) • Receptive fields in V1 • Visual processing beyond V1 V1 (Primary visual cortex) V1 or Primary visual cortex V1 (Primary visual cortex) V1 receives its input from LGN V1 (Primary visual cortex) • Also known as striate cortex ... because its stripy • Stimulus presented to animal • An electrode, inserted into a neuron (e.g. V1 cell), measures electrical activity of a SINGLE neuron time voltage Stimulus Record from V1 cell Measure activity over time Single cell recording (physiological approach) Hubel and Weisel V1 cell response Retina Response of V1 cell V1 cells have a level of baseline activity when no stimulus is presented. V1 cell response Retina Response of V1 cell In 1950s, Hubel and Wiesel could not find a stimulus to excite the V1 cell V1 cell response Retina Response of V1 cell Increase in response Until they found they got a big response when the edge of the slide moved across the receptive field From this they realised V1 cells like lines instead of spots! Orientation selectivity in V1 • V1 cells prefer lines of a particular orientation Y Organisation of V1 • Retinotopic mapping • Cortical magnification • Orientation columns • Ocular dominance columns Retinotopic mapping in V1 O bjects close together in the visual scene are analysed by neighbouring parts of V1. Organisation of V1 • Retinotopic mapping • Cortical magnification • Orientation columns • Ocular dominance columns Cortical magnification in V1 • A mount of cortex devoted to representing each part of the retinal field is distorted • Fovea represented by large area of cortex Retinal image Representation in V1 Fovea accounts of 0.01% of the retina but is represented by 8 - 10% of V1 Cortical magnification in V1 Organisation of V1 • Retinotopic mapping • Cortical magnification • Orientation columns • Ocular dominance columns Orientation columns 2. Recording from an electrode penetrating the cortex at an angle to the surface 1. Recording from an electrode penetrating the cortex perpendicular to the surface • Orientation preferences of V1 cells arranged in ordered way • This organisation was investigated by : Orientation columns Perpendicular to the surface all cells have same orientation preference (an orientation column) At an angle to the surface the cells’ orientation preferences vary systematically Organisation of V1 • Retinotopic mapping • Cortical magnification • Orientation columns • Ocular dominance columns Ocular dominance columns • Cells in LGN are monocular (respond to input from left or right eye, but not both) • 80% cells in V1 are binocular (respond to input from both eyes) • Most binocular cells respond better to one eye than other = ocular dominance • Cells with the same ocular dominance (i.e. same eye preference) are arranged in columns Columns in V1 Orientation columns Ocular dominance columns Lecture Overview • Recap – Ganglion cell receptive fields • LGN • V1 (Primary visual cortex) • Receptive fields in V1 • Visual processing beyond V1 Receptive fields in V1 • 3 different types of cell in V1 each with distinct receptive field organisation – Simple cells – Complex cells – Hypercomplex cells Photoreceptors Ganglion cells LGN cells Simple cells Complex cells H ypercomplex cells V1 Simple Cell Receptive Fields Simple cells respond to oriented bars and edges The receptive field has excitatory and inhibitory regions, but they are elongated - - - - - - + + + + + + Excitatory area Inhibitory area - - - - - - + + + + + + Simple Cell Receptive Fields A vertical bar covers only the excitatory region causing a big excitatory response - - - - - - + + + + + + Simple Cell Receptive Fields A bar tilted slightly away from vertical covers some of the excitatory region but also some inhibitory region causing a weaker excitatory response - - - - - - + + + + + + Simple Cell Receptive Fields A horizontal bar covers only a small part of the excitatory region but a larger part of the inhibitory region causing an inhibitory response + + - Orientation Response Retinal ganglion or LGN cell - - - - + O rientation selectivity of simple cells Response - - - - - - + + + + + + A B C + Simple cell - - O rientation selectivity of simple cells Orientation Response - - - - - - + + + + + + A B C RF of simple cell tuned to vertical bars Orientation tuning Orientation tuned neurons respond best to their preferred orientation but also respond to other similar orientations Orientation Response RF of simple cell tuned to 30deg bars O rientation selectivity of simple cells Orientation Response - - - - - - + + + + + + A B C RF of simple cell tuned to vertical bars ? Simple Cell Receptive Fields • Some simple cells have ON -centre RFs and some have Off -centre RFs, but all have a preferred orientation + + + + + + - - - - - - - - - - - - + + + + + + - - - - - - - - - - - - + + + + + + + + + + + + - - - - - - Simple Cell Receptive Fields • Some simple cells only have one excitatory and one inhibitory region + + + + + + - - - - - - - - - - - - + + + + + + - - - - - - + + + + + + - - - - - - + + + + + + - - - - - - Bar detectors Edge detectors + + + + + + - - - - - - - - - - - - + + + + + + + + + + + + - - - - - - Receptive fields in V1 • 3 different types of cell in V1 each with distinct receptive field organisation – Simple cells – Complex cells – Hypercomplex cells Photoreceptors Ganglion cells LGN cells Simple cells Complex cells H ypercomplex cells V1 Complex Cell Receptive Fields Respond to oriented lines but no discrete ON and OFF regions No discrete ON -OFF -regions +/ - +/ - +/ - +/ - +/ - +/ - Complex Cell Receptive Fields Respond to oriented lines but no discrete ON and OFF regions No discrete ON -OFF -regions +/ - +/ - +/ - +/ - +/ - +/ - Complex Cell Receptive Fields Complex cells are phase insensitive + + + + + + - - - - - - - - - - - - +/ - +/ - +/ - +/ - + + + + + + - - - - - - - - - - - - +/ - +/ - +/ - +/ - Simple cells are phase sensitive (response depends on position of bar within RF) Complex cells are phase insensitive (response does not depend on position within RF) Complex Cell Receptive Fields - respond to moving oriented bars and edges - respond best to a particular direction of movement +/ - +/ - +/ - +/ - +/ - Receptive fields in V1 • 3 different types of cell in V1 each with distinct receptive field organisation – Simple cells – Complex cells – Hypercomplex cells Photoreceptors Ganglion cells LGN cells Simple cells Complex cells H ypercomplex cells V1 Hypercomplex Cell Receptive F ields • Also called End -stopped cells • Respond to bars of: – particular orientation AND – moving in a particular direction AND – particular length +/ - +/ - +/ - +/ - +/ - +/ - Hypercomplex C ell Receptive Fields • Respond to bars of: – particular orientation AND – moving in a particular direction AND – particular length Response Response of complex and hypercomplex cells Length of line Complex cell Hypercomplex cell Complex and Hypercomplex cells Receptive field Stimulus Receptive fields increase in complexity Photoreceptors Ganglion cells LGN cells Simple cells Complex cells H ypercomplex cells V1 - - - - - - + + + - - - - - ++ - - - - - - - - + + + -- - - - - + + + + - - - - - ++ - - Jumbled summary chart (and break!) Type of cell Retinal Ganglion Cell LGN Cell V1 Simple Cell V1 Complex Cell V1 Hypercomplex Cell Receptive field Excitatory and inhibitory areas arranged side -by - side. Responds best to bars of particular orientation Responds to bars of light of a particular orientation and length moving in particular direction Centre -surround receptive field Responds best to spots of light Responds best to movement of a bar of particular orientation. Many respond best to particular direction of movement. Centre -surround receptive field Responds best to spots of light Lecture Overview • Recap – Ganglion cell receptive fields • Optic nerve, optic tract and LGN • V1 (Primary visual cortex) • Receptive fields in V1 • Visual processing beyond V1 Visual areas beyond V1 • Over 30 visual areas beyond V1 • Areas seem to be specialised e.g. – v3 form – v4 colour – v5 motion • All areas interconnected – no simple separation of function Processing streams: What vs. Where ‘What’ stream • Travels ventrally to inferotemporal cortex • Important for recognising and discriminating objects ‘Where’ stream • Travels dorsally to posterior parietal cortex • Important for determining where an object is and how to act upon it • Sometimes referred to as the ‘How’ stream Evidence: Lesions • Monkey lesion study (Ungerleider and Mishkin , 1982) Task 2: Landmark discrimination. Food is always close to the cylinder Task 1: Object discrimination. Food is always under the triangular prism Evidence: Lesions Task 2: Landmark discrimination. Task 1: Object discrimination. Lesion to posterior parietal cortex (where p’way ) causes problems for landmark discrimination task but not object discrimination task Lesion to inferotemporal cortex (what p’way ) causes problems for object discrimination task but not landmark discrimination task Evidence: Neuropsychology Visual form agnosia • Damage to ventral pathway (‘what’ stream): • C annot identify objects despite knowing their features Milner & Goodale (1998) "Visual brain in action," Psyche 4(12) Milner and Goodale (1998) Evidence: Neuropsychology Optic ataxia • Damage to dorsal pathway (‘where/ how’ stream): • Cannot reach to grasp objects, but can recognise and describe them • O pposite deficits to those shown patients with visual form agnosia (e.g. patient DF) Processing streams • Pathways not totally separate – many connections between them • Signals flow both ‘upwards’ and ‘backwards’. Feature Detectors • Cells in V1 can be thought of as ‘feature detectors’ – they respond to particular features of a image • As we move higher up in the visual system receptive fields get more complex and features they respond to become more specific • E.g. in area IT we find cells that respond to faces Feature Detectors • Monkey Area IT contains face sensitive cells Recap: Retinotopic mapping & cortical magnification in V1 Recap: Receptive fields increase in complexity Photoreceptors Ganglion cells LGN cells Simple cells Complex cells H ypercomplex cells V1 - - - - - - + + + - - - - - ++ - - - - - - - - + + + -- - - - - + + + + - - - - - ++ - - Recap: Processing streams: What vs. Where Key Concepts • Primary visual cortex (V1) • Retinotopic mapping • Cortical magnification • Orientation selectivity • Simple/ Complex/ Hypercomplex cells • What & Where processing streams Reading For this week’s lecture: • Goldstein chapter 3. Neural Processing and Perception • Goldstein chapter 4. Cortical Organization Next week: Form Perception How do we perceive the form or shape of objects? Revision questions… 1. What is ‘orientation selectivity’? 2. What type of stimuli do hypercomplex cells in V1 respond best to? 3. In what way are V1 cells in an ocular dominance column similar? 4. The ‘what’ processing stream is specialised for carrying out which types of task? Lecture 3: Receptive Fields PSYC11312: Sensation and Perception Dr. Becky Champion [email protected] Last week...The eye and the retina • How does the eye begin the process of visual perception? Questions from last week… • Which parts of the eye are responsible for focusing light on the retina? • Why does visual perception rely on rods at night? • Why do cones enable higher acuity vision? Today: Receptive fields • How do ganglion cells select the most important information to process? • How can ganglion cells explain some classic illusions? The Hermann Grid Simultaneous contrast illusion Four equally bright grey patches Simultaneous contrast illusion The same four grey patches Overview • Retinal ganglion cells • Receptive fields • Lateral inhibition • Centre -surround antagonism • Illusions explained by lateral inhibition Photo receptor cells Bipolar cells Retinal ganglion cells light Recap… The Retina Horizontal cells Amacrine cells The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Retinal Ganglion Cells • Far fewer ganglion cells than photoreceptors (about 1:126) • Ganglion cells must condense raw information from the photoreceptors • Aim to extract important information from retinal image • So how do they accomplish this task? Single c ell recording • Physiological approach • An electrode inserted into a neuron measures electrical activity • Activity is that of a SINGLE neuron Nerve impulse Recording electrode Single cell recording Nerve impulse Recording electrode time voltage Action potential Single cell recording Nerve impulse Recording electrode time voltage Action potential time voltage time voltage Increased rate of action potentials indicates increased activity of the neuron Single cell recording from ganglion cell • Stimulus presented to animal • Activity of ganglion cell recorded time voltage Stimulus Record from ganglion cell Measure activity over time Ganglion cell response Retina Response of Ganglion cell Before you put anything, at all, on the screen – you notice that the ganglion cell is already active i.e. there is some baseline activity from the cell. Experimenters try to find a stimulus that changes the activity of that ganglion cell Time Action potential Ganglion cell response Retina Response of Ganglion cell Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response (increase frequency of action potentials) Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response (increase frequency of action potentials) Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response (increase frequency of action potentials) Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response Decrease in response Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response Decrease in response Baseline activity Ganglion cell response Retina Response of Ganglion cell Increase in response Decrease in response Baseline activity Receptive Field Retina Response of Ganglion cell A cell's receptive field is the area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell. Receptive Field Receptive Field Retina Response of Ganglion cell 2 types of region: excitatory and inhibitory Receptive Field Excitatory response (increase in cell’s response rate) Inhibitory response (decrease in cell’s response rate) Overview • Retinal ganglion cells • Receptive fields • Lateral inhibition • Centre -surround antagonism • Illusions explained by lateral inhibition Receptive Fields LIGHT Receptors Ganglion cells X Receptive field of ganglion cell X • Why are ganglion cells influenced by a region on the retina? • Answer: Convergence! Photo receptor cells Bipolar cells Retinal ganglion cells light Convergence Horizontal cells Amacrine cells Receptive fields Excitatory response Inhibitory response How can convergence create 2 types of region – excitatory and inhibitory? Photoreceptor cells Bipolar cells Retinal ganglion cells light Lateral inhibition Horizontal cells Amacrine cells • Inhibition that is transmitted across the retina by horizontal and amacrine cells Lateral inhibition Baseline firing rate • Inhibition that is transmitted across the retina by horizontal and amacrine cells B 3 4 5 1 2 A 7 6 C + + + + + + + - - Inhibitory synapse 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4 3-5 2-6 1-7 Firing rate of cell B Receptors stimulated Centre - surround antagonism Inhibitory Excitatory + + + - - - - - - - - - Lateral inhibition Photoreceptors Ganglion cell Centre - surround antagonism Inhibitory Excitatory + + + - - - - - - - - - + + + - - - + + + + + ON -centre OFF -surround OFF -centre ON -surround Lateral inhibition Recap... Receptive fields • The receptive field (RF) is a region of the retina that affects the response of the ganglion cell • Stimulation of RF can have excitatory or inhibitory effect on ganglion cell response. On centre – off surround - - - - - + + + - - - - - - + + + - - - - - - + + + - - - - - - + + + - Baseline Excitation Inhibition Baseline On centre – off surround: Test? - - - - - + + + - - - - - - + + + - - - - - - + + + - - - - - - + + + - ? ? Excitation Baseline A B C D Off centre – on surround + + + + + - - - + + + + + + + - - - Baseline Excitation + + + + + - - - + Inhibition + + + + + + - - - Baseline Receptive fields • Each photoreceptor is part of the receptive field of more than one ganglion cell • Receptive fields of neighbouring ganglion cells overlap • Receptive fields of all ganglion cells together cover the whole visual field Quick Quiz (and break!) 1. If light falling on a region of the retina causes an increase in the response of a ganglion cell – what type of region is this? 2. What term describes the organisation of the receptive field of a ganglion cell? 3. What pattern of activity do you get if the whole receptive field is illuminated? 4. How is it possible for a single ganglion cell to be influenced by a large number of photoreceptors? Why have this organisation? - - - - - + + + - - - - - - + + + - - - - - - + + + - Ganglion cells respond to changes in light falling within receptive field - - - - - + + + - Why have this organisation? Ideal for detecting spots of light & edges NOT able to detect orientation of bars - - - - - + + + - - - - - - + + + - - - - - - + + + - Light Dark Light Dark Dark Why have this organisation? • No response to changes in overall level of illumination - - - - - + + + - - - - - - + + + - - - - - - + + + - Ganglion cells What do they do? • Answer – respond to changes in pattern of light Why? • Changes carry the most important information Ganglion cell Bipolar cell Rods Cones Ganglion cells • Ganglion cells reduce the amount of information in a stimulus by finding contours and boundaries Excitatory or inhibitory effect on ganglion cell response Baseline firing of ganglion cell Ganglion cells • Importance of boundaries between areas of light and dark explains why line drawings so effective Overview • Retinal ganglion cells • Receptive fields • Lateral inhibition • Centre -surround antagonism • Illusions explained by lateral inhibition Some classic illusions can be explained by lateral inhibition e.g. Hermann Grid • Two on -centre cells centred on light regions of grid • When RF at intersection - more light falls on the surround (OFF region) so receives more inhibition and cell fires less • Less firing interpreted as less bright so we perceive a dark spot How receptive fields explain Hermann Grid -+ - - -+ - - Larger Response Smaller Response Hermann grid – fixation? • Why do grey illusory spots reduce/disappear when we look directly at (fixate) them? Remember from last week….. Neural convergence in the retina On average... • 120 rods send signals to 1 ganglion cell • 6 cones send signals to 1 ganglion cell Ganglion cell Bipolar cell Rods Cones Receptive field size varies with eccentricity Receptive field diameter Fovea Periphery Retinal eccentricity Small Large Not to Scale! Receptive field sizes/eccentricity But maybe this isn’t the whole picture? • According to our centre -surround antagonism & receptive field explanation we should get the same illusion in both cases, but we don’t. But maybe this isn’t the whole picture? Typically explained in terms of 1. Centre -surround antagonism 2. Varying receptive field sizes Explanation is probably correct in essentials, but there might be additional processes (which we don’t fully understand yet) playing a role too. Hermann Grid Simultaneous contrast illusion Same square – different background Simultaneous contrast illusion Same square – different background Simultaneous contrast illusion + - + - + - + - + - + - + - + - - - - - + - + - + - + - + - + - + - + - - - - - Can you work out the explanation? Why is the inner square on the left perceived as darker than the one on the right? Simultaneous contrast illusion + - + - + - + - + - + - + - + - - - - - + - + - + - + - + - + - + - + - - - - - Can you work out the explanation? Why is the inner square on the left perceived as darker than the one on the right? Recap…Receptive Field Retina Response of Ganglion cell A cell's receptive field is the area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell. Receptive Field Recap...Lateral inhibition and centre - surround antagonism Inhibitory Excitatory + + + - - - - - - - - - + + + - - - + + + + + ON -centre OFF -surround OFF -centre ON -surround Lateral inhibition Key concepts • Receptive field • Excitatory and inhibitory response • Convergence • Lateral inhibition • Centre -surround antagonism • Hermann grid • Simultaneous contrast illusion Next week: Visual cortex • How does the brain process complex visual stimuli? • Which visual areas are responsible for processing different types of visual stimuli? Primary visual cortex Reading For this week’s lecture: • Goldstein chapter 3. Neural Processing and Perception For next week’s lecture: • Goldstein chapter 4. Cortical Organization Questions for next week… 1. If light falling on a region of the retina causes a decrease in the response of a ganglion cell – what type of region is this? 2. If a spot of light falls outside the receptive field of a ganglion cell what firing rate will be produced? 3. Why do receptive fields increase in size with eccentricity (distance from fovea)? 4. What kind of stimuli are ganglion cells good at detecting? Lecture 2: The Eye and Retina PSYC11312: Sensation and Perception Dr. Becky Champion [email protected] Last week… • Sensation and Perception – Why should we study them? • The perceptual process • Bottom -up vs. Top -down processing • Approaches to studying perception – Physiological and psychophysical Questions from last week… • What is a distal stimulus? • Which cells carry out transduction? • Why is knowledge important in perception? • What is an absolute threshold? Today: The eye and the retina • How does the eye begin the process of visual perception? Overview • Light • T he Eye • The Retina • Rods and cones The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Light Light is a form of electromagnetic energy Wavelength Intensity Light physical psychological Wavelength Colour Intensity Brightness Light is a form of electromagnetic energy The electromagnetic spectrum Light • Light is reflected from objects and into the eye. Distal stimulus Proximal stimulus Overview • Light • T he Eye • The Retina – Rods and cones – Ganglion cells The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge T he E ye The Eye • Receptors located in the retina • The function of the eye is to focus the image on the retina Distal stimulus Retinal image (Proximal Stimulus) The Eye Iris and Pupil • A djustable aperture • L imit the amount of light passing through • A llows us to deal with great range of light levels – Pupil between ~2 mm and ~9 mm diameter Pupil Iris Pupil Iris By Artwork by Holly Fischer http ://open.umich.edu/education/med / resources/second -look -series/materials https ://commons.wikimedia.org/w/index.php?curid=24367145 The Eye Cornea and Lens • Role is to focus light on the retina • Cornea – 80% focusing power • Lens – 20% but can change shape due to action of ciliary muscles Lens Cornea Ciliary muscles The Eye • Accommodation – Lens becomes fatter to focus close objects – Lens becomes thinner to focus far objects The Eye • Refractive errors – Myopia (nearsightedness ) – Hyperopia (farsightedness) Hyperopia Myopia Overview • Light • T he Eye • The Retina – Rods and cones – Ganglion cells The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge The Retina • Light - (photo -) sensitive layer at back of the eye • Different types of cells Retinal ganglion cells Bipolar cells Photoreceptors Horizontal cells Amacrine cells Optic nerve https://commons.wikimedia.org/wiki/File:Figure_36_05_02.png Photoreceptors • Photoreceptors = light sensitive cells • Carry out transduction = transforming light into electrical impulses • Transduction occurs by visual photopigments reacting to light and trigger electrical signals The Retina • Photoreceptors form the layer furthest from incoming light • Light must pass blood vessels, cells and axons before reaching photoreceptors Retinal ganglion cells Bipolar cells Photoreceptors Horizontal cells Amacrine cells Optic nerve https://commons.wikimedia.org/wiki/File:Figure_36_05_02.png Photoreceptors • 2 types: 1. Rods (~120 million) 2. Cones (~6 million) • Note different lengths/shapes Rod Cone Rods vs. Cones: The differences Rods and Cones differ in terms of: • Number • Sensitivity • Involvement in colour perception • Retinal distribution • Neural convergence & acuity Rods vs. Cones: T he differences • Why is it harder to see colour at night? • Why is it easier to see stars in the night sky when we don’t look directly at them? • Why do we move our eyes to look directly at an object we’re interested in? • Why is it hard to see anything when we go from bright sunlight into a dark room? • Why does everything look a bit blurry at night? Rods vs.Cones : Summary chart Cones Rods Number in retina Retinal distribution Sensitivity Acuity Number of types Enable colour perception? Luminance range operate in Rods vs. Cones: Sensitivity • Rods – Very sensitive, respond well in very dim light – Most useful at night, useless in daylight • Cones – Less sensitive – Work best in daylight, useless at night Rods vs. Cones: Sensitivity • Scotopic – only rods active • Photopic – only cones active • Mesopic – both rods and cones active Rods vs. Cones: Sensitivity • Bright light bleaches photopigments so photoreceptors stop responding • Going from bright to dark the photoreceptors have to ‘recover’ or regain sensitivity. • Dark adaptation = increase in eye’s sensitivity in the dark Dark adaptation • After 20 -30 minutes in the dark, sensitivity is about 100,000 times greater than the sensitivity in light • Rods and Cones adapt at different rates High sensitivity Low sensitivity Rods vs. Cones: Colour perception Cones • Responsible for colour vision Cones Rods vs. Cones: Colour perception Cones • Responsible for colour vision • 3 types of cone, sensitive to different wavelengths of light: – Red (long wavelengths) – Green (medium wavelengths) – Blue (short wavelengths) https://commons.wikimedia.org/wiki/File:1416_Color_Sensitivity.jpg Rods vs. Cones: Colour perception Rods • Produce monochromatic vision Cones Rods vs. Cones: Colour perception Rods • Produce monochromatic vision • 1 type - most sensitive to medium wavelengths (green light) Rods Rods vs. Cones: Colour perception Rods • Produce monochromatic vision • 1 type - most sensitive to medium wavelengths (green light) • Purkinje shift - at night red looks darker than green Rods vs. Cones: Retinal distribution • Photoreceptors are not evenly distributed across the retina • Fovea = small central area of the retina that contains only cones • When looking directly at an object the image falls on the fovea Fovea Rod vs. Cones: Retinal distribution Blind spot demo Blind spot demo 2 Quick Break • Why is it harder to see colour at night? • Why is it easier to see stars in the night sky when we don’t look directly at them? • Why do we move our eyes to look directly at an object we’re interested in? • Why is it hard to see anything when we go from bright sunlight into a dark room? • Why does everything look a bit blurry at night? Recap…Rods vs. Cones: The differences Rods and Cones differ in terms of: • Number • Sensitivity • Involvement in colour perception • Retinal distribution • Neural convergence & acuity Rods vs. Cones: Neural convergence • Convergence – one neuron receives signals from many other neurons Photoreceptors Ganglion cell Neural convergence in the retina On average... • 120 rods send signals to 1 ganglion cell • 6 cones send signals to 1 ganglion cell Ganglion cell Bipolar cell Rods Cones Rods vs. Cones: Acuity Neural convergence determines acuity • Acuity = the ability to detect fine details of a stimulus – High acuity – can detect fine details – Low acuity – can detect only course details Rods vs. Cones: Acuity Rods Cones Rods have greater convergence - leads to lower acuity Cones have less convergence - leads to higher acuity Photo receptors Ganglion cells Rods vs. Cones: Acuity Highest acuity at the fovea Rods vs. Cones: Acuity • Foveal vs. peripheral acuity A L F U V T W B X O C U V R S M R Q L X Keep your eyes fixated on A. How many letters can you read? Rods vs. Cones: Acuity Acuity decreases away from the fovea Rods vs. Cones: Acuity • Eye -movements ensure objects of interest are imaged on the fovea Yarbus A.L. (1967) Eye Movements During Perception of Complex Objects. In: Eye Movements and Vision. Springer, Boston, MA. Acuity decreases in low lighting conditions Rods vs. Cones: Acuity Rods vs. Cones: Summary chart Cones Rods Number in retina Retinal distribution Sensitivity Acuity Number of types Enable colour perception? Luminance range operate in Rods vs. Cones: Quick Quiz • Why is it harder to see colour at night? • Why is it easier to see stars in the night sky when we don’t look directly at them? • Why do we move our eyes to look directly at an object we’re interested in? • Why is it hard to see anything when we go from bright sunlight into a dark room? • Why does everything look a bit blurry at night? Recap…The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Key concepts • Eye • Retina • Photo -receptors – Rods and cones – Sensitivity – Neural convergence – Acuity Next week: Receptive fields • How do ganglion cells select the most important information to process? • How can ganglion cells explain some classic illusions? Reading For this week’s lecture: • Goldstein chapter 2 . The beginning of the perceptual process For next week’s lecture: • Goldstein chapter 3. Neural p rocessing Questions for next week … • Which parts of the eye are responsible for focusing light on the retina? • Why does visual perception rely on rods at night? • Why do cones enable higher acuity vision? Lecture 1: Introduction to sensation and perception PSYC11312: Sensation and Perception Dr. Becky Champion [email protected] Lecture Overview • Course information and overview • What are sensation and perception? • Why study perception? • The perceptual process • Approaches to the study of perception Course information Key People: • Unit Lead: Dr. Luke Jones ([email protected] ) • Lecturers: – Dr. Becky Champion ([email protected] ) – Dr. Luke Jones ([email protected] ) – Dr. Ellen Poliakoff ([email protected] ) • Lab Class Leads: – Dr . Becky Champion ([email protected] ) – Dr. Matt Checketts ([email protected] ) Course information • Lab Classes – Weekly lab class (weeks 1 -5) – Topic: Visual Illusions – Lab Report Deadline – Friday 3 rd March • Assessment – Lab Report (60%) – MCQ exam (40 %) covering lecture content Course information • Blackboard discussion boards – general course queries – lecture specific questions – lab related questions • Textbook – Goldstein “Sensation and Perception” – eTextbook on blackboard – Copies also in library Sensation and perception • Lecture 1: Introduction (RC) • Lecture 2: The eye and retina (RC) • Lecture 3: Receptive fields (RC) • Lecture 4: Visual cortex (RC) • Lecture 5: Form perception (LJ) • Lecture 6: Depth perception (LJ) • Lecture 7: Colour perception (LJ) • Lecture 8: Motion perception (LJ) • Lecture 9: Sound the ear and auditory perception (EP ) • Lecture 10: Visual Illusions (LJ ) • Lecture 11: Touch and pain (EP) • Lecture 12: Chemical senses and multisensory perception (EP) Lectures 1 - 4 Lecture 1: Introduction to sensation and perception (RC) Lecture 2: The eye and retina (RC) Lecture 3: Receptive fields (RC) Lecture 4: Visual cortex (RC) What are sensation and perception and why should we study them? How does the eye begin the process of visual perception? How does the visual system respond to changes in patterns of light? How does the brain process complex visual stimuli? Primary visual cortex Lectures 5 - 8 Lecture 5: Form perception (LJ) Lecture 6: Depth perception (LJ) Lecture 7 : Colour perception (LJ) Lecture 8: Motion perception (LJ) How do we perceive the form or shape of objects? How can we perceive in 3 - dimensions? How and why do we perceive colour? How do we perceive the motion of objects? Lectures 9 - 12 Lecture 9: Sound, the ear and auditory perception (EP) What is sound and how do the ear and brain achieve auditory perception? Lecture 11: Touch and pain (EP) How is it possible to feel things? Lecture 12: Chemical senses and multisensory perception (EP) How do we smell and taste? How do we combine information across senses? Lecture 10: Visual Illusions (LJ) Common visual illusions and how they work How does this course fit into the B.Sc. Psychology programme? • Mind and Brain Theme 1st year Introduction to Cognition Sensation and Perception Brain and Behaviour 2nd year Cognition Perception and Action Cognitive Neuroscience 3rd year Perception: From lab to life Cases in Clinical Neuropsychology Emotion Understanding Dementia: Brain and Behaviour Lecture Overview • Course information and overview • What are sensation and perception? • Why study perception? • The perceptual process • Approaches to the study of perception What do we mean by sensation and perception? The goal of sensation and perception is to find out about the external world Our senses – Vision – Hearing – Touch – Smell – Taste Our senses – Vision – Hearing – Touch – Smell – Taste – Temperature – Pain – Balance – Acceleration – Body position What do we mean by sensation and perception? • Sensation is the starting point - receiving information about the world via our senses. – Sensory receptor cells are sensitive to physical properties of the world Receptor cells – specialised neurones which respond to a particular physical property of environmental stimuli By Cenveo (https://courses.lumenlearning.com/austincc - ap1/chapter/special -senses -vision/) What do we mean by sensation and perception? • Sensation is the starting point - receiving information about the world via our senses. – Sensory receptor cells are sensitive to physical properties of the world • Perception is the end point – our experience of the world. 18 The illusion of perception • Perception seems easy …. But its hard! • Perceptual systems are incredible – Nothing man - made is even close https://commons.wikimedia.org/wiki/File:Kismet -IMG_6007 -gradient.jpg Q1: Is the top line to the left or right of the bottom line ? Q2: What is the cubed root of 571787 ? Q1 seems much easier than Q2, because perception so effortless but in fact very complex process The illusion of perception Lecture Overview • Course information and overview • What are sensation and perception? • Why study perception? • The perceptual process • Approaches to the study of perception Why study perception? • Perception is our only source of information about the world: everything we learn, is learned through perception • Perception underlies all our interactions with the environment • Perception allows survival Why study perception? Practical Applications • Understand changes in ageing, disease, injury etc. • Demands of driving, interacting with technology etc. • Design of artificial perceptual systems Why study perception? Practical Applications • Why do people drive too fast in fog? • Why do schizophrenic patients experience hallucinations? • How can driverless cars avoid collisions? • How do eye -witnesses recognise a face? • Why do crisps taste better when you hear a ‘crunch’? • How could food be made more appealing to people who have lost their sense of smell? • How can we make environments more acceptable for people with autism? Why should Psychology UGs study Perception? • Sensation and perception is the starting point for all psychological processes – Cognition – Social – Mental health – Developmental/ Educational Lecture Overview • Course information and overview • What are sensation and perception? • Why study perception? • The perceptual process • Approaches to the study of perception Perceptual Systems – Vision – Hearing – Touch – Smell – Taste – Temperature – Pain – Balance – Acceleration – Body position Perceptual Systems • Each perceptual system has its own functions e.g. – Vision - Object Identification/recognition & Navigation & Motion Perception – Audition - Object Identification/recognition & Object Localization – Touch - Object Identification/recognition & Pain (detection of tissue damage) – Smell and Taste - Chemical detection/ identification & nutrition and poison avoidance • However, all follow the same perceptual process…. The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Distal Stimulus • Physical object in the environment The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Proximal Stimulus • Information about the distal stimulus is received by the sensory receptor cells • The proximal stimulus is a representation of the distal stimulus Distal Stimulus Proximal Stimulus LIGHT SOUND WAVES PRESSURE CHEMICALS CHEMICALS Proximal Stimulus Each sense receives information about the distal stimulus via a different type of environmental physical energy The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Receptor Processes • Receptor cells carry out transduction • Transduction is the transformation of environmental physical energy into electrical energy in the nervous system PHYSICAL ENERGY e.g. LIGHT or SOUND ELECTRICAL IMPULSE Receptor Processes • Vision – receptors in the retina transform light into electrical impulses • Audition – receptors in the inner ear transform sound into electrical impulses Physical energy Receptors The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Neural Processing • Electrical signals are transmitted from one neuron to the next Neural Processing • The signal is changed as neurons interact The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Perception, Recognition and Action • Perception – conscious sensory experience • Recognition - placing an object in a category Perception, Recognition and Action • Perception – conscious sensory experience • Recognition - placing an object in a category – Visual form agnosia is an inability to recognise objects. This disorder highlights the distinction between perception and recognition. Perception, Recognition and Action • Perception – conscious sensory experience • Recognition - placing an object in a category • Action – movement: eyes, head, body ... The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Knowledge • Existing knowledge, assumptions, memories can influence perception, recognition and action. Knowledge Knowledge Knowledge • Existing knowledge, assumptions, memories can influence perception, recognition and action • The effect of knowledge is referred to as ‘Top down processing’: – Bottom -up processing – processing based on incoming sensory information – Top -down processing – processing based on prior knowledge/ experience/ assumptions • Perception involves both bottom -up and top - down processing. Top - down Processing Why do we need top -down processing? • Top -down processing is important for helping simplify the very complex perceptual process. Recap…The Perceptual P rocess Distal Stimulus Proximal Stimulus Receptor Processes Neural Processing Perception Recognition Action Knowledge Quick quiz (and break!) 1. How does the distal stimulus differ from the proximal stimulus? 2. What do we call the process of changing physical energy into electrical energy? 3. What is the difference between perception and recognition? 4. What does top -down processing involve? Lecture Overview • Course information and overview • What are sensation and perception? • Why study perception? • The perceptual process • Approaches to the study of perception Approaches to the study of perception 1. Physiological – what’s going on in the brain? 2. Psychophysical – what do we perceive? The physiological approach • Studying anatomy Photo by Bob Jacobs, Laboratory of Quantitative Neuromorphology Department of Psychology Colorado College http://www.ColoradoCollege.edu/IDProg/Neuroscie nce/ The physiological approach • Studying anatomy • Recording brain activity • Single cell recording The physiological approach • Studying anatomy • Recording brain activity • Single cell recording • Imaging • fMRI • MEG • EEG • PET https://en.wikipedia.org/w/index.php?curid=3335842 The physiological approach • Studying anatomy • Recording brain activity • Single cell recording • Imaging • Micro stimulation Micro stimulation Recording The physiological approach • Studying anatomy • Recording brain activity • Single cell recording • Imaging • Micro stimulation • Lesioning & TMS By Baburov -Own work, CC BY -SA 4.0, https://commons.wikimedia.org/w/in dex.php?curid=39754391 The psychophysical approach • Study what people actually perceive • Measures the relationship between: Stimulus (physical world) Perception (psychological world) The psychophysical approach • A typical visual psychophysics lab The psychophysical approach Types of threshold 1. Absolute (Detection) – what is the smallest magnitude that we can perceive? The psychophysical approach Types of threshold 1. Absolute (Detection) – what is the smallest magnitude that we can perceive ? 2. Difference (Discrimination) – what is the smallest difference that we can perceive? Measuring an absolute threshold • Example question: How bright must a spot of light be to detect it? • Detection experiment – measures an absolute threshold Measuring an absolute threshold • How bright must a spot of light be to be detected? • Method 1: Method of adjustment • Problem – different people have different criteria for saying ‘yes I see it!’ Measuring an absolute threshold • How bright must a spot of light be to detect it? • Method 2: Forced Choice – which stimulus contains the dot? A B Trial number: 1110987654321 Measuring an absolute threshold % correct Trial number 100 50 0 5 6 7 8 4 3 2 1 9 10 NB: Psychophysical studies usually use a small number of participants, each doing many repetitions of each trial. Psychometric function Chance performance Measuring an absolute threshold Stimulus intensity (i.e. light intensity in candelas) % correct 100 50 0 170 175 180 185 165 160 155 150 190 195 Absolute threshold taken as intensity that gives 75% correct performance Detection threshold = 172 candelas 75 Absolute thresholds Sense Absolute Threshold Vision A candle flame seen at 30 miles on a dark, clear night Hearing The tick of a watch under quiet conditions at 20 feet Taste One teaspoon of sugar in two gallons of water Smell One drop of perfume diffused into the entire volume of a three -room apartment Touch The wing of a bee falling on your cheek from a distance of 1 cm So how good are humans are detecting stimuli (i.e. how low are our absolute thresholds)? Galanter (1962) translated human absolute thresholds into more meaningful quantities The psychophysical approach Types of threshold 1. Absolute (Detection) – what is the smallest magnitude that we can perceive ? 2. Difference (Discrimination) – what is the smallest difference that we can perceive? The psychophysical approach Types of threshold 1. Absolute (Detection) – what is the smallest magnitude that we can perceive ? 2. Difference (Discrimination) – what is the smallest difference that we can perceive? • Not a constant value / quantity • Instead, its related to the baseline level – e.g. Adding a book to a bag of cotton wool vs. a bag of bricks • However the difference as a proportion of the baseline level is constant – Weber’s law Measuring a difference threshold • How different must two lines be to detect the difference ? • Forced Choice – which line is shorter? A B Trial 1 Measuring a difference threshold Difference in line length (cm) % correct 0 100 50 0 15 20 10 5 25 Difference threshold = 16cm 75 Psychophysics • Psychophysics uses carefully controlled experiments to test perceptual performance • Results highlight the relationship between physical world and perceptual experience Summary • Perception is a complex, multi -stage process • We need to understand perception to understand how we interact with the world • Perception involves incoming sensory information (bottom -up) and prior knowledge and experience (top -down) • Two main approaches to studying perception: physiological and psychophysical Key concepts • Perception • Sensation • Distal stimulus • Proximal stimulus • Transduction • Recognition • Action • Bottom -up and Top -down processing • Approaches to perception: physiological and psychophysical Next week: The first steps in visual perception • How does the eye begin the process of visual perception? Reading For this week’s lecture: • Goldstein chapter 1. Introduction to perception For next week’s lecture: • Goldstein chapter 2. The beginning of the perceptual process Questions for next week… • What is a distal stimulus? • Which cells carry out transduction ? • Why is knowledge important in perception? • What is an absolute threshold?