Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 43

Fig. 3.31 Optic flow: the
changing pattern moving
across the retina caused by
the images of objects as a
result of the environment
moving towards an observer
with speed V. The speed of
change across the retina is v 1 .
Drawing not to scale

V

v 1

line of movement and view

v 1

V

perception of movement. This changing pattern of images across the retina has since
been called “optic flow”. Figure 3.31 illustrates the changing pattern on the retina
for two visible objects moving towards a static observer. This situation is equivalent
to the situation where a dynamic observer moves towards stationary objects, but the
first situation is easier to illustrate. The objects can, for example, be obstacles on a
road or small parts of the white lines bordering a carriageway or traffic lane. The
speed, V, of the moving environment moving towards the observer (or, identically,
the speed of the moving observer through the environment) determines the speed,
v 1 , of the radial outward movement of the images on the retina.
The optic flow therefore contains the information about the changing distance of
objects in the environment for a constant speed of movement. At non-constant speeds
of the observer, the rate of change of the optic flow is the perception tool that enables
advance determination of time-to-contact with the object (Lee 1980 ). It seems that
a specialized region in the brain analyses the optic flow (Wurtz 1998 ). The brain
performs the required computations so rapidly that indeed the optic flow enables the
control of steering and braking at high speeds. In fact the perception tool's mechanism
is even more complicated. Firstly, motorists, cyclists and pedestrian “meet” not only
with stationary objects but with moving objects as well (such as other vehicles,
cyclists and pedestrians). The information from all these movements is contained in
the optic flow. Secondly, the eye and the head are always slightly in motion. The
retinal flow caused by this motion is superimposed over the optic flow caused by the
movement in space.
Since the early investigations into the mechanism of perception of motion and
action, many other researchers have confirmed and extended our knowledge on the
subject (Blake and Sekuler 2006 ; Vaina et al. 2004 ). In the previous sections of this
chapter some visual-performance tests have been discussed where the observer or
observers were actually moving, as in the tests concerning visibility in the road's
surrounds (Sect. 3.4) and on traffic flow (Sect. 3.5). Fundamental research on motion
processing as a function of light level is only rarely carried out (Billino et al. 2008a ).
Various studies have shown age-related deficits in motion perception (Billino et al.
2008b ). The author of this topic is not acquainted with research specifically devoted