Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTouchPSY 310Greg FrancisLecture 34Why can’t you tickle yourself?Purdue UniversityTouch receptors Last time we talked about the mechanoreceptorsinvolved touch perception It is important to remember that touch is a percept thatfundamentally is created by the brain Consider phantom limbs Referenced in Moby Dick by Captain Ahab Look ye carpenter, I dare say thou callest thyself a right goodworkmanlike workman, eh? Well, then, will it speak thoroughlywell for thy work, if, when I come to mount this leg thou makest,I shall nevertheless feel another leg in the same identical placewith it; that is, carpenter, my old lost leg; the flesh and bloodone, I mean. Canst thou not drive that old Adam away?Purdue UniversityPhantom limbs Estimates are that between 50-80% ofamputees suffer from some form ofphantom limb Here’s a description by neuropsychologistRamachandran (1998) As a result of this gruesome mishap [amotorcycle accident], Tom lost his left armjust above the elbow…In the weeksafterward, even though he knew his armwas gone, Tom could still feel its ghostlypresence below the elbow. He couldwiggle each “finger,” “reach out” and “grab”objects that were within arm’s reach.Indeed, his phantom arm seemed to beable to do anything that the real arm wouldhave done automatically, such as wardingoff blows, breaking falls or patting his littlebrother on the back.Purdue UniversityBrain representation The brain does notrepresent all areas of thebody equally Some areas (such asfingertips) have manymechanoreceptors withsmall receptive fields Other areas (such as theback) have fewmechnoreceptors withlarge receptive fields Two-point thresholdsreveal these differences Larger thresholdsmean less sensitivityPurdue UniversityBrain representation Neural fibers for touch go through several intermediate areas(spinal cord, thalamus, medial lemniscus) and then reach theparietal lobe of the cortexPurdue UniversitySomatotopic representation In the somatosensory cortex, neighboring neurons tend to respond totouch from neighboring parts of the body It’s not perfect, though, there are some gaps There are multiple maps for parts of the body, running along strips from topto bottom of the somatosensory areaProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySomatotopic representation Areas of the body are notrepresented equally in cortex Size of drawn body area indicatesthe number of cells that respond totouch at that location Note, your textbook has the headrepresentation upside down! Note, the genitals are just below thefeet Every wonder why playing “footsie”is arousing?Purdue UniversitySomatotopic homonculus Useful to make a 3-Drepresentation of the body maps Again, the size of the body partindicates the amount of brainarea dedicated to responding totouch at that area Compare to the two-point touchthresholds and you’ll see thatthere is pretty goodcorrespondence Lower thresholds are for thoseareas with more brain arearepresentationPurdue UniversityPhantom limbs The structure ofsomatosensory cortexpartially explains someaspects of phantomlimbs Before amputation, aperson’s brain might beorganized like thisPurdue UniversityPhantom limbs The structure ofsomatosensory cortexpartially explains someaspects of phantom limbs After arm amputation, thebrain starts to re-organize The region that used torespond to the fingers, startsto respond to touch from thefacePurdue UniversityPhantom limbs For some people withphantom limbs A touch on the face is felt asa touch on the phantom limb You can map out a map ofthe phantom limb on the faceof the personPurdue UniversityTickling Tickling is an odd sensation Both pleasure (laughter) And pain (a method of torture in Roman times) Similar to sexual stimulation It is almost impossible to tickle yourself Try it on the palm of your friend If you are comfortable with it, ask you neighbor to tickleyou in the same spot Note the difference in sensation Even though the same mechanoreceptors respond aboutthe same The difference is in the brainProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySorting out sources We need to be able to distinguish event in theenvironment from self-induced events Think of corollary discharge for eye movements Same issue for touch For the motor system, we call it an “efference copy” It produces a predicted touch sensationPurdue UniversitySelf-tickling When we try to self-tickle, the predicted sensoryfeedback is similar to the actual sensory feedback,and the sensory feedback is filtered out When someone else tickles us, we don’t have thepredicted sensory feedback, so the sensation isstrongerPurdue UniversityTickling machine To study anything you need to be able to reproduce the stimulus This is a tickling machine It can be run by a person or a computer If run by the person being touched, it does not produce a tickle Unless the motion of the machine is delayed unpredictablyPurdue UniversityPain perception Complex perceptual experience Warning of biological damage But not always, often people do not feel pain until after theyrecognize the damage Experience of pain depends tremendously on The instigating condition The person’s history with pain The person’s mental state Many aspects of pain are related to a class of receptors knownas nociceptors Found among free nerve endings in the skin, eye, teeth, andsome internal organsPurdue UniversityNociceptors Here are shown free nerve endings in the skin They are less sensitive to stimuli than other mechanoreceptors fortouch or temperaturePurdue UniversityNociceptors There are several types of nocicpetor fibers Mylenated (fast) Unmylenated (slow) Fast-conducting fibers Respond to» strong mechanical stimulus (a pin prick)» Thermal stimulation (intense heat) Produce a sensation of sharp acute pain Slow-conducting fibers Respond to» Mechanical and thermal stimulation» Chemical stimuluation (acid on the skin) Produce slow-growing sensation fo pain that can be long-lastingProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityGate control theory The responses from nociceptors do not alwaysreach the brain They are