CHAPTER 3 VISUAL PERCEPTION.pdf

CHAPTER 3 VISUAL PERCEPTION PRE PARED B Y: RODE L D. HAL IM RPM, MA FACULT Y – PS YCHOL OGY DEPART MENT  FROM SENSATION TO PERCEPTION ◦ PERCEPTION ◦ Is the set of processes by which we recognize, organize, and make sense of the sensations we receive from environmental stimul...

CHAPTER 3 VISUAL PERCEPTION PRE PARED B Y: RODE L D. HAL IM RPM, MA FACULT Y – PS YCHOL OGY DEPART MENT  FROM SENSATION TO PERCEPTION ◦ PERCEPTION ◦ Is the set of processes by which we recognize, organize, and make sense of the sensations we receive from environmental stimuli. ◦ HOW DO YOU RECOGNIZE THESE ITEMS?  FROM SENSATION TO PERCEPTION ◦ We do not perceive the world exactly as our eyes see it. Instead, our brain actively tries to make sense of the many stimuli that enter our eyes and fall on our retina. HOW DOES OUR VISUAL SYSTEM WORK? ◦ The precondition for vision is the existence of light. Light is electromagnetic radiation that can be described in terms of wavelength. Humans can perceive only a small range of the wavelengths that exist; the visible wavelengths are from 380 to 750 nanometers. ◦ Vision begins when light passes through the protective covering of the eye. This covering, the cornea, is a clear dome that protects the eye. The light then passes through the pupil, the opening in the center of the iris. It continues through the crystalline lens and the vitreous humor. ◦ The vitreous humor is a gel-like substance that makes up the majority of the eye. ◦ Eventually, the light focuses on the retina where electromagnetic light energy is transduced-----that is, converted-----into neural electrochemical impulses. Vision is most acute in the fovea, which is a small, thin region of the retina, the size of the head of a pin. When you look straight at an object, your eyes rotate so that the image falls directly onto the fovea. The retina contains the photoreceptors, which convert light energy into electrochemical energy that is transmitted by neurons to the brain. ◦ There are two kinds of photoreceptors-----rods and cones. Each eye contains roughly 120 million rods and 8 million cones. Rods and cones differ not only in shape but also in their compositions, locations, and responses to light. Within the rods and cones are photo pigments, chemical substances that react to light and transform physical electromagnetic energy into an electrochemical neural impulse that can be understood by the brain. The Rods are long and thin photoreceptors. They are more highly concentrated in the periphery of the retina than in the foveal region. The Rods are responsible for night vision and are sensitive to light and dark stimuli. STRUCTURE OF THE GANGLION CELLS & BIPOLAR CELLS STRUCTURE OF A ROD STRUCTURE OF A CONE ◦ The Cones are short and thick photoreceptors and allow for the perception of color. They are more highly concentrated in the foveal region than in the periphery of the retina. ◦ The rods and cones are connected to the brain. The neurochemical messages processed by the rods and cones of the retina travel via the bipolar cells to the ganglion cells. The axons of the ganglion cells in the eye collectively form the optic nerve for that eye. The optic nerves of the two eyes join at the base of the brain to form the optic chiasma. ◦ At this point, the ganglion cells from the inward, or nasal, part of the retina-----the part closer to your nose-----cross through the optic chiasma and extend to the opposite hemisphere of the brain. The ganglion cells from the outward, or temporal area of the retina closer to your temple go to the hemisphere on the same side of the body. The lens of each eye naturally inverts the image of the world as it projects the image onto the retina. In this way, the message sent to your brain is literally upside-down and backward. ◦ After being routed via the optic chiasma, about 90% of the ganglion cells then go to the lateral geniculate nucleus of the thalamus. From the thalamus, neurons carry information to the primary visual cortex (V1 or striate cortex) in the occipital lobe of the brain. The visual cortex contains several processing areas. Each area handles different kinds of visual information relating to intensity and quality, including color, location, depth, pattern, and form.  APPROACHES TO PERCEPTION ◦ BOTTOM – UP THEORIES ◦ Describe approaches in which perception starts with the stimuli whose appearance you take in through your eye. You look out onto the cityscape, and perception happens when the light information is transported to your brain. Therefore, they are data-driven (i.e., stimulus driven) theories.  FOUR MAIN BOTTOM-UP THEORIES ◦ DIRECT PERCEPTION ◦ Perception comes from the stimuli in the environment. ◦ Parts are identified, put together, and then recognition occurs. ◦ Gibson’s Direct Perception (Ecological Perception) ◦ All the information needed to form a perception is available in the environment. ◦ Perception is immediate and spontaneous. ◦ Existing beliefs or high-level inferential thought processes are not necessary for perception. What you see is what you get. ◦ TEMPLATE THEORIES ◦ Suggests that our minds stored myriad sets of templates. ◦ Templates are highly detailed models for patterns we potentially might recognize. ◦ We then choose the exact template that perfectly matches what we observe. ◦ Examples: fingerprints, barcodes, and chess board ◦ Storing, organizing, and retrieving many templates in memory would be difficult. ◦ FEATURE – MATCHING THEORIES ◦ We attempt to match features of a pattern to features stored in memory, rather than to match a whole pattern to a template or a prototype. ◦ We have brain cells that respond to specific features such as lines and angles referred to as “feature – detectors”. Elements are detected and assemble into more complex forms. ◦ Oliver Selfridge’s Pandemonium Model describes “image demons”. They pass on a retinal image to “feature demons”. Each feature demons calls out where there are matches between the stimulus and the given feature. OLIVER SELFRIDGE’S PANDEMONIUM MODEL ◦ RECOGNITION BY COMPONENT THEORIES ◦ We quickly recognize objects by observing the edges of them and then decomposing the objects into geons (geometric ions). ◦ We are able to recognize objects by separating them into geons. ◦ The geons are simple and function regardless of viewpoint-----they are viewpoint invariant. ◦ The object constructed from geons thus are recognized easily from many perspectives, despite visual noise. ◦ TOP – DOWN THEORIES ◦ Describe approaches in which perception is driven by high-level cognitive processes, existing knowledge, and the prior expectations that influence perception. These theories then work their way down to considering the sensory data, such as the perceptual stimulus. ◦ It is a constructive approach. ◦ In Constructive Perception, people actively construct perceptions using information based on expectations and experiences. ◦ We use our knowledge and expectations to influence what we see. ◦ This viewpoint is also known as “Intelligent Perception” because it states that higher-order thinking plays an important role in perception. ◦ Some investigators have pointed out that the world we experience is actually formed by our perception. ◦ During perception, we quickly form and test various hypothesis regarding percepts. ◦ The percepts are based on three things: ◦ What we sense (the sensory data). ◦ What we know (knowledge stored in memory). ◦ What we can infer (using high-level cognitive processes). PERCEPTION OF OBJECTS AND FORMS ◦ VIEWER-CENTERED PERCEPTION ◦ Perception changes with the position of the viewer. ◦ It depends on where the viewer is located. ◦ What matters is the appearance of the object to the viewer. ◦ The shape of the object changes depending on the angle from which we look at it. ◦ OBJECT-CENTERED PERCEPTION ◦ Description of object is independent of where the viewer is located. ◦ The shape of the object will stay stable across different orientations. ◦ THE PERCEPTION OF GROUPS---GESTALT LAWS ◦ It is useful particularly for understanding how we perceive groups of objects or even parts of objects to form integral wholes. ◦ Law of Pragnanz ◦ We tend to perceive any given visual array in a way that most simply organizes the different elements into a stable and coherent form. ◦ We do not merely experience a jumble of unintelligible, disorganized sensations. ◦ Gestalt principles that support the abovementioned law: figure-ground perception, proximity, similarity, continuity, closure, and symmetry. GESTALT PRINCIPLES OF VISUAL PERCEPTION ◦ TWO DIFFERENT PATTERN RECOGNITION SYSTEM ◦ Feature-analysis system ◦ It specializes in recognition of parts of objects and in assembling those parts into distinctive wholes. ◦ Configurational system ◦ It specializes in recognizing larger configurations. ◦ It is not well equipped to analyze parts of objects or the construction of the objects. But it is especially well equipped to recognize configurations. ◦ It is most relevant to the recognition of faces.  THE ENVIRONMENT HELPS YOU SEE ◦ PERCEPTUAL CONSTANCIES ◦ Occurs when our perception of an object remains the same even when our proximal sensation of the distal objects changes. ◦ The physical characteristics of the external distal object are probably not changing. ◦ TWO MAIN PERCEPTUAL CONSTANCIES ◦ Size constancy ◦ Is the perception that an object maintains the same size despite changes in the size of the proximal stimulus ◦ Shape constancy ◦ Is the perception that an object maintains the same shape despite changes in the shape of the proximal stimulus. ◦ DEPTH PERCEPTION ◦ Depth ◦ Is the distance from a surface, usually using your won body as a reference surface when speaking in terms of depth perception. ◦ Depth cues ◦ Monocular depth cues ◦ Can be represented in just two dimensions and observed with just one eye. ◦ These includes texture gradient, relative size, interposition, linear perspective, aerial perspective, location in the picture plane, and motion parallax. ◦ Binocular depth cues ◦ Based on the receipt of sensory information in three dimensions from both eyes. ◦ It provides two kinds of information to your brain: binocular disparity, your two eyes send increasingly disparate (differing) images to your brain as objects approach you. Binocular convergence, your two eyes increasingly turn inward as objects approach you.  DEFICITS IN PERCEPTION ◦ DIFFICULTIES PERCEIVING THE “WHAT” ◦ Agnosia ◦ People who suffer from an agnosia have trouble perceiving sensory information. ◦ It is often caused by damage to the border of the temporal and occipital lobes or restricted oxygen flow to the areas of the brain, sometimes as a result of traumatic brain injury. ◦ Generally, people with agnosia have normal sensations of what is in front of them. They can perceive the colors and shapes of objects and persons, but they cannot recognize what the objects are. They have trouble with the what pathway. ◦ KINDS OF AGNOSIA ◦ Simultagnosia ◦ Disturbance in the temporal region of the cortex can lead to simultagnosia. ◦ In simultagnosia, an individual is unable to pay attention to more than one object at a time. ◦ A person with simultagnosia would not see each of the objects depicted in Figure 3.28. Rather, the person might report seeing the hammer but not the other objects. ◦ Prosopagnosia ◦ Results in a severely impaired ability to recognize human faces. ◦ A person with prosopagnosia might not recognize her or his own face in the mirror. ◦ DIFFICULTIES IN KNOWING THE “HOW” ◦ Ataxia ◦ It is a kind of perceptual deficit which is an impaired ability to use the visual system to guide movements. ◦ People with this deficit have trouble reaching for things. ◦ It results from a processing failure in the posterior parietal cortex, where sensorimotor information is processed. ◦ People with ataxia can improve their movements toward a visible aim when they hold off with their movements for a few seconds. ◦ ANOMALIES IN COLOR PERCEPTION ◦ COLOR BLINDNESS ◦ Achromacy ◦ People with this condition have no color vision at all. ◦ It is the only true form of pure color blindness. ◦ Dichromacy ◦ Only two of the mechanisms for color perception work, and one is malfunctioning. ◦ FORMS OF DICHROMACY ◦ Protanopia ◦ The extreme form of red-green color blindness. ◦ Deuteranopia ◦ Trouble seeing greens. ◦ Tritanopia ◦ Confusion of blues and greens, and yellows that disappear or appear as light shades of red. THE END. 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