Haptic Perception & Human Response to Vibrations

Transcription

1 Sensing HAPTICS Manipulation Haptic Perception & Human Response to Vibrations Tactile Kinesthetic (position / force) Outline: 1. Neural Coding of Touch Primitives 2. Functions of Peripheral Receptors B R A I L L E 1. Neural Coding of Touch Primitives Touch Receptors Mechanoreceptors and their Function Other skin receptors: thermal, pain Kinesthetic receptors Pathways from receptors to brain Touch Receptors Touch sensations are mediated by receptors that respond to pressure, vibration, and heat flow. The receptors are found in two regions: -- Within skin: cutaneous sensing pressure, temperature, pain -- Beneath skin in muscles, tendons, joints: kinesthetic sensing limb position and movement 1

2 Classes of Receptors Skin Mechanoreceptors have specialized endings Receptors that respond to pressure and vibration are called mechanoreceptors Mechanoreceptors are found in skin (cutaneous) and muscles, tendons, and joints (kinesthesis) The skin also includes other receptors that signal skin warming and cooling (thermo-receptors) that signal pain (nociceptors) Epidermis Dermis Meissner Merkel Ruffini Subcutis Pacinian Johannson & Valbo, 1983 Functional characteristics of Skin Mechanoreceptors: Receptive field size (I = small, II = large) and adaptation rate (FA = fast adapting, SA = slow adapting) I I Distribution of Mechanoreceptors on the Palm: Receptors with small receptive fields are densely packed on fingertips. Receptors Meissner s Merkel Pacinian Ruffini Corpuscle Cell Complex Corpuscle Ending Result: ~.5 mm 2-point discrimination on fingertip Receptive Field Kandel et. al., 2000 Intensity and Time Course of Neural Signal (adaptation) Neural Spike train Stimulus 2

3 Frequency Sensitivity: One-channelper-mechanoreceptor model Thermo-Receptors Amplitude required for threshold response at given frequency (db relative to 1 m) Average data I I (PC) Thermo-receptors lack specialized endings; respond to temperature change. Two populations of nerve fibers: warm and cold Cold fibers Normal Body temp Warm fibers Bolanowski et al., 1988 Kenshalo, 1976 The brain: Primary and Secondary Somatosensory Cortex Somatosensory Homunculus in SI results from somatotopic mapping* Central Sulcus SI Areas 5 & 7 SI SI includes Brodmann areas: 3a: Muscle 3b: Skin (SA and FA) 1: Skin (FA) 2: Pressure, joints There are several such maps in SI and SII. SI projects to SII, 5, 7 SII Lateral Sulcus SII Penfield & Rasmussan, 1950 *Adjacent on skin Adjacent in SI 3

4 Somatosensory areas in brain are plastic: Re-assignment of receptive fields after amputation of a digit Cortical plasticity causes phantom limb pain Areas in SI that once responded to 3rd fingertip are now activated by finger 2 and 4, plus base of 3 Amputation point Thumb Index finger 5th digit face arm Merzenich et al., 1984 Stimulating face and arm activates cortical areas previously responsive to fingers, now taken over by face and arm. 2. Functions of peripheral receptors Grasping is supported by FAI mechanoreceptors that detect incipient slip Slowly adapting cutaneous mechanoreceptors provide array (tactile) sensing. Fast adapting cutaneous receptors signal pressure changes on the skin. Load Force N Grip Force N Position mm Grasp, lift Return Load Force N Grip Force N Position mm Force Ratio FAIs fire under near-slip Kinesthetic mechanoreceptors provide a sense of limb position. Grip: Load ratio Time Slip threshold I I Therefore, the mechanoreceptor populations support different human abilities. Johansson & Westling, 1984; Westling,

5 Tactile Pattern Perception is based on SAI mechanoreceptors Spatial plot of the response of a skin mechanoreceptor to a Braille pattern swept through its receptive field: Each tick mark is a neural impulse given contact from that stimulus location. Curvature perception also reflects responses Data are shown for 7 curves, ranging from radius zero to radius 1.44 mm Input pattern R e s p o n s e I I Pattern preservation Mean Response Distance of receptive field from center of indentation (mm) Phillips, Johansson & Johnson, 1990 Goodwin et al., 1995 PC receptors (I) signal causes of deep vibrations Transient vibrations from contact Micron-element textural variations, as in this stimulus Stimulus plate Receptor Responses Therefore, PCs underlie roughness percept at micro-scale. Impact on functions of cutaneous receptors when skin is covered with a rigid sheath This simulates some force-feedback environments. Only Pacinian Corpuscles respond continuously to the textured portion of a half-textured plate with ht. 1 m Decrements in spatial resolution, differential force sensing, pattern perception -- but vibration (sensed by deeper PCs) remains essentially intact Srinivasan, Whitehouse, & LaMotte, 1990 Lederman & Klatzky,