Introduction

If a hornet flies toward your face, you might duck, squint, and lift your hand to block it. If the insect touches your hand, you might withdraw your hand, even pulling it behind your back. These defensive movements have a reflexive quality. They are fast and can occur without conscious planning or thought. They are similar in all people (Fig. 27.1; Color Plate 6). Although they seem reflexive, however, defensive movements are also highly sophisticated. They can be elicited by touch, sight, or sound. They involve coordination between different body parts, such as the arm and head. They are spatially specific: the body parts that move and the direction of movement are appropriate for the location of the threat. The movements can be stronger or weaker, depending on external context or the internal state of the person. For example, someone whose “nerves are on edge” may give an exaggerated alerting response to an unexpected stimulus.

Figure 27.1.  

Detail from Michelangelo's Fall and Expulsion from Eden. Both Adam and Eve are in classic defensive poses, with the head turned and the hands raised to defend the face. Compare with Figure 27.5B. (See Color Plate 6).

What sensorimotor pathways in the brain coordinate this rich and complex behavior? We suggest that a special set of interconnected areas in the monkey brain monitors the location and movement of objects near the body and controls flinch and other defensive responses. This hypothesized “defensive” system, shown in Figure 27.2, includes the ventral intraparietal area (VIP), parietal area 7b, the polysensory zone (PZ) in the precentral gyrus, and the putamen. These brain areas are monosynaptically interconnected (Cavada & Goldman-Rakic, 1989a, 1989b, 1991; Kunzle, 1978; Luppino, Murata, Govoni, & Matelli, 1999; Matelli, Camarda, Glickstein, & Rizzolatti, 1986; Mesulam, Van Hoesen, Pandya, & Geschwind, 1977; Parthasarathy, Schall, & Graybiel, 1992; Weber & Yin, 1984). Of the four areas, PZ is closest to the motor output, sending direct projections to the spinal cord (Dum & Strick, 1991). Electrical stimulation of PZ evokes defensive movements, such as withdrawal of the hand, squinting, turning of the head, ducking, or lifting the hand as if to defend the side of the head (Graziano, Taylor, & Moore, 2002).

Figure 27.2.  

Side view of a macaque monkey brain showing the location of four interconnected multisensory areas.

In the following sections we review experimental results on this system of areas and discuss the evidence that they are involved in representing the space near the body and in controlling defensive movements. We concentrate mainly on areas VIP and PZ in the monkey brain, because they are the most thoroughly studied of these multisensory areas. We then discuss the general question of coordinate transformations from sensory input to motor output. Finally, we discuss the evidence that the human brain contains a similar set of multisensory areas processing the space near the body.

The brain contains many multisensory areas in addition to the set of areas described in this chapter. These other areas are thought to have a variety of specific functions. For example, the superior colliculus contains neurons that respond to tactile, visual, and auditory stimuli. This structure is thought to be involved in orienting of the eyes, ears, or body toward salient stimuli (see Stein, Jiang, & Stanford, Chap. 15, this volume; Meredith, Chap. 21, this volume; Van Opstal & Munoz, Chap. 23, this volume). Regions of the parietal and premotor cortex appear to be involved in the multisensory task of coordinating hand actions for grasping objects (see Fogassi & Gallese, Chap. 26, this volume; Ishibashi, Obayashi, & Iriki, Chap. 28, this volume). Work in human stroke patients and normal human subjects suggests that multisensory processing is crucial for directing spatial attention around the body (see Spence & McDonald, Chap. 1, this volume). A common view a century ago was that the brain contained association areas, regions that served the general purpose of combining the senses. These association areas did not have specific functions; they provided a general understanding of the environment and helped in choosing a path of action. Work over the past 20 years on multisensory integration paints a different picture, one in which the brain contains many distinct multisensory areas, each with its specific set of functions.