Perception PDF - Course of Experimental Psychology

PERCEPTION COURSE OF EXPERIMENTAL PSYCHOLOGY Università Cattolica del Sacro Cuore Milan CLAUDIA REPETTO TOC — Perception: definition and functions — Localization: separation of objects — Localization: perceiving distance — Localization: perceiving motion — Recognition...

PERCEPTION COURSE OF EXPERIMENTAL PSYCHOLOGY Università Cattolica del Sacro Cuore Milan CLAUDIA REPETTO TOC — Perception: definition and functions — Localization: separation of objects — Localization: perceiving distance — Localization: perceiving motion — Recognition: global-to-local processing — Recognition: what an object is — Perceptual constancies How are raw sensations translated into perception? What do you see in these pictures?  PERCEPTION Is what we perceive a copy of physical reality? According to naive psychology (naive realism) YES HOWEVER Perceived (phenomenical) reality does not always coincide with physical reality, as demonstrated by the Gestalt PHENOMENICAL REALITY = PHYSICAL REALITY "seen" through the eyes of the subject Physical reality and phenomenical reality Mimetism: phenomenon of the absence of the phenomenical object à what exists in the physical world does not appear on a perceptual level Anomalous figures: phenomenon of the absence of the physical object à what we perceive on the phenomenical level does not exist on the physical level Optical illusions: discrepancies between physical and phenomenical object Absence of the phenomenical object: mimetism Absence of the phenomenical object: mimetism Absence of the physical object: anomalous figures KANIZSA’S TRIANGLE Optical illusions MULLER-LYER ILLUSION La catena psicofisica HOW MANY LEGS DOES THIS ELEPHANT HAVE?  Psychophysical chain DISTAL STIMULUS: physical stimulus, as it is in reality PROXIMAL STIMULUS: receptors’ response (i.e. image on the retina) PERCEPT: at the cerebral level, the recognition of the phenomena experienced by our sense organs occurs (i.e. an object in movement, music) Attention To decide which incoming information should be further processed Localization Where the objects of interest are Recognition What an object is Abstraction The ability to extract the main features from an object Perceptual constancies the ability to maintain certain inherent features of objects Five functions of perception  LOCALIZATION: separation of objects The image projected on our retina is a mosaic of varying brightnesses and colors. Somehow our perceptual system organizes that mosaic into a set of discrete objects projected against a background Grouping principles Gestalt’s principles of organization Figure and ground relation Grouping principles Determinants favouring the unification of perceptual elements into groups Each principle triggers the perception of a configuration If several principles can be applied to a given stimulus, only one organization can be seen at a time, but it is possible to switch from one configuration to another. Proximity Spatial or temporal proximity between elements may induce the brain to consider them as a whole Proximity Spatial or temporal proximity between elements may induce the brain to consider them as a whole  Similarity The brain groups together elements which are similar to each other. Similarity could refer to different features, such as size, shape, colour, brightness... Closure All other conditions being equal, elements closed or tending to close are perceived as belonging to the same figure. Continuity All other conditions being equal, lines are united that are characterized by continuity of spatial or motor orientation Pregnanz or Good form The sets of stimuli are perceived as objects if they are characterized by simplicity, symmetry, and regularity. Pregnanz or Good form The sets of stimuli are perceived as objects if they are characterized by simplicity, symmetry, and regularity. Pregnanz or Good form The sets of stimuli are perceived as objects if they are characterized by simplicity, symmetry, and regularity. Pregnanz or Good form The sets of stimuli are perceived as objects if they are characterized by simplicity, symmetry, and regularity. Figure-ground relation Rubin (1915): “There is no figure without a background”. Compared to the background, the figure: has a shape has an outline has a defined extension Stands out Inclusion The included region becomes a figure Convexity The convex region becomes figure with respect to the concave one Relative area The portion with the smallest extention becomes figure Orientation Regions whose intrinsic axes are aligned with the principal directions of space (horizontal and vertical) become figures  Simmetry Regions with bilateral symmetry with respect to an axis become figures What if these rules don’t work? When these principles fail to dominate we are experiencing REVERSIBLE FIGURES, or figures in which there is a systematic inversion between figure and background. FIGURE REVERSIBILI FIGURE REVERSIBILI LOCALIZATION: perceiving distance The retina is a two- dimensional surface onto which a three-dimensional world is projected How do we go from a 2D image to a 3D image? We use depth cues BINOCULAR CUES They arise from the difference of the two retinal images projected by the same stimulus BINOCULAR DISPARITY LET’S TRY! Take a pencil and hold it in front of your eyes with your arm outstretched. Now look at it first with one eye and then with the other. Repeat the same procedure after bringing the pencil close to the face. What difference do you notice? Binocular disparity doesn’t work for longer distances…. MONOCULAR RELATIVE SIZE: If an image contains an array of similar CUES objects that differ in size, the viewer interprets the smaller objects as being farther away MONOCULAR INTERPOSITION: If one object is CUES positioned so that it obstructs the view of the other, the viewer perceives the overlapping object as being nearer MONOCULAR RELATIVE HEIGHT: Among CUES similar objects, those that appear closer to the horizon are perceived as being farther away MONOCULAR PERSPECTIVE: When parallel CUES lines in a scene appear to converge in the image, they are perceived as vanishing in the distance MONOCULAR SHADING AND SHADOWS: CUES shadows provide important cues about object shapes, distances between objects and the position of the light source MONOCULAR MOTION PARALLAX: The CUES difference in speed with which objects closest and farthest away from us move when we are in motion LOCALIZATION: perceiving motion An object is perceived in motion when the image on the retina moves in relation to an immobile background à presence of a reference system à RELATIVE MOTION If it moves towards us, the image on the retina is enlarged HOWEVER.... LOCALIZATION: perceiving motion Although the retinal image changes constantly as we move, we do not see a world in constant motion We also take into account the movements of our head and our eyes! We see movement even where there is none! SEE THE FOLLOWING EFFECTS…. INDUCED MOTION (Wallach, 1935) à a rectangle with a bright spot inside. If I move the rectangle, I have the perception that the point is moving in the opposite direction https://www.youtube.com/watch?v=lvvcRdwNhGM SEE THE FOLLOWING EFFECTS…. STROBOSCOPIC MOTION: Wertheimer discovered that it is possible to induce motion perception by pairing static images at a certain rate of succession (about 60 ms). AUTOKINETIC MOTION: dark place, static bright spot. The observer perceives a luminous trail. Phenomenon due to the absence of reference points (eye movements erroneously attributed to the external stimulus) RECOGNITION: global–to- local processing Objects appear in a huge number of shapes, colors, orientations and sizes: how do we recognize them and manage the infinite set of combinatorial possibilities? Global understanding processing what the scene is Using knowledge Local about the scene to assist in processing identifying single objects RECOGNITION: global–to- local processing The context helps us to disambiguate different alternative objects  RECOGNITION: determining what an object is The SHAPE plays a critical role. It is determined by VISUAL PROCESSING Early stages, using later stages, information on the comparing with retina visual memory for best match RECOGNITION: determining what an object is Features detectors in the cortex include – simple cells (responding when eye is exposed to line stimulus at particular orientation and position within receptive field) – complex cells (also respond to bar or edge in particular orientation but do not need specific position à they respond for stimuli in motion) – Hypercomplex cells (require stimulus to be in a particular orientation and of particular length) RECOGNITION: determining what an object is How does it work for natural objects? Bottom- up processes Top-down processes Bottom- up processes Biederman (1987): recognition by components theory. The features of objects include a number of geometric forms, such as cylinder, cone, block, wedge à GEONS (geometric ions). They are combined according to a small set of relations to create most of the objects people can recognize  Top-down processes Perception is guided by processes "from above»: by previous experience, context, expectation and motivation Any example? A VOLUNTEER, PLEASE! Aoccdrnig to rscheearch at Cmabrigde uinervtisy, it deosn’t mttaer waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteres are at the rghit pclae. The rset can be a tatol mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae we do not raed ervey lteter by it slef but the wrod as a wlohe. ABSTRACTION We do not need to process all the infinite details of an object to recognize it or to interact with it. The process of ABSTRACTION allows to extract the relevant information to carry out whatever task with the perceived object Abstraction has several advantages – Less required storage à faster – It is more efficient to encode an abstraction and to store it in memory than an exact object representation – information retained is the critical information needed PERCEPTUAL CONSTANCIES The brain never has direct access to physical reality (distal stimulus), but through subsequent physiological modifications performed by proximal stimulation The information filtered by our receptors is partially indeterminate The brain must therefore have organizational criteria that make it possible to reconstruct a representation as close to the truth as possible Color and brightness constancies Color constancy: ability of visual system to perceive the reflectance characteristic (an inherent property of the object) no matter what the source wavelengths (light source – sun/light bulb, etc.) Brightness constancy: the perceived lightness of an object changes very little, if at all, even when the intensity of the source light changes dramatically Color and brightness constancies  Color and brightness constancies The brain «corrects» for the background illumination and we are led to perceive gray as blue under a yellowish light and as yellow under a bluish light. Shape constancy It consists in attributing the same shape to objects despite the different shapes they project onto the retina SIZE constancy An object’s perceived size remains relatively constant no matter how far away it is (think about when you greet a friend and he walks away from you.... do you see him becoming smaller and smaller?) We do not only rely on the retinal image SIZE constancy: dependence on depth cues Emmert law: keeping the optical angle constant, greater distances correspond to greater sizes SIZE constancy: dependence on depth cues Size-distance dependence: when the distance to an object increases, the object’s retinal size decreases, BUT if distance cues are present (as in the previous picture - linear perspective), the perceived size remains constant