- seCtIon tWo: tHe BRAIn
Binocular rivalry, by contrast, is found when different images are presented to the
two eyes. For example, a picture of the seaside might be shown to the right eye
and a face to the left, or a vertical grating to the left eye and a horizontal grating
to the right. In such cases the face and ocean are not combined into one picture,
nor do the gratings fuse into a plaid. Instead perception seems to flip between
the two.
What is going on? Early theories suggested that the flipping was due to eye
movements or other peripheral effects, but keeping the eyes still does not stop
the alternation, and peripheral theories have not generally fared well. It seems
more likely that the flipping occurs further up the visual system. But how does
this relate to the subjective experience? It feels as though the two views are
competing for consciousness. It seems as though first one, and then the other,
gains access to consciousness and thus you become aware of it. This simple
phenomenon provides an ideal situation for investigating the relationship
between the objective facts (input to the eye, events in the visual system, and
so on), and the subjective facts (being conscious of first one of the pictures and
then the other).
The first experiments to use binocular rivalry to look for the NCCs were done
with macaque monkeys (Logothetis and Schall, 1989; Sheinberg and Logothetis,
1997). Macaques can be trained to report which of two pictures they are seeing
by pressing a lever, and their responses are much like ours. For example, when
shown a vertical grating to one eye and a horizontal grating to the other, or
gratings moving in different directions, they can press a lever to indicate when
what they see flips from one to the other. Logothetis and his colleagues trained
monkeys in this way, recording from single cells in various brain regions. They
were looking for areas where the activity corresponded
not to the unchanging visual input, but to the changing
perceptions reported by the monkey’s behaviour.
Cells in early visual cortex, such as area V1, responded to
the unchanging input. For example, some cells responded
to vertical stripes, some to motion in different directions,
and some to particular stimuli, but their behaviour did not
change when the monkey’s perception changed. Further
along the visual pathway (for example in MT and V4), some
cells responded to what the monkey reported seeing.
Finally, in the inferior temporal cortex (IT), almost all the
cells changed their response according to what the mon-
key reported seeing. So, if the monkey pressed the lever
to indicate a flip, active cells stopped firing and a different
set started. It looked as though activity in this area corre-
sponded to what the monkey was consciously seeing.
Does this mean that the NCC lies in IT? One problem is
that the connection with consciousness depends on assuming that the monkeys
are consciously perceiving. This seems reasonable given the way they respond,
but of course we cannot know for sure, and those who believe that language
is necessary for consciousness might argue that the monkey’s responses tell
us nothing about human consciousness (Chapter 10). Since these early experi-
ments, technology has made similar experiments on humans possible, and weFIGURE 4.3 • The Necker cube: a simple
example of rivalry. Keep your gaze
on the central spot while looking
at the cube. There are two equally
likely interpretations that tend
to alternate: one with the front
face up and to the left, the other
with the front face down and to
the right. You may be able to flip
views deliberately and vary the
speed of alternation.
FIGURE 4.4 • The principle behind Logothetis’s
experiments. When monkeys are
shown a different display to each
eye they report binocular rivalry
just as humans do. They cannot
speak, but they can indicate which
display they are currently seeing
by pressing a lever.