Chapter
Three
The grand illusion
us in difficulty once we start trying to apply them to understanding how the brain
contributes to visual, or any other, experience. If we assume all three, we end up
having to explain how all the neural processing in all the parallel pathways in the
human visual system results in that rich, definite, representation-based conscious
experience. We also have to work out what it is that distinguishes all that mass of
‘unconscious’ processing from the final ‘conscious’ representation. What creates the
‘magic difference’ between some representations being conscious and others not?
One way to approach this question – which amounts to a version of the hard
problem – is to stick with the idea of a stream of conscious visual representation
and look for its neural correlates (Chapter 4). The basic principle is simple. If you
believe that some visual representations in the brain are conscious and others
are not, then you should be able to take examples of each and study them in
detail until you discover the difference. In this light Francis Crick asks, ‘What is the
“neural correlate” of visual awareness? Where are these “awareness neurons” – are
they in a few places or all over the brain – and do they behave in any special
way?’ (1994, p. 204). He goes on to consider synchronised behaviour in widely
separated neurons (Chapter 6), but adds that ‘so far we can locate no single region
in which the neural activity corresponds exactly to the vivid picture of the world
we see in front of our eyes’ (p. 159).
Research on the neural correlates of vision has since progressed far enough that,
using fMRI to build up a library of correspondences between what someone is
viewing and their brain activity, scientists can infer backwards from new patterns
of activity to the stimuli being perceived (Nishimoto et al., 2011; Poldrack, 2011).
A similar technique has been used to match up brain activation with people’s
reports of what they were dreaming about (Horikawa et al., 2013). But however
good we get at finding these correspondences, remember they are finding the
NCs of particular visual experiences, not the NCs of consciousness itself.
It is easy to imagine that this clever method is reading off the brain’s own internal
pictures, but in fact it relies on complex patterns widely distributed across the
cortex. Perhaps we should challenge the natural trio of ideas that conscious vision
is as detailed as a hyper-realist painting or an HD 3D film, with things categorically
in or out of the frame, and all dependent on picture-like representations.
FILLING IN THE GAPS
The perceptive William James noticed something very odd, although it is obvious
once someone points it out: when we look around we do not, and cannot, take in
everything at once, and yet we are unaware of any gaps. Imagine you have been
sitting in your friend’s living room for an hour and suddenly notice that there is a
vase of flowers on the table. What was there before? More wallpaper? A flower-
shaped gap?
It is true that we may sometimes be tempted to exclaim, when once a lot
of hitherto unnoticed details of the object lie before us, ‘How could we
ever have been ignorant of these things and yet have felt the object, or
drawn the conclusion, as if it were a continuum, a plenum? There would
have been gaps – but we felt no gaps. . . .’
(1890, i, p. 488)