Perception PDF

Perception si ty er U niv ni Topics co The topics dealt with in this lesson are as follows: r – Definition of perception; Ma – Perception of form and bottom-up/top...

Perception si ty er U niv ni Topics co The topics dealt with in this lesson are as follows: r – Definition of perception; Ma – Perception of form and bottom-up/top-down processing; – Gestalt psychology and its principles; – Direct perception and Gibson's affordances; – Face perception; – Perception of motion; – Perceptual disorders.  Objectives The learning objectives of this lesson are: – To provide a definition of perception; – To review bottom-up/top-processing in the perception of form; – To briefly analyze Gestalt psychology and its principles; – To provide an overview of Gibson’s affordances in relation to direct perception; ty – To analyze face perception and perception of motion; – To briefly review the main perceptual disorders. si er U niv ni Definition of Perception co Perception: r – is the process by which we recognize what is Ma represented by the information provided by our sense organs; – is a rapid, automatic, unconscious process;  Perception of Form Most of what we see can be classified as: – Objects: things having particular shapes and particular locations in space; – Background: is formless and helps us to judge the location of objects we see in front of it. In psychology the terms figure and ground are used to ty label an object and its background, respectively. si The classification of an item as a figure or as a part of the background is not an intrinsic property of the item. er Rather, it depends on the observer’s behavior. U niv ni r co Ma  Perception of Form: the Principles of Gestalt Gestalt (German for “form”) psychology: – The task of perception is to recognize objects in the environment according to the organization of their elements; – In perception the whole is more than the sum of its parts; – Elements of a visual scene can combine in various ways to produce different forms; ty – Several principles of grouping can predict the combination of these elements: si The adjacency/proximity principle; The similarity principle; Good continuation; The law of closure; er The principle of common fate. U niv ni Perception of Form: Bottom-Up and Top- Down Processing co Bottom-up processing: the perception is constructed r out of the elements - the bits and pieces - of the Ma stimulus, beginning with the image that falls on the retina. The information is processed by successive levels of the visual system until the highest levels are reached, and the object is perceived. Top-down processing: refers to the use of contextual information - to the use of the «big picture».  Direct Perception: Gibson's Affordances Gibson’s theory of perception: “perceiving is an act, not a response; an act of attention, not a triggered impres- sion; an achievement, not a reflex”. Gibson abandoned the depth/space perception view of the world and, instead, suggested that our perception of surfaces was more important. ty Gibson’s affordances: the meanings that the environment gives us in respect of what an object furnishes us with, si what it affords us. er U niv ni Face Perception r co Being able to recognize and identify faces is one of the Ma most important social functions human beings can perform. It helps us form relationships with people, spot faces in a crowd and provides us with potential non-verbal cues as to what a person is thinking or feeling. Psychologists in the nineteenth century were interested in what makes a face attractive and constructed composites - averages of several different images - to produce a face which they believed was attractive. Recent work has indicated which features of the face best allow us to remember a face or which make a face distinctive.  Face Perception: Distinctiveness and Attractiveness Distinctiveness of the face - defined as the deviation from the norm - is unrelated to attractiveness. Galton’s hypothesis is that averageness was attractiveness. si ty er U niv ni Theories of Face Perception co The mechanisms that allow us to perceive faces are r considered to be different from those that allow us to Ma perceive objects. Face perception involves a number of operations. Bruce and Young (1986) have suggested that face pro- cessing is made up of three functions: – perception of facial expression; – perception of familiar faces; – perception of unfamiliar faces.  Perception of Space and Motion In addition to being able to perceive the forms of objects in our environment, we are able to judge quite accurately their relative location in space and their movements. Perceiving where things are and perceiving what they are doing are obviously important functions of the visual system. si ty er U niv ni Perception of Space and Motion: Depth Perception co Depth perception requires that we perceive the distance r of objects in the environment from us and from each Ma other. We do so by means of two kinds of cues: – binocular (“two-eye”) cues: the visual fields of both eyes overlap; – monocular (“one-eye”) cues: typical of animals that have eyes on the sides of their heads  Binocular Cues Convergence provides an important clue about distance: the eyes make conjugate movements so that both look at (converge on) the same point of the visual scene. Another important factor in the perception of distance is the information provided by retinal disparity: whenever the eyes are pointed towards a particular point, ty the images of objects at different distances will fall on different portions of the retina in each eye. The amount si of disparity produced by the images of an object on the two retinas provides an important clue about its distance from the viewer. er U niv ni Monocular Cues co One of the most important sources of information about r the relative distance of objects is interposition Ma (meaning “placed between”). It works best when we are familiar with the objects and know what their shapes should look like. Another important monocular distance cue is provided by our familiarity with the sizes of objects. The patterns of light and shadow in a scene - its shading - can provide us with cues about the three-dimensional shapes of objects.  Constancies of Visual Perception Despite the changing nature of the image the visual environment casts on our retinas, our perceptions remain remarkably constant. However, perception is not absolute in so far as it varies across cultures. Ecological variables such as those associated with geography, cultural codes and education influence perception. si ty er U niv ni Constancies of Visual Perception: Form Constancy co The unchanging perception of an object's size and shape r when it moves relative to us is called form constancy: Ma – It is achieved by unconscious inference, a mental computation of which we are unaware.  Perception of Motion Detection of movement is one of the most primitive aspects of visual perception. Our visual system can detect more than the mere presence of movement: we can see what is moving in our environment and can detect the direction in which it is moving. ty Motion, like other kinds of stimuli, can give rise to adaptation and after-effects. si er U niv ni Perceptual Disorders co Perceptual disorder: the brain is damaged and visual r perception is impaired. Ma There are several perceptual disorders (there is no underlying impairment in sensation) and each is associated with damage to different parts of the visual system: – blind sight; – agnosia; – spatial neglect.  Perceptual Disorders: Blindsight Blindsight: when the primary visual cortex is damaged, a person becomes blind in some portion of the visual field: – although patients are unable to see properties of objects, they are aware of other aspects such as movement of objects. si ty er U niv ni Perceptual Disorders: Visual Agnosia co Agnosia (literally “without knowledge”): patients with r posterior lesions to the left or right hemisphere Ma sometimes have considerable difficulty in recognizing objects, despite having intact sensory systems. A distinction is usually made between two types of visual agnosia: – Associative: the inability to make meaningful associations to objects that are visually presented; – Apperceptive: the inability to recognize objects.  Perceptual Disorders: Spatial Neglect Spatial neglect: patients with lesions in the right or left parietotemporal cortex sometimes have difficulty in perceiving objects to their left or right respectively. Neglect for the left side is more common than right neglect si ty er U niv ni Conclusions co In this lesson we have: r – Defined what is meant by perception; Ma – Reviewed bottom-up/top-processing in the perception of form; – Briefly analyzed Gestalt psychology and its principles; – Provide an overview of Gibson’s affordances in relation to direct perception; – Analyzed face perception and perception of motion; – Briefly reviewed the main perceptual disorders.  Copyright si ty er This didactic material is meant for the personal use of the student only, and is copyrighted. Its reproduction, even for a partial utilization, is strictly forbidden in compliance with and in force of the law on Authors rights. U niv ni r co Ma