23: Touch

Introduction

• Active and Passive Touch Evoke Similar Responses in Mechanoreceptors

• The Hand Has Four Types of Mechanoreceptors

  • Receptive Fields Define the Zone of Tactile Sensitivity

  • Two-Point Discrimination Tests Measure Texture Perception

  • Slowly Adapting Fibers Detect Object Pressure and Form

  • Rapidly Adapting Fibers Detect Motion and Vibration

  • Both Slowly and Rapidly Adapting Fibers Are Important for Grip Control

• Tactile Information Is Processed in the Central Touch System

  • Cortical Receptive Fields Integrate Information from Neighboring Receptors

  • Neurons in the Somatosensory Cortex Are Organized into Functionally Specialized Columns

  • Cortical Columns Are Organized Somatotopically

• Touch Information Becomes Increasingly Abstract in Successive Central Synapses

  • Cognitive Touch Is Mediated by Neurons in the Secondary Somatosensory Cortex

  • Active Touch Engages Sensorimotor Circuits in the Posterior Parietal Cortex

• Lesions in Somatosensory Areas of the Brain Produce Specific Tactile Deficits

• An Overall View

In this chapter on the sense of touch, we focus on the hand because of its importance in the sensory appreciation of object properties and its role in skilled motor tasks. The hand is one of evolution's great creations. The fine manipulative capacity provided by our fingers is possible because of their fine sensory capacity; if we lose tactile sensation in our fingers we lose manual dexterity.

When we become skilled in the use of a tool, such as a scalpel or a pair of scissors, we feel conditions at the working surface of the tool as though our fingers were there because two groups of mechanoreceptors monitor the vibrations and forces produced by those distant conditions. When we scan our fingers across a surface we feel its form and texture because another group of mechanoreceptors has high spatial and temporal acuity. A blind person uses this capacity to read Braille at a hundred words per minute. When we grip and manipulate an object we do so delicately, with only as much force as is required, because yet another group of mechanoreceptors continually monitors slip and adjusts our grip appropriately.

We are also able to recognize objects placed in the hand from touch alone. When we are handed a baseball we recognize it instantly without having to look at it because of its shape, size, weight, density, and texture. We do not have to think about the information provided by each finger to deduce that the object must be a baseball; the information flows to memory and instantly matches previously stored representations of baseballs. Even if we have never previously handled a baseball, we perceive it as a single object, not as a collection of discrete features. The somatosensory pathways of the brain have the daunting task of integrating information from thousands of sensors in each hand and transforming it to a form suitable for cognition.

Sensory information is extracted for the purpose of motor control as well as cognition, and different kinds of information are extracted for those purposes. We can, for example, shift our ...