Page 104 - Introduction to AI Robotics

87
3.4 Perception in Behaviors

Figure 3.6 A collection of artificial bait, possibly the first example of humans ex-
ploiting affordances. Notice that the lures exaggerate one or more attributes of what
afish mighteat.

OPTIC FLOW you haven’t noticed it, called optic flow. Optic flow is a neural mechanism
for determining motion. Animals can determine time to contact quite easily
with it. You probably are somewhat familiar with optic flow from driving in
a car. When driving or riding in a car, objects in front seem to be in clear focus
but the side of the road is a little blurry from the speed. The point in space
that the car is moving to is the focus of expansion. From that point outward,
there is a blurring effect. The more blurring on the sides, the faster the car is
going. (They use this all the time in science fiction movies to simulate faster-
than-light travel.) That pattern of blurring is known as a flow field (because
it can be represented by vectors, like a gravitational or magnetic field). It is
TIME TO CONTACT straightforward, neurally, to extract the time to contact, represented in the
cognitive literature by .
Gannets and pole vaulters both use optic flow to make last-minute, pre-
cise movements as reflexes. Gannets are large birds which dive from high
altitudes after fish. Because the birds dive from hundreds of feet up in the
air, they have to use their wings as control surfaces to direct their dive at the
targeted fish. But they are plummeting so fast that if they hit the water with
their wings open, the hollow bones will shatter. Gannets fold their wings just
before hitting the water. Optic flow allows the time to contact, ,to be a stim-
ulus: when the time to contact dwindles below a threshold, fold those wings!