Neurons in the lateral prefrontal cortex represent the content of consciousness. The red trace depicts neural activity (neuronal discharges) in the lateral prefrontal cortex when a stimulus is consciously perceived for 1 second while the green trace depicts neural activity when the same stimulus is suppressed from awareness. Image: MPI for Biological Cybernetics |
Consciousness
is a selective process that allows only a part of the sensory input to
reach awareness. But up to today it has yet to be clarified which areas
of the brain are responsible for the content of conscious perception.
Theofanis Panagiotaropoulos and his colleagues—researchers at the Max
Planck Institute for Biological Cybernetics in Tübingen and University
Pompeu Fabra in Barcelona—have now discovered that the content of
consciousness is not localized in a unique cortical area, but is most
likely an emergent property of global networks of neuronal populations.
The
question of which parts of the brain are responsible for the things
that reach our awareness is one of the main puzzles in neurobiology
today. Previous research on the brains of primates has shown that
neurons in primary and secondary cortices provide poor representation of
visual consciousness. In contrast, the neurons in the temporal lobe
seem to reliably reflect the actual conscious perception of a visual
stimulus. These findings indicated that not all parts of the brain are
responsible for the content of conscious awareness. Nevertheless, the
question whether only one of the brain’s areas is responsible for the
content of perception or whether more regions are involved in the
process has so far remained unanswered.
The
Max Planck scientists in Tübingen led by Nikos Logothetis have now
addressed this issue using electrophysiological methods to monitor the
neural activity in the lateral prefrontal cortex of macaque monkeys
during ambiguous visual stimulation. The visual stimuli used allow for
multiple perceptual interpretations, even though the actual input
remained the same. In doing so, Panagiotaropoulos and his team were able
to show that the electrical activity monitored in the lateral
prefrontal cortex correlates with what the macaque monkeys actually
perceive.
They
thus concluded that visual awareness is not only reliably reflected in
the temporal lobe, but also in the lateral prefrontal cortex of
primates. The results depict that the neuronal correlates of
consciousness are embedded in this area, which has a direct connection
to premotor and motor areas of the brain, and is therefore able to
directly affect motor output. These findings support the “frontal lobe
hypothesis” of conscious visual perception established in 1995 by the
researchers Crick (the co-discoverer of the structure of the DNA
molecule) and Koch that awareness is related to neural activity with
direct access to the planning stages of the brain.
The
results support this theory insofar as they show that the lateral
prefrontal cortex is involved in the process of visual awareness.
However, the fact that neural activity in two different cortical areas
reflects conscious perception shows that the decision which sensory
input reaches our awareness is most likely not made in a unique cortical
area but, rather, that a global network of neurons from different areas
of the brain is responsible for it.
“Our
results therefore broaden the hypothesis and create new questions
regarding the cortical mechanisms of visual awareness”,
Panagiotaropoulos explains. In the near future the group is going to
record the electrical activity in both regions simultaneously.
They
will try to find out which of the two areas is activated first and draw
conclusions on how the two areas interact with each other during
conscious perception. This may lead to a better understanding of why
only certain things reach our awareness and others remain suppressed.
