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Chapter 14: Touch

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Psychology 2115A/B
Christine Tsang

Lecture 14 Review: Touch By the end of this section you should know:  Touch receptors  Body maps  Tactile acuity  Vibration and texture  Object recognition using touch  Models of pain The Somatosensory System  Three parts: o Cataneous senses: the perception of touch and pain that comes from the stimulation of the skin o Proprioception: our (unconscious) ability to sense the position of our bodies and limbs in respect to one another o Kinesthesis: our ability to sense the movement of our body and limbs  If you don‟t have these senses you would be severely disabled  Text talks about a man who „lost his body‟ o Not able to sense his body despite the fact that all his muscles and nerves are in tact in respect to movement but he is unaware of their movement o His whole body from the neck down is insensitive – doesn‟t move under full control o Had some form of viral infection that eliminated some of the somatosensory system and left him with no sense of proprioception or kinesthesis – was not able to know where his body was at any moment of time o Now takes him a lot of effort to move about the world The Skin  Heaviest organ in the body o Skin weighs approximately 20 pounds on average o Its role is to protect the inside parts of us by keeping damaging agents in the environment from penetrating our bodies  Epidermis: the outer layer we can see o Made up of dead skin cells  Dermis: lies below the epidermis and contains all the receptor cells o Mechanoreceptors: the receptor cells  Several types that respond to different stimulation  Pressure  Stretching  Vibration Mechanoreceptors – Near Skin Surface  Merkel Receptors o Continuous firing receptors – fire continuously as long as the stimulus is present o Responsible for sensing fine detail o Tend to resemble regular receptors  Meissner Corpuscle o Fire on/off – detectors of when a stimulus is detected/removed o Responsible for grip o Tends to look like a bulb Mechanoreceptors – Deeper in the Skin  Ruffini Cylinder o Responds continuously to stimulus o Responsive to stretching of skin  Pacinian Corpuscle o On/off receptor o Responsible to vibrations and fine texture by moving fingers Somatosensory Pathways: Skin to Cortex  Medial Lemniscal Pathway o Responsible for proprioceptive and touch information o Large nerve fibres – big bundles o Takes a route from the spine through medial lemniscal fibres before getting to the thalamus  Spinothalamic pathway o Responsible for temperature and pain information o Smaller bundles of nerve fibres o Up the spine to the thalamus  Both considered to responsible for the peripheral nervous system because they lay outside of the spinal chord  The fibres cross over to the opposite side – sensations of the receptors of your left hand go to the right side of your brain and vice versa  All synapse in the thalamus because they go to the higher somatosensory cortex  The thalamus is a critical part of the brain this is responsible for all sensory signals  Body homunculus: we have a body map in the somatosensory cortex  A little bit of training trains the cortical mapping within our homunculus and the proportions that are given to a certain area Tactile Acuity  Learning through feeling/touch  Three methods of measurement o Two point thresholds  Calculation of the minimum amount of separation between two points needed to sense them as two separate points as opposed to one point  Poke a person on the skin and see if they can sense that it is two points or one point  Different two point thresholds across the body  We have really good two point threshold around the mouth and the fingers/hands and really poor two point threshold on the legs and back o Grating acuity  Feeling how close together the bumps and raises are in a grating o Raised pattern identification  Using raised patterns on a surface and seeing how fine a pattern you can detect  E.g. brail Merkel Receptors and Acuity  There is a high density of Merkel receptors in the fingertips o Responsible for touchy acuity o Merkel receptors very sensitive to grooves in comparison to pacinian corpuscles o The Merkel receptor is signaling the physical details of the object – correlated to the firing rate of the receptor  Merkel receptors are densely packed on the fingertips – similar to cones in the fovea o We get less acute in terms of our perception with most acuity at the fingertips, less at the base of the finger, and the least at the palm of the hand o Positive relationships between density and tactile acuity and it changes across the hand o We have equally sense receptors cross all of our fingertips – the density of Merkel receptors on each fingertip is similar to one another but have better perception of the thumb and middle finger vs. pinky o Therefore, there is some other kind of mechanism cortically too that is also feeding into our abilities of tactile acuity o Areas with higher acuity have smaller receptors on the skin  Both two-point thresholds and grating acuity studies show these results o Looking at two point thresholds at different points on a monkey arm and the relative area in cortex o Two point thresholds from the finger take up more space in the cortex than areas in the arm o The idea of body mapping as important to how many receptors are devoted to that particular space on your skin Pacinian Corpuscles and Vibration  Pacinian corpuscle (PC) is primarily responsible for sensing vibration o Nerve fibres associated with PCs respond best to high rates of vibration  It is something about the physiology of the PC itself that makes it so sensitive to vibration  When vibration is applied to the PC, each of the individual pressures to the corpuscle inside the bulb transferred to the fibre extend outwards  Continuous pressure does not cause the fibre to fire in any way – no activity in the tail o The structure of the PC is responsible for the response to vibration – fibres without the PC only respond to continuous pressure  Looking at the physiology between the bulb and the fibre extending out of it  If you take the receptor and give it a push at one point (point A), the push is signaled to B  When it is let go, the signal is transferred through the rest of the nerve fibre  Essentially each push is signaled from A to B which causes the rest of the fibre to active  If you have a continuous push at A and never let go, nothing fires at B  This is probably what is going on in terms of vibration detection – an on/off movement vs. a continuous push  Rapid vibration does the trick but continuous pressure does not Texture Perception  Duplex Theory of Texture Perception - two main sets of cues: o Spatial cues are determined by the size, shape, and distribution of surface elements o Temporal cues are determined by the rate of vibration as skin is moved across finely textured surfaces  Hollins and Reisner (2000) o Looked at movement-based cues – do we just need to touch the object or is movement a critical part in sensing what an object is? o Have an object that varied in its roughness (fine vs. rough texture) and a static condition (can only touch the object) or moving condition (can move your fingers/hand across the object) o Have to make a judgment of the level of roughness o In the static condition, our judgment of texture is the same no matter the texture – not particularly distinguishable when only allowed to touch the object o In the movement condition, the two objects become very distinct o Therefore, temporal cues (movement) are critical to texture perception – adds in richness of detail that you cant get from static  Hollins et al. (2001) o Which receptors were active at any given point o Looked at the different corpuscles o Did an adaption based study – took the skin of the participant and did 6 minutes of stimulus of either 10 Hz (slow) or 250 Hz (fast) of stimulation o Found that only adaption to the fast stimulus affects the perception of fine textures o Adaption changes the pacinian corpuscle o Temporal information gives us a lot more information about texture but is still related to the spatial patterning of the object itself How do we Perceive Objects with Touch?  Haptic perception: the active exploration of 3D objects with the hand  It uses three distinct systems in the brain o Sensory system o Motor system o Cognitive system  Humans are very active in our interaction with the environment – are morel likely to pick up an object and move it around as a means of figuring out what the object is  Our recognition of objects just with the haptic sense is very good Object Recognition Physiology  Distributed coding of shape at the level of the mechanoreceptor o Probably using a distributed code in terms of its code in the cortex at the level at the mechanical receptors o The firing pattern of the mechanical receptors probably signal the shape of the object  Neuron
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