Perception - PDF

What is perception? ▪ Perception is generally defined as the process of acquiring knowledge about environmental objects or events via the senses ▪ The perceptual process is often broken down into two stages: ▪ Sensation is the process of transforming physical stimuli to electrical signal...

What is perception? ▪ Perception is generally defined as the process of acquiring knowledge about environmental objects or events via the senses ▪ The perceptual process is often broken down into two stages: ▪ Sensation is the process of transforming physical stimuli to electrical signals ▪ Perception is the process of interpreting these signals for conscious awareness and for action 4 The perceptual process Perception: signal processing and interpretation Sensation: conversion to Proximal Distal stimuli neural signals stimuli which are sent to 5 the brain  Evolutionary utility - what is perception for? ▪ Aristotle (384-322 BC): animals must have perception if they are to live ▪ Perception has evolved to aid the survival Somatosensory perception and reproduction of organisms (touch) ▪ All of our senses help us to seek out desirable objects Olfactory Visual and situations and avoid perception perception dangerous ones (smell) (seeing) Auditory Gustatory Humans: 5 senses perception perception or more? (hearing) (taste) 6 Also: Proprioception (sense of body position & movement), Nociception (pain), Thermoception (temperature) Evolutionary utility - what is perception for? ▪ Importance of different types of energy in the environment determines which senses have evolved ▪ Some species sense energies that humans cannot: Caribou can sense light into Elephants are sensitive to very Snakes can detect infrared the UV spectrum, enabling low frequency sounds and radiation, enabling them to them to detect camouflaged vibrations, allowing them to generate a ‘thermal image’ predators communicate over large of prey distances 7  Is perception veridical? ▪ Senses would not evolve if they did not provide reasonably accurate information about the world ▪ However, this is not to say that perception is necessarily a clear window onto reality The eye has some similarities to a camera However, the eye does not transmit a picture – information about the pattern of light reaching the eyes must be interpreted by the brain. 8 “What is real?”. Morpheus’ answer to Neo in The Matrix, 1999. Illusions ▪ Loosely defined, illusions are situations in which perception differs from reality ▪ Let’s look at examples of two illusions in which we perceive objects at locations where no visual stimulus exists The ‘Lilac chaser’ ▪ Keep your eyes fixed on the cross. ▪ Throughout the movie there are always 11 pink blobs and one gap. ▪ What do you perceive? 9  Illusions The Hermann grid illusion ▪ Notice the illusory grey spots at the intersections of the white lines. ▪ Illusions can provide insight into the processes of sensation and perception 10 More illusions ▪ In some illusions, our perception of objects is systematically distorted Müller Lyer illusion Ponzo illusion which horizontal line is longer? which horizontal line is longer? Zöllner illusion Ebbinghaus illusion (Titchner circles) are the long lines parallel? which central circle is larger? 11  More illusions Café Wall illusion Shepard’s table illusion are the horizontal lines parallel? are the tables the same size? 12 Ambiguous figures ▪ Ambiguous figures are images that can give rise to two or more distinct perceptions The ‘Necker cube’ ▪ Note, our perception is rarely ambiguous, but tends to alternate over time or ▪ These figures are sometimes also referred to as bistable images 13 Ambiguous figures Old Lady Young/Old Lady Young Lady 14 Ambiguous figures Rubin’s vase Jastow’s duck/rabbit 15  Ambiguous figures ▪ Ambiguous figures sometimes produce #thedress different perceptions between different people that are stable over time 16 Ambiguous sounds ▪ Ambiguous sounds can also give rise to multiple bistable and stable perceptions Auditory stream ▪ Most people switch between hearing the sound segregation on the right as triplets of an A-B-A pattern or as two streams of A-A-A pattern and B-B-B pattern ▪ Laurel or Yannie? or Original High- Low- version frequency frequency shifted shifted 17  Impossible objects ▪ Sometimes sensory input is interpreted by the brain as representing objects or scenarios that are physically impossible Penrose triangle Schuster’s conundrum Endless stairs (Devil’s fork) Shepard scale Rissett rhythm illusion 18 Perception as interpretation Knowledge ▪ These illusions illustrate that perception is not a clear window onto reality Top-down ▪ Objects are not perceived directly Perception ▪ The brain is doing its best to figure out what is out there based on the available or Bottom-up information Sensory ▪ Two sources of information are available - input current sensory input and existing knowledge about the environment 19  Top-down and bottom-up processes Top-down ▪ use knowledge about the structure of the world to influence perception (sometimes referred to a ‘conceptually driven’ processes) Bottom-up ▪ take information from the senses and make judgements about the nature of the world solely based on this information (sometimes referred to as ‘data- driven’ processes) ▪ Both types of process are important - perception is frequently modified by knowledge, but knowledge can’t always override perception ▪ Historically, some theories of perception emphasise one component more than the other 20 Constructivist theories of perception ▪ Emphasise the importance of top-down processing ▪ Helmholtz (1821-1894) argued that the inadequate information provided by the senses is augmented by unconscious inference ▪ But Ibn al-Haytham (aka Alhazan) (c. 965-1040) is claimed to have had similar ideas much earlier… 21  Constructivist theories of perception ▪ Helmholtz (1821-1894) argued that the inadequate information provided by the senses is augmented by unconscious inference ▪ Details of the image are unconsciously completed ▪ This constructivist approach (see also Bruner, Neisser, Gregory) has a number of assumptions ▪ Perception is an active and constructive process ▪ Perception is an end-product of the presented stimulus and internal factors (e.g., hypotheses, expectations, motivations) ▪ It is prone to error 22 Constructivist theories of perception ▪ Gregory (1923-2010) elaborated the theory of perception as inference: ▪ “Perception is not determined simply by stimulus patterns; rather it is a dynamic searching for the best interpretation of the available data… perception involves going beyond the immediately given evidence of the senses” (Gregory, 1966) 23  Constructivist theories ▪ According to this approach, many illusions are better described as rational inferences rather than ‘perceptual errors’ Ponzo illusion Size constancy Hollow mask illusion The most common occurrence of converging lines In the real world faces are almost real 3D world is perspective distortion of parallel always convex, so our brains are lines. The brain tries to estimate the true properties reluctant to interpret face images of objects, so objects assumed to be further away as convex (even if they are) are perceived as longer/larger 24 Direct theories of perception ▪ Emphasise the importance of bottom-up processing ▪ James Gibson (1904-1974) argued that the constructivist approach may underestimate the richness of the sensory evidence we receive ▪ There are a great variety of cues in the natural world that provide much information about the structure of the environment ▪ The perceiver is not a passive observer but interacts with the environment – this interaction is also the key to picking up useful information 25  The modern approach – information processing paradigm ▪ Since the 1950s and 60s, perception has typically been approached as a computational process ▪ Focus on understanding the acquisition, processing, storage and recall of data in the brain Physiology Stimuli (neural Perception (sensory representations & & action input) processes) (output) 26 Key scientific approaches ▪ Modern approaches to studying perception probe the perceptual process in different ways Psychophysiology & brain Psychophysics Neurophysiology imaging / stimulation Stimuli Physiology Perception Stimuli Physiology Perception Stimuli Physiology Perception & action & action & action Measure relationship Measure the Measure the relationship between stimulus and relationship between a between physiological perception stimulus and the responses and physiological perception response 27  Summary Learning objective: Describe the perceptual process ▪ When thinking about the perceptual process, it can be useful to distinguish between two separate stages: ▪ Sensation ▪ Energy from physical stimuli in the environment stimulates sensory receptors ▪ Converted to neural impulses, which are sent to the brain ▪ Perception ▪ The brain processes and interprets this input 28 Summary Learning objective: Identify situations in which perception departs from reality ▪ Most of the time our perceptions are reasonably accurate, otherwise we would have difficulty navigating and interacting with the world ▪ However, our brain’s interpretation does not always coincide with the properties of the sensory input. Types of illusions include ▪ failure to perceive objects (e.g. Lilac chaser) ▪ perception of objects in the absence if a stimulus (e.g. Lilac chaser, Hermann grid) ▪ perceptual distortions (e.g. Müller-Lyer illusion) ▪ multiple perceptions of the same stimulus (e.g. Necker cube, Rubin’s vase) ▪ perceptions that are physically impossible (e.g. Penrose triangle) 29  Summary Learning objective: Distinguish between top-down and bottom-up processing ▪ Top-down ▪ use of context and prior knowledge in perception ▪ emphasised by constructivist theories (e.g. Helmholtz, Gregory) ▪ Bottom-up ▪ processing of sensory information as it is received ▪ emphasised by Gibson’s direct theory of perception 30 Summary Learning objective: Describe the modern scientific approach to studying perception ▪ The modern ‘information processing approach’ to perception focusses on the computational steps required to acquire, select, recall and process sensory information ▪ This is informed by complementary scientific approaches that probe relationships between sensory input, neural representations/processes and perception 31  Light: the stimulus for vision ▪ Visible light is a band of energy within the electromagnetic spectrum (wavelengths from 400-700nm) ▪ Different wavelengths of light are associated with different colour perceptions ▪ Light can also be described as consisting of small packets of energy called photons ▪ Light intensity or luminance (number of photons per unit space) is associated with perception of Note: brightness and colour brightness are perceptual properties, NOT physical properties 3 Light: the stimulus for vision ▪ Light interacts with objects and surfaces in the environment, e.g.: ▪ Absorption as photons collide with particles of matter ▪ Reflection as light strikes opaque surfaces ▪ Transmission as light passes through transparent matter ▪ Different material properties are associated with different perceptions. e.g. ▪ ‘bright’ objects reflect more light than ‘dark’ objects ▪ ‘red’ objects selectively absorb short wavelength light and reflect longer wavelengths 4  The human eye ▪ Single-chambered eye uses convex cornea and lens to project an image onto the retina ▪ Enables directional sensitivity – can represent the spatial structure rather than sum total of light ▪ Photoreceptors transduce light into an electrical potential ▪ These signals then flow through a network of neurons to retinal ganglion cells and then out the back of the eye via the optic nerve 5 Two types of photoreceptor: rods and cones Rods ▪ Located primarily in peripheral retina ▪ Capable of operating in low light levels (can detect single photon) Cones ▪ Concentrated in centre of retina (fovea) ▪ Require higher light levels (daylight, 100s of photons) to respond ▪ 3 different photopigments, sensitive to short, medium and long wavelengths of light  The visual pathways ▪ Visual information is transmitted from the retina to the brain ▪ The main pathway consists of: Retina Optic nerve Optic chiasm LGN Primary visual cortex (V1) 7 Brightness perception ▪ Light intensity is related to perceived brightness (higher intensity regions tend to be perceived as brighter) ▪ However, this relationship is not straightforward – brightness perception is influenced by both bottom-up and top-down processes. Bottom-up Top-down ▪ The retina does not simply record ▪ The brain also uses knowledge light intensities about how light interacts with objects ▪ Responses are shaped by processes when determining perceived occurring within the retina, most brightness (e.g. shadows) notably light/dark adaptation and lateral inhibition 8  Brightness perception – light/dark adaptation ▪ Consider a piece of paper with black text viewed either under indoor lighting or outside on a bright sunny day ▪ We perceive the text as black and paper as white in both situations (we call this brightness constancy) ▪ Why is this? The white paper will actually reflect less light indoors than the black paper will reflect outdoors The luminance of a retinal image depends on both the amount of light falling onto an object and the relative reflectance of the surface(units: cd/m2) 9 Brightness perception – light/dark adaptation Light / dark adaptation Example of a retinal ganglion cell ▪ The sensitivity of the retina is constantly response under different lighting conditions adjusted to compensate for changes in mean 100 luminance 80 Percentage response ▪ Sensitivity is reduced when the mean intensity of the image is high and increased 60 when it is low 40 ▪ This process, termed light/dark adaptation dictates that the retina encodes contrast 20 (the ratio of an object’s luminance relative to 0 the mean or background luminance) and 10000 100000 1 10 100 1000 Luminance plays a critical role in achieving brightness (cd/m2) constancy 10  Brightness perception – negative afterimages ▪ While light/dark adaptation is essential for our visual systems to function in different lighting conditions, it can produce illusions under some circumstances + 11 Brightness perception – lateral inhibition Lateral inhibition ▪ early form of information processing in retina ▪ retinal ganglion cells receive both excitatory (+) and inhibitory (-) input from neighbouring photoreceptors ▪ arranged in a centre-surround configuration across the retinal image - - - - - ++ Example receptive field of an ON-centre OFF- - + surround ganglion cell - - - - 12  Brightness perception – influence of lateral inhibition ▪ Lateral inhibition makes the visual system sensitive to changes in luminance (important for detecting edges and borders of objects) ▪ Can have dramatic effects on perceived brightness Hermann grid illusion - - - - - - - - - - ++ ++ - + - + - - - + - - - - - The intersections are surrounded by more high intensity (white). This results Notice the illusory grey spots at the in more inhibition from the surround in 13 intersections on-center, off-surround receptive fields Brightness perception – top-down influences ▪ Our visual systems also uses knowledge of how light interacts with 3D objects in the world when determining brightness ▪ for example, it tries to maintain brightness constancy when the amount of light falling on a surface is affected by shadows ▪ this can result in ‘errors’ in 2D images portraying 3D scenes Checker-shadow illusion 14 A & B have identical luminance, but different brightness  Colour perception Why does the world appear devoid of colour under low-light conditions? ▪ only rod photoreceptors are sensitive enough to operate ▪ rods contain a single type of photopigment (rhodopsin) ▪ light of different wavelengths and intensities can elicit identical responses ▪ this makes it impossible to accurately signal different wavelengths 15 Colour perception - trichromacy Cone photoreceptors contain one of three different photopigments, each with different wavelength sensitivities ▪ S-cones: Cones that are preferentially sensitive to short wavelengths (blue cones) ▪ M-cones: Cones that are preferentially sensitive to middle wavelengths (green cones) ▪ L-cones: Cones that are preferentially sensitive to long wavelengths (red cones) 16  Colour perception - trichromacy The relative outputs of the three cone types allows unambiguous signalling of wavelength 17 Colour perception – variations from trichromacy Monochromacy ▪ individuals have either 0 or 1 functioning cone type, resulting in complete colour blindness ▪ extremely rare (approx. 1 person in 100,000) Dichromacy ▪ only 2 functioning cone types ▪ Protanopia (1% males, 0.02% females) = missing L-cones ▪ Deuteranopia (1% males, 0.01% females)= missing M-cones ▪ Tritanopia (0.002% males, 0.001% females) = missing S-cones Bunch of flowers Bunch of flowers as as seen by a seen by a typical trichromat protanope 18  Colour perception – variations from trichromacy Anomalous trichromacy ▪ more common form of colour perception deficiency ▪ characterised by defect in one of the cone types ▪ Protanomaly (1.3% males, 0.02% females) = L-cone defect ▪ Deuteranomaly (5% males, 0.35% females)= M-cone defect ▪ Tritanomaly (0.01% males, 0.01% females) = S-cone defect ▪ Commonly assessed using the Ishihara Colour Test Example test plate (warning – uncalibrated) - those with normal trichromacy typically see an 8 - deuteranomalous individuals typically see a 3 - A monochromat would be unable to discern any number 19 Colour perception – opponency Like brightness, perception of colour is also shaped by bottom-up processing of visual information in the retina and beyond Colour opponency ▪ retinal ganglion cells receive excitatory (+) and inhibitory (-) input from different cone types ▪ results in distinct Red/Green and Blue-Yellow pathways 20 Colour perception – negative afterimages ▪ colour opponency can be demonstrated using negative afterimages ▪ e.g., staring at a red object will result in a green afterimage ▪ adaptation to red causes a reduction in the sensitivity of long wavelength cones, creating an imbalance in the inputs to red/green opponent retinal ganglion cells 21 The ‘Lilac chaser’ revisited Can we now explain why this illusion occurs? 22  Colour perception – top-down influences ▪ Our visual systems also try to achieve colour constancy by accounting for the intensity and composition of light hitting different surfaces ▪ Colour constancy – the tendency for the perceived colour of objects to remain the same, even if the lighting changes ▪ This can give rise to illusions in which the same wavelength of light is perceived as different colours 23 #thedress revisited Why do people perceive the dress differently? 24  Summary Learning objective: Understand the stimulus for vision and basic structure of the human visual system ▪ Visible light is a band of energy within the electromagnetic spectrum, and can be described by ▪ its wavelength (difference in peaks of the electromagnetic waves) ▪ its intensity/luminance (amount of photons) ▪ The optics of the eye project light onto the retina, which converts the energy into electrical signals ▪ transduction achieved by different types of photoreceptors (rods & cones) ▪ information processing begins through circuitry of wiring between photoreceptors and retinal ganglion cells ▪ signals leave the eye via the optic nerve and are relayed to visual cortex at the back of the brain 25 Summary Learning objective: Describe the factors that shape the perception of brightness ▪ Brightness is associated with the intensity of light reflected by an object ▪ However, our perception of brightness is heavily influenced by: ▪ Bottom-up factors (how the visual system processes input to the eyes) ▪ Light/dark adaptation ▪ Lateral inhibition ▪ Top-down factors (knowledge about the environment) ▪ Impact of shadows ▪ These factors help vision function in normal environments (e.g. achieve brightness constancy) but can result in illusions under some circumstances 26  Summary Learning objective: Understand the principles of normal and abnormal colour perception ▪ Colour is a perceptual property associated with the wavelength of light ▪ Normal human colour perception relies on the relative output of three types of cones (trichromacy) ▪ Abnormal colour perception occurs ▪ in very rare instances where individuals have less than 3 functioning cone types (monochromacy, dichromacy) ▪ in rare instances where defects exist in one of the cone types (anomalous trichromacy) ▪ Colour perception is also shaped by both bottom-up and top-down factors ▪ Colour opponent processing ▪ Colour constancy 27  Why is it difficult to design a perceiving machine? ▪ Visual input provides ambiguous information about the 3D structure of the world The same 2D retinal image could be produced by an infinite This ambiguity is exploited by number of 3D objects anamorphic street artists 4 Why is it difficult to design a perceiving machine? ▪ Image complexity makes it increasingly tricky for computers to organise the visual scene into distinct objects Image Output of edge detection algorithm 5  Today’s lecture Learning objectives: ▪ Understand how we extract information about 3-dimensional space from 2-dimensional retinal images ▪ Compare the Structuralist and Gestalt approaches to perceptual organisation and describe the principles of object grouping 6 How do we perceive depth? ▪ Humans make use of a variety of sources of image information to infer depth in a visual scene ▪ Monocular cues (work with one eye) ▪ relative height ▪ relative size ▪ occlusion ▪ linear perspective ▪ texture gradient ▪ motion parallax ▪ shadows ▪ shading ▪ Binocular cues (require both eyes) ▪ disparity 7  Relative height and size ▪ Objects that are below the horizon and have their bases higher are typically perceived as being more distant ▪ If two objects are of equal physical size the more distant one will take up less of your field of view ▪ We need prior knowledge about the relative sizes of objects when judging distance 8 Occlusion ▪ Closer objects will occlude further away ones Relative height, relative size and occlusion cues all contribute to our perception of depth in this image 9  Linear perspective and texture gradient ▪ Parallel lines extending away from ▪ Texture elements get smaller observer converge in the distance and more dense with distance ▪ Foreshortening (circles become ovals) occurs when the surface is tilted away Motion parallax ▪ As we move, more distant objects will glide past us more slowly than nearer objects 11  Shadows and shading Brightness of a surface depends on its orientation with respect to the light source The addition of cast shadows to the Note – when the image above is viewed upside down, bottom video creates a strong many people perceive the footprints as flipping from perception of depth concave to convex (light-from-above assumption) 12 Binocular disparity Stereoscopic vision ▪ Our two eyes receive a slightly different image of the world Disparity ▪ This creates a differences in image location of an object seen by left and right eyes ▪ The size of the disparity depends on an object’s depth Try lining up your index fingers with one another, then closing each eye in turn 13  Binocular disparity Horopter ▪ Set of points in space that project to corresponding positions in the two retinas (i.e., zero disparity) ▪ Includes the fixation point When the lifeguard looks at Frieda, the images of Frieda, Susan and Harry fall on corresponding positions on the lifeguard’s retinas (if one retina could be slid on top of the other, the points would overlap). 14 Binocular disparity Uncrossed disparity ▪ Objects closer than the horopter have crossed disparities ▪ (You would have to cross your eyes to fixate on it) ▪ The image lies further to the right from the right eye’s viewpoint than from the left eye’s viewpoint ▪ Objects further than the horopter have Crossed uncrossed disparities disparity ▪ (You would have to uncross your eyes to fixate on it) ▪ The image lies further to the left from the right eye’s perspective 15  Summary Learning objective: Understand how we extract information about 3D space from 2D retinal image ▪ A variety of depth cues are contained in each monocular image, including relative object height and size, occlusion, linear perspective, texture gradients, motion parallax, shadows and shading ▪ Perception of depth relies on a combination of bottom up (extraction of the cues) and top-down (e.g. prior knowledge of object size, lighting direction) processes ▪ Depth information is also extracted from binocular disparities, via a process known as stereopsis 16 How do we perceive objects? Structuralism ▪ Approach pioneered by Wilhelm Wundt that was popular in mid to late 19th century Wundt (1832-1920) ▪ Proposed that perceptions are simply the sum of ‘atoms’ of sensation 17  How do we perceive objects? The Gestalt school ▪ Reaction against structuralism led by three central figures working at Frankfurt University: Wertheimer, Köhler & Koffka ▪ Argued that the whole form or configuration is greater than the sum of its parts Wertheimer Köhler Koffka 18 (1880-1943) (1887-1967) (1886-1941) How do we perceive objects? Illusory contours ▪ some images evoke the perception of edges in locations where there is no change in luminance or colour ▪ difficult to explain via the structuralist approach Do you perceive an upwards pointing arrow? Do you perceive a floating cube? 19  The Gestalt principles of perceptual organisation ▪ Having rejected structuralism, the Gestalt psychologists proposed a number of principles by which elements in an image are grouped to created larger objects ▪ Principles are all manifestations of the Law of Prägnanz (‘good figure’) ▪ “Of several geometrically possible organisations, that one will actually occur which possesses the best, simplest and most stable shape” (Koffka, 1935) 20 Proximity ▪ Things that are close together group together ▪ A perceived as rows as horizontal spacing is smaller ▪ B perceived as columns because vertical spacing is smaller ▪ C is ambiguous as dots are equally spaced A B C 21  Similarity ▪ Things that are similar group together ▪ Similarity could be in terms of any basic characteristic, such as shape (A) or orientation (B) ▪ The perception of (C) as columns occurs even though the proximity information suggests rows – similarity may override proximity A B C 22 Similarity versus proximity 23  Common fate ▪ Things that move together group together Strong grouping from common fate & proximity Motion can be a powerful way of segmenting objects from the background Weaker grouping common fate alone 24 Good continuation ▪ Group elements to form smoothly continuing lines rather than abrupt or sharp angles ▪ Helps preserve grouping of occluded objects 25  Closure ▪ Group elements to form complete figures, even if incomplete Grouping by closure requires top-down knowledge of forms 26 Symmetry ▪ Elements more likely to be formed into groups that are balanced or symmetrical 27  Assessment of Gestalt principles ▪ Seem correct about many things – perceptual objects are not simply the sum of their parts ▪ Gestalt principles generally hold across wide range of images ▪ However, some of the principles seem rather vague and imprecise ▪ not always clear what is meant by a ‘good’ or ‘simple’ shape ▪ No coherent workable account of the underlying neural mechanisms ▪ Köhler proposed an electrical field theory, in which lines of flow are created in the brain which match the structure of perception ▪ No empirical evidence for this! 28 Summary Learning objective: Compare the Structuralist and Gestalt approaches perceptual organisation and describe the principles of object grouping ▪ According to structuralist theorists, perceptual objects are formed by grouping a number of primary sensations ▪ Gestalt psychologists argued that objects are stable, organised wholes ▪ They proposed that features are grouped according to to key principles including ▪ proximity ▪ similarity ▪ common fate ▪ good continuation ▪ closure ▪ symmetry 29