Multisensory neurons for the control of defensive movements
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 (see Figure 1). However, although they seem reflexive, 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. What sensory-motor 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 startle, flinch and defensive responses. This hypothesized “defensive” system, shown in Figure 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 and Goldman-Rakic 1989a,b; Cavada and Goldman-Rakic 1991; Kunzle 1978; Matelli et al. 1986; Mesulam et al. 1977; Parthasarathy et al. 1992; Weber and Yin 1984; Luppino et al., 1999). Of the four areas, PZ is closest to the motor output, sending direct projections to the spinal cord (Dum and Strick 1991). Electrical stimulation of PZ evokes defensive movements, such as withdrawal of the hand, squinting and turning of the head, ducking, or lifting the hand as if to defend the side of the head (Graziano, Taylor and Moore 2002).