- seCtIon tWo: tHe BRAIn
multiple parallel streams of processing, and no magic moment at which input
turns into output or consciousness happens, nor any central timing mechanism
or ‘clock’ (Zeki, 2015). In Chapters 6 and 9 we will return in more detail to the tim-
ing of consciousness, and the idea that it takes time for consciousness to ‘build
up’. For now, here is the critical point: we naturally want to ask: ‘Which bits of
neural processing were the conscious ones and which the unconscious ones? Did
I become conscious of my thumb on my nose as soon as it got there?’ Dennett
argues that even asking such questions betrays a commitment to the CT. They
set us off looking for the special time or place where consciousness comes into
existence, and that time and place cannot be found.
This argument leads straight back to the hard problem. We assume that in some
way all this brain activity is responsible for the powerful feeling I just had that
I decided to move my thumb, the thumb did what I told it to, and then I con-
sciously felt the sensation on my nose without being aware of what all those
neurons were doing. So, either we have to find an answer to the question ‘how
does subjective awareness arise from the objective actions of all these neurons
and muscle cells?’, or we have to work out what mistake has led us into posing
such an impossible question in the first place. Inventing a place and time at which
subjectivity happens is not a viable answer.
THE MENTAL SCREEN
In 1971 the American psychologist Roger Shepard published a classic experiment
which changed forever how psychologists thought about mental imagery (Shep-
ard and Metzler, 1971). Participants were presented with pairs of diagrams like
those shown in Figure 5.3, and were asked to press a button to indicate whether
the two were different shapes or different views of the same shape. If you try this,
you will probably find that you seem to mentally rotate the objects in your mind’s
eye. Ask yourself where this mental rotation seems to be taking place.
Discussion of such private and unobservable experiences had been banished
from psychology by behaviourism, but the importance of this experiment was
that Shepard and Metzler made objective measurements. They found that the
time taken to reach a decision correlated closely with the time it would actually
take to rotate the objects in space. In other words, participants responded more
quickly if the object had been rotated only a few degrees, compared with a 180°
rotation. Later experiments on imagery showed similar effects. For example, when
people are asked to remember a map or drawing and then answer questions
such as ‘How do you get from the beach to the look-out tower?’, the time taken to
answer is related to the distance between the starting and finishing points on the
map (Kosslyn, Ball, and Reiser, 1978). In other words, it appears that something is
happening in the brain that takes time to traverse imagined distance.
The most obvious conclusion is that mental images are like pictures inspected by
some kind of mind’s-eye function (Kosslyn, 1980), but this was immediately chal-
lenged (Pylyshyn, 1973), leading to a long debate between pictorialist and prop-
ositionalist (language-like) theories. In essence, the challenge to pictorialism was
this: pictorialists correctly observe the similarities between imagery and vision,
but incorrectly take this to mean that there are pictures in the brain, painted on a
mental canvas.‘The visual buffer [. . .] is
the canvas upon which
images are painted;
it is the medium that
supports depictive
representations’
(Kosslyn, Thompson, and Ganis,
2006, p. 18)