A major challenge in neuroscience is to understand the neural basis of behavior. The problem is multifaceted. First one must understand which afferent type(s) and cortical pathways are involved in the aspect of perception that you want to understand. Then one must understand how information is represented and coded in the neural responses. The earliest attempts at addressing the neural pathways underlying perception relied on lesion studies in which animals were trained to perform specific behavioral tasks and then were retested following the ablations. If the animal could not perform the task then the area that was ablated was deemed essential for the behavior. More recently researchers use functional imaging techniques to address these questions. Understanding the neural codes underlying behavior has been elusive. The intellectual breakthrough came from studies that combined psychophysical studies in humans with neurophysiological studies in monkeys [1, 2]. In these pioneering studies, Mountcastle and his colleagues showed that there is a tight correspondence between human vibratory detection and the neural activity recorded in the peripheral afferents of non-human primates. These findings not only demonstrated that the neural mechanisms used by the two species are similar but more importantly provided a scientific approach for studying the neural mechanisms of behavior.
In Goodwin and Wheat’s chapter they describe the physiological basis of the peripheral receptor systems that underlie cutaneous perception. However, that is only part of the story. In addition to the four kinds of mechanoreceptors there are also other receptors that provide information about sensory inputs from the hand. These include receptors specialized for pain (two kinds), temperature (two kinds), itch (one kind) and, as noted by Goodwin, four kinds of afferents that are located in the muscles, tendons and joints that provide information about body position, movement and force. Together these afferents provide a rich multidimensional percept of the size, shape, texture, and temperature of objects that we hold and manipulate with our hands. In this chapter we describe the neural basis of size, shape, and texture perception. We first layout the basic architecture of the somatosensory system and briefly describe the regions of the brain that are involved in haptic perception. Next, we describe neural coding studies of texture and two- and three dimensional form and discuss how these aspects of haptic perception are represented in the somatosensory system. Finally we propose a working hypothesis of how we believe mechanoreceptive and proprioceptive information is integrated to form central representations of three-dimensional shape.