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Chapter 7

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Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 7: Mechanisms of Perception, Hearing, Touch, Smell, Taste, and Attention PRINCIPLES OF SENSORY SYSTEM ORGANIZATION • The sensory areas of the cortex are, by convention, considered to be of three fundamentally different types: primary. secondary, and association • The primary sensory cortex of a system is the area of sensory cortex the receives most of its input directly from the thalamic relay nuclei of that system • The secondary sensory cortex of a system compromises the areas of the sensory cortex that receive most of their input from the primary sensory cortex of that system or form other areas of the secondary sensory cortex of the same system • Association cortex is any area of cortex that receives input from more than one sensory system • The interactions among these three types of sensory cortex and among other sensory structures are characterized by three major principles: hierarchical organization functional segregation, and parallel processing • Hierarchical Organization • A hierarchy is a system whose members can be assigned to specific levels or ranks in relation to one another • As one moves through a sensory system from receptors, to thalamic nuclei, to primary sensory cortex, to secondary sensory cortex, to association cortex, one finds neurons that respond optimally to stimuli of greater and greater specificity and complexity • Each level of a sensory hierarchy receives most of its input from lower levels and adds another layer of analysis before passing it on up the hierarchy • The hierarchical organization of sensory systems is apparent from a comparison of the effects of damage to various levels: the higher the level of damage the more specific and complex the deficit • In recognition of the hierarchical organization of sensory systems, psychologists divide the general process of perceiving into two general phases: • Sensation - the process of detecting the presence of stimuli • Perception - the higher-order process of integrating, recognizing, and interpreting complete patterns of sensations • Functional Segregation • It was once assumed that the primary, secondary, and association areas of a sensory system were each functionally homogeneous • It was assumed that all areas of cortex at any given level of a sensory hierarchy acted together to perform the same function • Functional segregation, rather than functional homogeneity, characterizes the organization of sensory systems • It is now clear that each of he three levels of cerebral cortex in each sensory system contains functionally distinct areas that specialize in different kinds of analysis • Parallel Processing • A serial system is a system in which information flows among the components over just one pathway • Now evidence that sensory systems are parallel systems - systems in which information flows through the compnents over multiple pathways Chapter 7: Mechanisms of Perception, Hearing, Touch, Smell, Taste, and Attention • Parallel systems feature parallel processing - the simultaneous analysis of a signal in different ways by the multiple parallel pathways of a neural network • There appear to be two fundamentally different kinds of parallel streams of analysis in our sensory systems: one that is capable of influencing out behaviour without our conscious awareness and one that influences our behaviour by engaging our conscious awareness • Summary Model of Sensory System Organization • Sensory systems are characterized by a division of labor: multiple specialized areas, at multiple levels, are interconnected by multiple parallel pathways • One possible solution to the binding problem is that there is a single area of the cortex at the top of the sensory hierarchy that receives signals from all other areas of the sensory system and puts them together to form perceptions • However, there are no areas of cortex to which all areas of a single sensory system report • It seems, then, that perceptions must be a product of the combined activity of different interconnected cortical areas AUDITORY SYSTEM • The function of the auditory system is the perception of sound - or, more accurately, the perception of objects and events through the sounds that they make • Sounds are vibrations of air molecules that stimulate the auditory system; humans hear only those molecular vibrations between about 20 and 20,000 hz • The amplitude, frequency, and complexity of the molecular vibrations are most closely linked to perceptions of loudness, pitch, and timbre, respectively • Pure tones exist only in laboratories and sound recording studios; in real life, sound is always associated with complex patterns of vibrations • Fourier analysis is the mathematical procedure for breaking down complex waves into their component sine waves • For any pure tone, there is a close relationship between frequency of the tone and its perceived pitch; however, the relation between the frequencies that make up natural sounds and their perceived pitch is complex: the pitch of such sounds is related to their fundamental frequency • An extremely important characteristic of pitch perception is the fact that the pitch of a complex sound may not be directly related to the frequency of any of the sounds components • This is important aspect of pitch perception is referred to as the missing fundamental • The Ear • Sound waves travel from the outer ear down the auditory canal and cause the tympanic membrane to vibrate • These vibrations are then transferred to the three ossicles - the small bones of the middle ear: the malleus (the hammer), the incus (the anvil), and the stapes (the stirrup) • The vibrations of the membrane called the oval window, which in turn transfers the vibrations to the fluid of the snail-shaped cochlea • The cochlea is a long, coiled tube with an internal membrane running almost to its tip Chapter 7: Mechanisms of Perception, Hearing, Touch, Smell, Taste, and Attention • This internal membrane is the auditory receptor organ, the organ of Corti • The organ of Corti is composed of two membrances: the basilar membrane and the tectorial membrane • The auditory receptors, the hair cells, are mounted in the basilar membrane, and the tectorial membrane rests on the hair cells • A deflection of the organ of Corti at any point along its length produces a shearing force on the hair cells at the same point • This force stimulates the hair cells, which in turn increase firing in axons of the auditory nerve - a branch of cranial nerve VIII • The vibrations of the cochlear fluid are ultimately dissipated by the round window, an elastic membrane in the cochlea wall • The major principle of cochlear coding is that different frequencies produce maximal stimulation of hair cells at different points along the basilar membrane - with higher frequencies producing greater activation closer to the windows and lower frequencies producing greater activation at the tip off the basilar membrane • Thus, the many component frequencies that compose each complex sound activate hair cells at many different points along the basilar membrane, and the many signals created by a single complex sound carried out of the ear by m
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