- seCtIon one: tHe PRoBLem
Why don’t we notice the gaps? One answer might be that, in some sense, the
brain fills in the missing bits: it pastes in more stripy wallpaper behind the vase,
say. But if the brain already knows what needs to be filled in, who does it do the
filling-in for, and why? Another answer is that there is no need to fill anything in
because the gaps are just a lack of information. And an absence of information is
not the same as information about an absence.
Consider something that happens in vision all the time: we infer the presence
of whole objects from their visible parts. A car parked behind a tree looks like
a whole car, not two halves separated by a tree trunk; a cat sleeping behind a
chair leg looks like a whole cat, not two odd-shaped furry lumps. This ability to
see objects as whole is obviously adaptive, but what is going on? We don’t liter-
ally ‘see’ the hidden parts of the car, yet the car seems whole. This is sometimes
referred to as amodal perception or conceptual filling-in. The car is conceptually
completed but not visually filled in.
A more controversial kind of filling-in arguably happens in the blind spot. Where
the optic nerve leaves the back of the eye there are no photoreceptors, creating a
blind spot on the retina that subtends about 6 degrees of visual angle, roughly 15
degrees away from the fovea. Most people are unaware of their own blind spots
until shown a demonstration such as that in Figure 3.4. Partly this is because we
have two eyes, and the two blind spots cover different parts of the visual world,
though even with one eye the blind spot is normally undetectable. But experi-
ments can easily reveal it.
A small object can be made to disappear from sight by lining it
up precisely on the blind spot. What is seen where the object
should have been? Not a blank space or a gaping black hole, but
a continuation of the background. If the background is boring
grey, then boring grey fills the space where the object should
have been. If the background is black and blue stripes, the stripes
seem to cover the whole area. The obvious conclusion is that the
brain has somehow filled in the gap with grey or pink, or stripes
or checks (or more people in the crowd, or more pebbles on the
beach). But is this the right conclusion?
The American neuroscientist Christof Koch thinks so. ‘Unlike
electronic imaging systems, the brain does not simply neglect
the blind spot; it paints in properties at this location using one
or more active processes such as completion, interpolation, and
filling-in’ (2004, p. 54).
Dennett thinks not. This kind of thinking ‘is a dead giveaway of
vestigial Cartesian materialism’ (1991, p. 344) – i.e. pretending to be a materialist
but falling back into dualism. He challenges it with a thought experiment: imag-
ine walking into a room papered all over with identical portraits of Marilyn Mon-
roe. You can see at a glance (or a few glances) that the portraits are all the same.
If one had a moustache, or a silly hat, you would notice straight away. It seems
natural to conclude that you have seen the room in all its detail and now have a
rich representation of it in your head.
This cannot be right. As Dennett points out, in order to identify one of the por-
traits you would have to look straight at it so that the image fell on the fovea.
‘There would have been
gaps – but we felt no
gaps’
(James, 1890, i, p. 488)
FIGURE 3.4 • Shut your right eye and look steadily
at the small black spot with your left
eye. Starting at about half a metre
away, slowly bring the book towards
you until the striped area disappears.
It is then in your blind spot. You
may need to try a few times to find
it (remember to keep your eye on
the black spot). What can you see
there? Is the space filled in? Do
the black lines join up? (Based on
Ramachandran and Blakeslee,
1998, p. 95.)