Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTouchPSY 310Greg FrancisLecture 33Why is the Braille system better?Purdue UniversityTouch perception Vision and audition involve perception of objects from adistance Safe and dependent on the transfer of energy (light, airpressure) Touch inherently involves contact of your body withobjects in the world More dangerous and dependent on the transfer of mechanicalenergy You can recognize objects in the dark You can explore properties of an object that cannot beidentified by vision (strength, stretch,…) People are very sensitive about touch Higher status people are allowed to touch lower status people,not the other way aroundPurdue UniversityTouch perception In 1992 the Queen of England visited Australia At the parliament, the premier of Australia, Paul Keating,touched the Queen’s back This caused a huge uproar among the British press Keating was labeled the “Lizard of Oz”Purdue UniversityTouch perception In 2000 the Queen of England visited Australia again Something similar happened when the prime minister, JohnHoward, put his arm around the queen while introducing her tosome other people A spokeswoman for the prime minister said: "We firmly deny that there was any contact whatsoever."Purdue UniversityCutaneous sensations You skin is one of the largest organs of your body It includes a variety of sensory systems Cutaneous sensations are those that are based on stimulationof receptors in your skin Neural system connected to the skin is called thesomatosensory system Sensations include: Touch: tactile sensation Temperature Nociception: Pain Proprioception: position of the limbs in space Kinesthesis: movement of the limbsPurdue UniversityMechanoreceptors Embedded in the skinare four types ofreceptors that respond todifferent types of touch Merkel receptor Meissner corpuscle Ruffini cylinder Pacinian corpuscleProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityMechanoreceptors Embedded in the skinare four types ofreceptors that respond todifferent types of touch Merkel receptor Meissner corpuscle Ruffini cylinder Pacinian corpusclePurdue UniversityPacinian corpuscle Large enough to see with the naked eye Layered like an onionPurdue UniversityPacinian corpuscle Pressure on the connective lamina opens pathwaysfor exchange of potassium and sodium This leads to a neural responsePurdue UniversityNeural response The frequency of action potentials codes theamount of pressure above a thresholdPurdue UniversityStimulus responses Different receptors respond preferentially to different kinds of stimuli Meissner’s are found very near the surface and respond best tochanging details, giving a perception of flutter Merkel’s are near the surface and respond best to unchangingdetail (pressure) Ruffini’s are found deep in the skin and respond best to unchanginggross movement (stretching) Pacinian are found deep in the skin and muscles and respond bestto changes in gross movement (vibration)Purdue UniversityTemporal responses Merkel receptor – slow (~1 Hz) pressure (top layers of skin) Meissner corpuscle – medium fast (~10 Hz) flutter Ruffini cylinder – fast (~ 100 Hz) stretching Pacinian corpuscle – very fast (~400 Hz) vibrationProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReceptor adaptation The fibers connected to the receptors adapt toconstant pressure. Substantial differences acrossreceptor types Slow adapting (SA): Merkel and Ruffini Continue to fire throughout a sustained stimulus Rapid adapting (RA): Meissner and Pacinian Fire only at the onset and offset of a sustained stimulus We are mostly interested in changes in mechanicalcontact with our bodies Change is important to identifying threats or opportunities inour environmentPurdue UniversityReceptive field size Receptors respond differently to the spatial positionsof stimuli Small receptive field: Meissner and Merkel Large receptive field: Ruffini and PacinianPurdue UniversityNeural connections Different receptors have nerves thatkeep the responses separated The brain can know which receptoris responding by which neuron isfiring Similar to place coding on the basilarmembrane Similar to retinotopic coding on theretinaPurdue UniversityNeural fibers Often combine the adapting properties and receptivefield sizes to label the neural fibers connected tothese receptors RA1: Meissner (flutter) Rapid adaptation, small receptive field RA2: Pacinian (vibration) Rapid adaptation, large receptive field SA1: Merkel (pressure) Slow adaptation, small receptive field SA2: Ruffini (stretching) Slow adaptation, large receptive fieldPurdue UniversityReceptive field size The size of the receptive field is related to spatial resolution anddiscriminationPurdue UniversitySurface detection The properties of the receptors explains why we moveour hands to feel the properties of a surface Place the index finger of your right hand on the desktop Place the index finger of your left hand on a piece ofpaper If you do not move them, it is difficult to judge which ismore smooth Now gently move your hands to lightly touch thesurface It is now easy to tell that the paper is rougher than thedesk topProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySurface detection Different surfaces havedifferent physical properties Bumps and crevices Discriminations of thesebumps requires fine spatialresolution Meissner or Merkel The Meissner is moresensitive than the Merkel, soit’s behavior is what limits yourabilityPurdue UniversitySurface detection But the Meissner connects to the RA1 fiber It responds only to onset and offset of a stimulus So, keeping the hand still will not generate anyresponse You have to move the hand around to get the RA1 fiberto respond to the changes in the bumps For other kinds of discriminations (e.g, rapidly vibratingstimuli), other fibers will determine your resolutionPurdue UniversityBraille letters Raised dots that allow blind people to read Why not just use the standard alphabet letters?Purdue UniversityBraille letters Compare the roman letters A and C and theirBraille counterpartsPurdue