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Chapter 5

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Steve Joordens

Chapter 5: Sensory Processing: Sensory Processing: Experience is composed of sensation and perception Sensation: the detection of simple properties of stimuli (i.e. brightness, colour, warmth) Perception: the detection of objects (animate and inanimate), their locations, their movements, and their backgrounds) i.e. seeing the colour red is sensation but seeing a red apple is perception i.e. seeing movement is sensation but seeing a soccer ball and realizing that we have to move away is called perception sensations are simple, perceptions are complex Transduction : the only sense receptors in the brain detect temperature and salt concentration of the blood and do not tell us anything about what is going on outside information about the external world is gathered by sense organs located outside the brain sense organs detect stimuli provided by light, sound, odour, taste, and mechanical contact with the environment the brain then analyzes this information (from sensory organs) and reconstruct what has occurred transduction: leading across is the process by which sense organs convert energy from environmental events into neural activity each sense organ responds to a particular form of energy given off by an environmental stimulus and translates that energy unto neural firing to which the brain can respond the means of transduction are as diverse as the kinds of stimuli we can perceive specialized neurons called receptor cells (in our senses) release chemical transmitter substances that stimulate other neurons, thus altering the rate of firing of their axons in the somatosenses (body senses), dendrites of neurons respond directly to physical stimuli without the intervention of specialized receptor cells in the somatosenses however, some of the neurons have specialized endings that enable them to respond to particular kinds of sensory information Location of Sense Organ Environmental Stimuli Energy Transduced Eye Light Radiant energy Ear Sound Mechanical energy Vestibular system Tilt and rotation of head Mechanical energy Tongue Taste Recognition of molecular shape Nose Odour Recognition of molecular shape Skin, internal organs Touch, Mechanical energy temperature, thermal energy vibration mechanical energy Muscle Pain Chemical reaction stretch mechanical energy Sensory Coding: it is important for sensory information to accurately represent the environment however, we know from before that nerves are bundles of axons each of which all transmit action potentials that are fixed in size and duration this means different stimuli cannot be translated into different types of action potentials PROBLEM? However, we can still detect an enormous amount of stimuli each of our sense organs (i.e. we can discriminate among 7.5 million colours, 10 000 odours) HOW DO SENSE ORGANS TELL THE BRAIN ALL THIS DIFFERENT INFORMATION IF ACTION POTENTIALS CANNOT BE ALTERED? Solution: information from the sense organs must be coded in the activity of axons carrying information from the sense organs to the brain Code: a system of symbols or signals representing information Sensory systems transmit information to the brain through: a) anatomical coding b) temporal coding Anatomical Coding: A means by which the nervous system represents information; different features are coded by the activity of different neurons Ever since Johannes Muller formulated doctrine of specific nerve energies, we know that the brain learns what is happening through the activity of specific sets of neurons Sensory organs located in different places in the body send their information to the brain through different nerves Because the brain has no direct information about the physical energy on a sense organ, it uses anatomical coding to interpret the location and type of sensory stimulus according to which incoming nerve fibres are active Basically, to know the location and type of stimulus (i.e. light, sound, odour), the brain interprets the incoming active nerve fibres through anatomical coding i.e., if you rub your eyes, you will mechanically stimulate the light sensitive receptors located there this stimulation will produce action potentials in the axons of the nerves that connect the eyes with the brain (optic nerves) the visual system of the brain has no way of knowing that the light sensitive receptors of the eyes have been activated by a non visual stimulus thus, the brain acts as if the neural activity in the optic nerves was produced by light, which is why you see stars and flashes i.e. during surgery, experiments have shown that artificial stimulation for the nerves that convey taste produces a sensation of taste, electrical stimulation of the auditory nerve produces a buzzing noise, etc anatomical coding is present all over the body surface the primary somatosensory cortex contains a neural map of the skin receptors n the skin in different parts of the body send information to different parts of the primary somatosensory cortex basically, anatomical coding is when particular nerves fire to show the brain where the body is being affected by the stimuli Temporal Coding: the coding of information in terms of time a means by which the nervous system represents information (different features are coded by the pattern of activity of neurons) i.e. rate (is a form of temporal code) by firing at a faster or slower rate according to intenseness of stimuli, an axon communicates quantitative information to the brain i.e. a light touch to the skin can be encoded by a low rate of firing and a more forceful touch is communicated by a high rate How do they work together? the firing of a particular set of neurons (anatomical code) tells where the body is being touched and the rate at which these neurons fire (temporal code) tells how intense that touch is sensory systems use rate of firing to encode the intensity of stimulation Psychophysics: The systematic study of the relation between the physical characteristics of stimuli and the sensations they
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