Cutaneous Perception: Physiology

A major challenge in neurobiology is to understand how the brain constructs mental images of the world around us. The mental images that arise from the sense of touch are based on continuously changing patterns of electrical activity called action potentials that are evoked in the nerves that innervate the skin, muscles, and joints. The dynamic patterns of action potentials that come from the skin are the basis of cutaneous perception. These patterns are sent to the central nervous system via two main nuclei located in the brain stem and thalamus. Once the information reaches the cortex, it is systematically transformed through several processing stages into an alternate transformed pattern that is matched against previously stored patterns to evoke mental images of objects and surfaces in contact with the skin. The challenge facing neurobiologists is to understand the anatomical pathways and neural circuits that transform the patterns from the initial pattern into the representation that underlies memory, in other words, the challenge is to understand the neural code(s) that underlie behavior.

When exploring and manipulating an object with our hands, we readily appreciate many qualities or features of the object. These features include characteristics such as its size and shape, the texture of the surface, its weight, and dynamic properties, such as whether it is stationary or is moving in our hand. Many studies have shown that our ability to discriminate and identify objects is based on a rapid pattern recognition mechanism. For example, common everyday objects are recognized (typically in less than 3 seconds) without visual input at accuracy rates greater than 96%. In those experiments, subjects typically report that they identified the object using two to three features, such as its size and texture. In addition to be being highly accurate and rapid, the cutaneous system is also extremely sensitive with young adults being capable of detecting vibrations with amplitudes as low as 100 angstroms.

Discovering the neural code(s) that underlie cutaneous perception has been difficult for a number of reasons. First, the sense of touch is composed of multiple sub-modalities with individual [Page 349]features being coded by different afferent types. Somehow, these features are processed and integrated in the cortex to produce a single unified percept of objects. Second, in contrast to vision and audition, the cutaneous receptors are imbedded in a deformable sensory sheet (i.e., the hand) that dynamically changes as the hand moves around in space. Thus, the pattern matching is a dynamic process. Third, sensory inputs play a dual role and are important for both sensation and action. The sensory inputs provide information about the world, and the inputs related to action play important roles in motor control. Fourth, cutaneous perception is tightly linked to higher cognitive functions such as selective attention and drive what aspects of the inputs are given privileged access to perception.

To illustrate the role and complexity of the sensory processing from the hand, imagine what happens during the simple act of grasping and tossing a tennis ball. During the grasping phase, the fingers must spread apart to surround the ball and the hand moves to surround and enclose the ball. After contact, sensory afferents send their outputs to the cortex where they are transformed and matched against stored memories of tennis balls. The sensory inputs also play an important role during manipulation and ensure that the proper grip force is used to prevent the ball from being dropped. Finally, to toss the ball, the motor system must be activated in a way that is highly coordinated with the sensory inputs to allow the ball to be released at just the right time. How this simple task is performed in the nervous system is not known.

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