Touch PSY 310 Greg Francis Lecture 33 Why is the Braille system better? Vision and audition involve perception of objects from a distance Safe and dependent on the transfer of energy (light, air pressure) Touch inherently involves contact of your body with objects in the world More dangerous and dependent on the transfer of mechanical energy You can recognize objects in the dark You can explore properties of an object that cannot be identified by vision (strength, stretch, ) People are very sensitive about touch Higher status people are allowed to touch lower status people, not the other way around 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 In 2000 the Queen of England visited Australia again Something similar happened when the prime minister, John Howard, put his arm around the queen while introducing her to some other people A spokeswoman for the prime minister said: "We firmly deny that there was any contact whatsoever." Cutaneous 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 stimulation of receptors in your skin Neural system connected to the skin is called the somatosensory system Sensations include: Touch: tactile sensation Temperature Nociception: Pain Proprioception: position of the limbs in space Kinesthesis: movement of the limbs Embedded in the skin are four types of receptors that respond to different types of touch Merkel receptor Meissner corpuscle Ruffini cylinder Mechanoreceptors 1
Embedded in the skin are four types of receptors that respond to different types of touch Merkel receptor Meissner corpuscle Ruffini cylinder Mechanoreceptors Large enough to see with the naked eye Layered like an onion Pressure on the connective lamina opens pathways for exchange of potassium and sodium This leads to a neural response Neural response The frequency of action potentials codes the amount of pressure above a threshold Stimulus responses Different receptors respond preferentially to different kinds of stimuli Meissner s are found very near the surface and respond best to changing details, giving a perception of flutter Merkel s are near the surface and respond best to unchanging detail (pressure) Ruffini s are found deep in the skin and respond best to unchanging gross movement (stretching) Pacinian are found deep in the skin and muscles and respond best to changes in gross movement (vibration) Temporal responses Merkel receptor slow (~1 Hz) pressure (top layers of skin) Meissner corpuscle medium fast (~10 Hz) flutter Ruffini cylinder fast (~ 100 Hz) stretching very fast (~400 Hz) vibration 2
Receptor adaptation The fibers connected to the receptors adapt to constant pressure. Substantial differences across receptor 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 mechanical contact with our bodies Change is important to identifying threats or opportunities in our environment Receptive field size Receptors respond differently to the spatial positions of stimuli Small receptive field: Meissner and Merkel Large receptive field: Ruffini and Pacinian Neural connections Different receptors have nerves that keep the responses separated The brain can know which receptor is responding by which neuron is firing Similar to place coding on the basilar membrane Similar to retinotopic coding on the retina Neural fibers Often combine the adapting properties and receptive field sizes to label the neural fibers connected to these 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 field Receptive field size The size of the receptive field is related to spatial resolution and discrimination The properties of the receptors explains why we move our hands to feel the properties of a surface Place the index finger of your right hand on the desk top Place the index finger of your left hand on a piece of paper If you do not move them, it is difficult to judge which is more smooth Now gently move your hands to lightly touch the surface It is now easy to tell that the paper is rougher than the desk top 3
Different surfaces have different physical properties Bumps and crevices Discriminations of these bumps requires fine spatial resolution Meissner or Merkel The Meissner is more sensitive than the Merkel, so it s behavior is what limits your ability 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 any response You have to move the hand around to get the RA1 fiber to respond to the changes in the bumps For other kinds of discriminations (e.g, rapidly vibrating stimuli), other fibers will determine your resolution Raised dots that allow blind people to read Why not just use the standard alphabet letters? Compare the roman letters A and C and their Braille counterparts When blurred, similar to the spatial resolution of the fingertips The letters A and C are indistinguishable Their Braille counterparts can be discriminated There are also receptors in the skin that respond to hot and cold Neural fibers respond to changes in temperature Thermoreceptors 4
Why so many receptors? Many perceptual experiences are combinations of responses from several different receptors For example, there is no receptor for wet We experience the sensation of wetness when we get a response from Meissner: touch Merkel: pressure Cold fibers: temperature It s similar to how three cone receptors allow for millions of perceived colors There may also be millions of different touch-related perceptual experiences Conclusions Touch Receptors Fibers Adaptation Receptive fields resolution Braille system Temperature Next time Cortical representation of touch Two-point thresholds Sensory homunculus Pain perception 5