- seCtIon one: tHe PRoBLem
just what is on the retina, and this distinction is present early
in the visual system. In humans, fMRI has been used to inves-
tigate the filling-in of contours, and several studies suggest
that activity at the blind-spot region in V1 is closely linked to
changes in perception, for example when what is seen alter-
nates between two options in binocular rivalry (Meng and
Tong, 2004). However, colour and brightness filling-in seem to
be rather different from other kinds, with at least two studies
finding no evidence that early visual areas (V1 and V2) contain
map-like representations of brightness and colour that could
be filled in (Perna et al., 2005; Cornelissen et al., 2006).
Filling-in also happens with afterimages, which are unique in
retaining the same position on the retina when the eye moves.
It seems that filling-in works differently for the original image
and the afterimage, in the way the colours spread into each
other (Hamburger, Geremek and Spillmann, 2012). This adds
another dimension to evidence about whether the retina or the
cortex is responsible for filling-in, suggesting that afterimages
are treated by the brain as genuine stimuli that can create their
own perceptual effects, rather than just obeying the rules of the
retinal stimulation that first elicited them.
In other experiments, a background of English, Latin, or non-
sense text was used. The square was filled in all right but, like
Josh with his numbers, the participants could not read the text
produced. This also throws doubt on the idea that filling-in is
literally a process of completing a picture dot by dot. For how
would one create visible letters and numbers that could not be
read? But what then is it?
Contrary to the extreme sceptical view, these results clearly
indicate that there is a real effect to be explained. The brain
does not just ignore a lack of information, but responds in var-
ious ways at varying speeds. However, we cannot make sense
of the findings by assuming that somewhere inside the brain
there is a picture-like representation of the current object of
perception, which must be filled in all over or gaps will be
noticed.
Some kind of dynamic spreading of activation clearly does create
illusory contours and the like. But this does not mean that this
process is used to fill in an internal metric picture of the world. We
might instead think of the blind spot, like other constants of our
perceptual apparatus (such as how retinal resolution and colour
sensitivity drop off drastically towards the periphery) as being used in order to see.
For example, ‘if retinal sensation were not to change dramatically when an object
falls into the blind spot, then the brain would have to conclude that the object was
not being seen, but was being hallucinated’ (O’Regan and Noë, 2001, p. 951). Like
the curvature of the lens and the different functions of the rods and the cones, the
blind spot is just one of the sensorimotor contingencies that shape our perceptualACtIVItY 3.1
Filling-inWith some simple experiments you can experience
filling-in and explore its limits. In Figures 3.4 and 3.6,
shut or cover your right eye and fixate the small black
dot with your left eye. Hold the picture at arm’s length
and then move it gradually towards you until the larger
circle disappears. Do you see a gap or a continuation of
the background? Is the black line completed across the
gap? What happens to the pebbles?
You can also try the effect with real people. It is said
that King Charles II, who was a great promoter of
science, used to ‘decapitate’ his courtiers this way. To
do this in class, ask someone to stand in front while
everyone else aims their blind spot at the victim’s
head. If you have trouble doing this, try the following.
Hold up Figure 3.4 so that the circle disappears. Now,
keeping the book at the same distance away from
you, line up its top edge below the person’s chin with
the circle directly below. Now fixate whatever you can
see above the black dot and remove the book. Your
blind spot should now be on the person’s head. Does
the whole head get filled in? If not, why not? Does it
matter how well you know the person?
You can explore what can and cannot be filled in by
using your own pictures. Cut out a small fixation spot
and a larger circle, or find suitably sized stickers, and
stick them on. Alternatively fix them to a computer
screen and experiment with moving displays. If you are
doing several experiments it is worth putting a patch
over your eye. With a stop-watch, you can time how
long filling-in takes for different displays.
Can you deliberately prevent filling-in? Can you speed
it up by making an effort? Does what you see in the
gap ever surprise you? Can you explain the difference
between those things that do and do not get filled in?