Who Needs Emotions? The Brain Meets the Robot

152 brains
Movements performed by living organisms owe their specificity to the
fact that they usually have a goal. As a consequence, they display a number
of kinematic properties that reveal their “intentional” origin. One of these
properties is that goal-directed movements have an asymmetrical kinematic
profile—a fast acceleration followed by a much longer deceleration—as opposed
to the symmetrical profile of the ballistic motion of a projectile, for
example. Another property is that the tangential velocity of the moving limb
varies with the radius of curvature of the movement (Lacquaniti, Terzuolo, &
Viviani, 1983). A further characteristic of biological movements is that they
follow biomechanically compatible trajectories. Consider the perceptual effect
produced by fast sequential presentation of pictures of an actor with an
arm at two different postures. This alternated presentation is perceived as a
continuous apparent movement between the two arm postures. If, however,
the presentation of the two postures is such that the arm should go across
an obstacle (e.g., another body part), then the apparent movement is perceived
as going around and not across the obstacle. This striking effect
(Shiffrar & Freyd, 1990) reflects the implicit representation built from visual
perception of motion when it refers to a biological (or intentional) origin.
Obviously, this is not to say that a robot could not be programmed for
accurately reaching a goal with a different strategy (e.g., using movements
with a symmetrical velocity profile or violating biomechanical constraints):
these movements would simply look “unnatural” and would not match the
internal representation that a human subject has of an intentional movement.
As a matter of fact, a normal subject cannot depart from the relation
between geometry and kinematics which characterizes biological action: he
or she cannot track a target moving with a spatiotemporal pattern different
from the biological one (e.g., accelerating rather than decelerating in the
curves). According to Viviani (1990), the subject’s movements during the
attempts to track the target “continue to bear the imprint of the general
principle of organization for spontaneous movements, even though this is
in contrast with the specifications of the target.” Interestingly, the same
relation between velocity and curvature is also present in a subject’s perceptual
estimation of the shape of the trajectory of a luminous target. A target
moving at a uniform velocity is paradoxically seen as moving in a nonuniform
way and, conversely, a kinematic structure which respects the above
velocity–curvature relation is the condition for perceiving a movement at a
uniform velocity. According to Viviani and Stucchi (1992), perception is
constrained by the implicit knowledge that the central nervous system has
concerning the movements that it is capable of producing. In other words,
there is a central representation of what a uniform movement should be,
and this representation influences visual perception. Whether this implicit
knowledge is a result of learning (e.g., by imitation) or an effect of some innate