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Chapter 6

Chapter 6 Sensation and Perception.doc

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Ted Petit

Chapter 6 Sensation and Perception: Vision Module 6.1: Organization of sensory systems: Three main types of sensory cortex: 1) Primary Sensory Cortex: First part of the cortex that receives information relayed from the senses through the thalamus - Of the visual cortical areas, has the most direct access to the information provided by the sensory systems. 2) Secondary Sensory Cortex: - Receives much of its information from the primary sensory cortex - Is highly interconnected: Receives information from other areas of the secondary sensory cortex - Sends information to association cortex 3) Association cortex: - Any area of the cortex that receives information from more than one sense. Manner with which information is received and disbursed through the three cortical areas suggests that sensory systems can be categorized into three organizing principles: 1. Sensory systems are characterized by hierarchical organization 2. Each level of the organization contains functionally distinct cortical areas 3. The processing of sensory information occurs in parallel throughout the cortex Hierarchical Organization: At the level of the eye, there are large numbers of specialized receptors that have one function to transform light into signals that are meaningful in the nervous system - At the level of the association cortex, job is more complex to integrate information from a number of sensory systems into a perception of the outside world - As we move from the lowest to the highest level of sensory systems, the neurons changes from having simple on/off sensory functions to those that respond optimally to stimuli of greater complexity and specificity Deficits that arise when the various levels are damaged are informative as to the function of each of the levels Segregation by function: Within each of the three levels of the sensory cortex, there are specific areas that are involved in processing specific aspects of the same sensory stimulus Processing of information is parallel: - Information Is transmitted throughout the sensory system in parallel: each level receives some information from the level below it and some information from levels below that - When information is processed in parallel through multiple pathways, information flows through the levels of the sensory system really fast and decreases the reliance of the system on any one level of processing Areas of sensory systems are very interconnected with each other( Both within and between levels) Module 6.2: The visual system Light: Stimulus for the visual system: - Electromagnetic(EM) energy: Very short wavelengths: high energy gamma rays, X rays Very long Wavelengths: Low energy radio waves Wavelengths of about 400-700nm= Visible to human eye - All EM energy is light but light is usually referred to the relatively narrow band of EM energy that humans can sense with their eyes - Two potential sources of light: 1) Light coming directly from something that is producing is like sun, light bulb 2) Light that has travelled from a source to an object and is then reflected off the object or multiple objects - Different wavelengths of light are perceived as different colours. The eye and Retina: - Light enters the eye through the pupil and is focused on the retina by the curvature of the cornea and fine-tuned by the lens Retina: A layered structure at the back of the eye that contains 5 different types or cells ( Receptors, horizontal cells, bipolar cells, amacrine cells and retinal ganglion cells), each of which has different functions - Receptors: converts light energy into neural responses which are then transmitted to the brain via the retinal ganglion cells Receptors in retina (rods and cones) are specialized and are active under different types of lighting conditions Rods: Typically active is low light levels, very sensitive to movement (Has acuity, has multiple rods synapse on one bipolar cells, which in turn converges on the same retinal ganglion cell with other bipolar cells) Cones: typically active in medium to bright light, responsible for high acuity vision or vision that provides rich details and color (Synapses 1:1 on bipolar cells, which then synapse individually on retinal ganglion cells: Quite sensitive to the activity of that cone) - Amacrine and Horizontal cells: Lateral communication between the various cells ( Communication between cones and other cones or retinal ganglion cells and other retinal ganglion cells) - Bipolar cells: Synapse on the receptors which n turn synapse on the retinal ganglion cells, whose axons leave the retina via the optic nerve) Light tends to travel in straight line SIDES OF THE RETINA: - Nasal: Closest to the nose - Temporal: Closest to temples The retinal ganglion cells in the nasal hemiretina project contralaterally to the lateral geniculate nucleus of the thalamus and the retinal ganglion cells in the temporal hemiretina project ipsilaterally to the LGN Ensure that information about the right side of space from either eye stays together in the left LGN and that information about the left side of space from either eye stays together in the right LGN Left and right visual fields are segregated into different hemispheres in the brain Retino-Geniculate-Striate System Beings in the eye and ends in the primary visual cortex Although the visual system conforms to the three principles of sensory organization, visual information is perceived as an integrated whole The Lateral Geniculate Nucleus of the Thalamus A 6-layered nucleus of the thalamus that receives information from the retina Each layer receives different sensory information 1) Top 4 layers Parvocellular / P layers composed of neurons that have small cell bodies 2) Bottom two layers Magnocellular / M layers composed of ne
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