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Chapter 6

Chapter 6- Perception.doc

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Chapter 6- Perception • The primary function of the sense organs is to provide info to guide behaviour but they cannot achieve this function by themselves Perception- a rapid, automatic, unconscious process by which we recognize what is represented by the info provided by our sense organs • Gives unity and logic • The distinction between sensation and perception in not easy to make Brain Mechanisms of Visual Perception • Circuits of neurons analyze particular aspects of visual info and send the results of their analysis o In each step, successively more complex features are analyzed and eventually, the process leads to perception of the scene and all objects in it The Primary Visual Cortex • Early stages of visual analysis has come from in the thalamus and the primary visual cortex • David Hubel and Torsten Wiesel inserted microelectrodes (extremely small wires with microscopically sharp points) into various regions of the visual systems of animals to detect the action potential produced by individual neurons o After positioning a microelectrode close to a neuron, they presented various stimuli on a large screen in front of the open- eyed but anaesthetized animal, which made them unconscious but does not prevent neurons in the visual system from responding o They moved a stimulus around on the screen until they located a point where it had the largest effect on the electrical activity on the neuron, and then presented stimuli of different shapes to learn which ones produced the greatest response from the neuron o Concluded that the geography of the visual field is retained in the primary visual cortex (the surface of the retina is “mapped” on the surface of the primary visual cortex) Module/tile- a block of cortical tissue that receives info from the same group of receptor cells • Approx. 0.5 X 0.7 mm in size • Contains approx. 150,000 neurons • The neurons within a module receive info from the same small region of the retina 1 • The primary visual cortex contains approx. 2500 modules Receptive field- that portion of the visual field in which the presentation of visual stimuli will produce an alteration in the firing rate of a particular neuron The Visual Association Cortex • To perceive objects and entire visual scenes, the info from these individual modules must be combined and it takes place at different levels in the visual association cortex Two Streams of Visual Analysis • Visual info analyzed by the primary visual cortex is further analyzed in the visual association cortex • Neurons in the primary visual cortex send axons to the region of the visual association cortex that surrounds the striate cortex, and that’s when the visual association cortex divides into 2 pathways: o Ventral stream- the flow of info from the primary visual cortex to the visual association area in the lower temporal lobe  Used to form the perception of an object’s shape, colour, and orientation (the “what” system)  Continues forward and ends in the inferior temporal cortex o Dorsal system- the flow of info from the primary visual cortex to the visual association area in the parietal lobe  Used to form the perception of an object’s location in 3- dimensional space (the “where” system)  Rises into the posterior parietal cortex The Ventral Stream: Perception of Form • Recognition of visual patterns and identification of particular objects takes place in the inferior temporal cortex (at the end of the ventral stream) Visual agnosia (“failure to know”) - the inability of a person who is not blind to recognize the identify of an object visually, caused by damage to the visual association cortex Prosopagnosis- a form of visual agnosia characterized by difficulty in the recognition of people’s faces • Caused by damage to the visual association cortex • Can recognize that they are looking at a face, but cannot say whose face it is, even if it was a friend or relative Fusiform face area (FFA) – a region of the ventral stream of the visual system that contains face-recognizing circuits 2 • Located at the base of the brain • Face-recognition circuits develop as a result of experience with seeing people’s faces • 2 functional imaging studies found that when an expert viewed pictures of what they are experts of, the FFA was activated, but this didn’t occur when the non-experts viewed the pictures • Tarr and Gauthier suggested that the FFA be relabelled as the flexible fusiform areas, given its participation in the visual recognition of diverse objects o People with autism show a deficit in the ability to recognize faces and that looking at faces failed to activate the FFA Extrastriate body area (EBA) – a region of the occipital cortex, next to the primary visual cortex, that responds to forms resembling the human body • Activated by photographs, silhouettes, or stick figures of human bodies or body parts, not by control stimuli such as photographs or drawing of tools, scrambled silhouettes, or scrambled stick drawings of bodies • When the EBA was inactivated by transcranial magnetic stimulation, people lost the ability to recognize photographs of body parts but not parts of faces or motorcycles Parahippocampal place area (PPA) – a region of the ventral stream, below the hippocampus, that is activated by visual scenes (collection of several objects) and backgrounds • Able to recognize both natural and human-made scenes (Ex: beaches, forests, cities, etc) The Ventral Stream: Perception of Colour • Damage to the ventral stream disrupts the ability to distinguish different colours, but can still distinguish between different shades of grey • Lesions of a particular region of the human ventral stream can cause loss of colour vision without disrupting visual activity Cerebral achromatopsia (“vision without colour”) - the inability to discriminate among different hues • Caused by damage to the visual association cortex • If the brain damage occurs on only ones side of the brain, people will lose their colour vision in only half of the visual field • If the damage is bilateral, they lose all colour vision and cannot even imagine colours, or remember the colours of objects they saw before their brain damage occurred The Dorsal Stream: Perception of Spatial Location 3 • Damage to the parietal lobe disrupts performance on a variety of tasks that requires: o Perceiving and remembering the location of objects o Controlling the movement of the eyes and the limbs • End of the dorsal stream is located in the posterior parietal cortex • Neurons are involved in visual attention and control of eye movements, the visual control of reaching and pointing, and the visual control of grasping and other hand movements, and the perception of depth • Melvyn A. Goodale and his colleagues suggested that the primary function of the dorsal stream of the visual cortex is to guide actions rather than simply to perceive spatial locations (how to perform the action) The Dorsal Stream: Perception of Movement • Damage in the extrastriate cortex, which surrounds the primary visual cortex, of the visual association cortex, severely disrupts the ability to perceive moving stimuli Akinetopsia- an inability to see motion • Walsh, Ellison, Battelli, and Cowey found that participants were able to recognize different-shaped objects displayed on a computer screen but were unable to detect which of the objects are moving, when using transcranial magnetic stimulation to temporary inactivate the extrastriate cortex in humans Form from Motion Form from motion- perception of movement that help perceive 3-dimensional forms • Grossman and colleagues found that when people view a video that show form from motion, a bilateral brain region (the ventral bran of the posterior end of the superior temporal sulcus) becomes active • Form from motion involves brain mechanisms different from those involved in the perception of objects Visual Perception of Objects Figures and Ground Objects- things that have particular shapes and particular locations in space • Figure- a visual stimulus that is perceived as a self-contained object Backgrounds- essentially formless and serve to help us judge the location of objects we see in front of them 4 • Ground- a visual stimulus that is perceived as a formless background against which objects are seen • The classification of an item as a figure or as part as a background is not an intrinsic property but depends on the behaviour of the observer • Most important aspects of form perception is the existence of a boundary Gestalt Laws of Perceptual Organization Illusory contours- lines that does not exist Gestalt psychology (“form” psychology) – a branch of psychology that asserts that the perception of objects is produced by particular configurations of the elements of stimuli • Recognize objects in the environment according to the organization of their elements • What we see depends on the relationships of these elements to one another, instead of simply understanding the visual perception just by analyzing the scene into elements • Components of a visual scene can combine in various ways to produce different forms • Gestalt psychologists have observed that several principles of grouping can predict the combination of these elements Law of proximity- elements located closest to each other are perceived as belonging to the same figure Law of similarity- similar elements are perceived as belonging to the same figure Good continuation- given 2 or more interpretations of elements that form the outline of the figure
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