Touch

Excerpt

As discussed later in this chapter, the initial peripheral representation of spatial form in touch is isomorphic, as individual receptors on the skin have small localized receptive fields. While these point sensors indicate the strength of a stimulus at a particular location on the skin, they do not individually convey salient information about features of the stimuli (e.g., whether the point is part of a spider creeping up your arm). It is possible that the central representation is also isomorphic. However, if this were the case, then even minor changes in the position, orientation, or size of the stimulus would require the activation of a different set of central neurons for perception of the object to occur—essentially requiring a unique complement of cells to represent each and every possible version of every tactile stimulus. This is clearly not reasonable since it would require an infinite set of stored representations of every object for perception to occur. A more realistic alternative is to transform the sensory input into a central representation that maintains perceptual constancy of object size, orientation, and position. Understanding how the brain transforms the sensory inputs into an invariant representation is a major thrust of contemporary neurophysiological research and in our opinion is key to deciphering the neural bases of sensation, reward, and behavior (Phillips et al. 1988).