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Chapter 14

Sensation and Perception Psych 367 Chapter 14.docx

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Douglas Wylie

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Chapter 14 the cutaneous senses Overview of the cutaneous system The skin - Comel called the skin the monumental facade of the human body - Heaviest organ - In addition to its warning function, the skin also prevents bodily fluids from escaping and at the same time protects us by keeping bacteria, chemical agents and dirt from penetrating our bodies - Epidermis: layer of dead cells in the outer layer - Dermis: found below the epidermis - Mechanoreceptors: are found in the two layers o They respond to mechanical stimulation such as pressure, stretching and vibration Mechanoreceptors - Four types of mechanoreceptors that are located in the epidermis and the dermis - We can distinguish between these receptors by their distinctive structures and by how fibres associated with the receptors respond to stimulation - Two mechanoreceptors o Merkel receptor o Meissner corpuscle - Both located close to the surface - The merkel receptor: fires continuously as long as the stimulus is on o Sensing fine details - The meissner corpuscle: fires only when the stimulus is first applied and when it is removed o Controlling handgrip - Found deeper in the skin o Ruffini cylinder  Is associated with perceiving stretching of the skin o Pacinian corpuscle  Sensing rapid vibrations and fine texture Pathways from skin to cortex - Peripheral nerves: Nerve fibres from receptors in the skin travel in bindles o They enter the spinal cord through the dorsal root - They then go up the spinal cord along two major pathways o Medial lemniscal pathway  Has large fibres that carry signals related to sensing the position of the limbs (proprioception) and perceiving touch o Spinothalamic pathway  Smaller fibres that transmit signals related to temperature and pain - The case of Ian waterman illustrates this separation in function , because although he lost the ability to feel touch and to sense the position of his limbs he was still able to sense pain and temperature - Most of these fibres synapse in the ventrolateral nucleus in the thalamus, but some synapse in other thalamic nuclei - Because the signals in the spinal cord have crossed over to the opposite side of the body on their way to the thalamus, signals originating from the left side of the body reach the thalamus in the right hemisphere of the brain, and signals from the right side of the body reach the left hemisphere Maps of the body on the cortex - From the thalamus, signals travel to the somatosensory receiving area (S1) in the parietal lobe of the cortex and possibly also to the secondary somatosensory cortex (S2) - Signals also travel between S1 and S2 - Important characteristic of the somatosensory cortex is that is organized into maps that correspond to locations on the body - When penfield stimulated points on S1 and asked patients to report what they perceived - Homunculus: The resulting body map o Some areas on the skin are represented by disproportionately large area of the brain o Results analogous to the magnification factor in vision - Similarity, parts of the body such as the fingers, what are used to detect details through the sense of touch, are allotted a large area on the somatosensory cortex The plasticity of cortical body maps - Experiments that demonstrated experiment dependent plasticity were carried out in the somatosensory system - Jenkins and Merzenish showed that increasing stimulation of a specific area of the skin causes an expansion o the cortical area receiving signals from that area of skin - Comparison of the cortical maps of the fingertip measured just before the training and 3 months later shows that the area representing the stimulated fingertip was greatly expanded after the training - The effect of plasticity is determined by measuring how special training affects the brain - What this plasticity means is that while we can specify the general area of the cortex that represents a particular part of the body, the exact size of the area representing each part of the body is not totally fixed Perceiving details - Examples of perceiving details with the skin is provided by Braille, the system of raised dots that enables blind people to read with their fingertips - Experienced Braille readers can read at the rate of 100 words per minute - Normal readers 300 words per minute - Measuring tactile acuity o Tactile acuity: the ability to detect details on the skin o Two point threshold  The minimum separation between two points on the skin that when stimulated is perceived as two points o Grating acuity  Measured by pressing a grooved stimulus onto the skin and asking the person to indicate the orientation of the grooves Receptor mechanisms for tactile acuity - Merkel receptor is sensitive to details - Fibres associated with a Merkel receptor fires in response to a grooved stimulus pushed into the skin - Merkel receptors fibre signals details - High density of merkel receptors in the fingertips, because the fingertips are the parts of the body that are most sensitive - Better acuity is associated with less spacing between merkel receptors - While receptor spacing is part of the answer, the cortex also plays a role in determining tactile acuity Cortical mechanisms for tactile acuity - Representation of the body in the brain and the acuity at different locations on the body - The map of the body on the brain is enlarged to provide extra neural processing that enables us to accurately sense fine details with our fingers and other parts of the body - Demonstrate the connection between cortical mechanisms and acuity by Determine the receptive fields of neurons in different parts of the cortical homunculus - The receptive field for a neuron in the visual system is the area on the retina that when stimulated, influences the firing of the neuron - The receptive field for a neuron in the cutaneous system is the area on the skin that, when stimulated influences the firing of the neurons - Cortical neurons representing parts of the body with better acuity, such as the fingers, have smaller receptive fields - This means that two points that are closer together on the fingers might fall on different receptors - Having small receptive fields of neurons receiving signals from the fingers translates into more separation on the cortex, which enhances the ability to feel too close together points on the skin as two separate points Perceiving vibration - The mechanoreceptor that is primarily responsible for sensing vibration is the pacinian corpuscle - Recording from fibres associated with the PC show that these fibres respond poorly to slow or constant pushing, but respond well to high rates of vibration - The presence of the PC determine which pressure stimuli actually reach the fibres - PC which consists of a series of layers, like an onion, with fluid between each layer, transmits rapidly applied pressure like vibration to the nerve fibres - But it does not transmit constant pressure - The corpuscle causes the fibres to receive rapid changes in pressure, but not the receive continuous pressure - If the PC does not transmit continuous pressure to the fire, then pressing continuous pressure to the PC should cause no response in the fibre - Lowenstein o Showed that when pressure was applied to the corpuscle o The fibre responded when the pressure was first applied and when it was removed, but did not respond to continuous pressure o When lowenstien dissected away the corpuscle and applied pressure directly to the fibre, the fibre fired to the continuous pressure o That properties of the corpuscle cause the fibre to respond poorly to continuous stimulation Perceiving texture - Katz proposed that our perception of texture depends on both spatial cues and temporal cues - Spatial cues: are caused by relatively large surface elements, such as bumps - Temporal cues: occur when the skin move across a textured surface o This cue provides information in the form of vibrations that occur as a result of the movement over the surface o Responsible for our fine grain textures - Hollins o Provided evidence that temporal cues are responsible for our perception of fine textures o Duplex theory of texture perception  Two types of receptors - Hollins and Risner o Presented evidence for the role of temporal cues by showing that when participants touch surfaces without moving their fingers, they couldn’t tell texture o Movement which generates vibration as the skin scans a surface, make it possible to sense the roughness of fine surfaces - Selective adaptation o Involves presenting a stimulus that adapts a particular type of receptor and then testing to see how inactivation of that receptor by adaptation affected perception - Hollins o Presenting two adaptation conditions o First was 10Hz adaptation in which the skin was vibrated for 6 minutes o The frequency adaptation for meissner corpuscle which responds to low frequencies o 250 Hz adaptation  This was to adapt the pacinian corpuscle which responds to high frequencies o Following each type of adaptation, participants ran their fingers over two fine textures o Results indicate that participants could tell the difference between the two textures when they had not been adapted or had received that 10Hz o But they couldn’t tell the difference when adapted to the 250 Hz o Adapting pacinian corpuscle receptor, which respond to vibration, eliminates the ability to sense fine textures - The most remarkable thing about perceive texture with a tool is that what you perceive is not the vibration, but the texture of the surface Perceiving objects - Ability to identify objects and their features by touch is an example of active touch o Touch in which a person actively explores an object - Active touch contrast with passive touch o When touch stimuli are applied to the skin - Haptic perception: perception in which 3D objects are explored with the hand Identifying objects by haptic exploration - Provides good example of a situation in which a number of different system are interacting with each other - The sensory system, which was involved in detecting cutaneous sensations such as touch, temperature and texture - Motor system, which was involved in moving your fingers and hands - The cognitive system, which was involved in thinking about the information provided by
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