Wednesday, July 04, 2012

Research - Conscious Perception is a Matter of Global Neural Networks



Representation of consciousness from the seventeenth century


For centuries, there was an assumption that consciousness resides in a single location, once thought to the the heart or lungs (Greek Classical period), then the brain (by the Renaissance), then a specific structure of the brain, such as the frontal cortex, the supposed seat of "executive function."

On the other hand, there has been a parallel belief that consciousness is not of the body, but is something more ephemeral, often seen as a universal presence (Roman period, some Buddhist schools, and modern writers like Deepak Chopra and B Alan Wallace).

More recently, there has been a move toward viewing consciousness as an emergent process of whole brain function - this new research, posted by Neuroscience News, supports that view.

Conscious Perception is a Matter of Global Neural Networks

New findings support the view that the content of consciousness is not localised in a unique cortical area.

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 localised in a unique cortical area, but is most likely an emergent property of global networks of neuronal populations.

The question 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.
A brain is shown with two electrical traces next to it.
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 by MPI for Biological Cybernetics.
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 in so far 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.

By this 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.

Notes about this consciousness research and article
Contacts:  Dr. Theofanis Panagiotaropoulos, Prof. Dr. Nikos K. Logothetis & Stephanie Bertenbreiter - (Press and Public Relations) – Max Planck Institute for Biological Cybernetics
Source: Max Planck Institute press release
Image Source: Image from Max Planck Institute press release image credited to MPI for Biological Cybernetics.

Original Research: 
Panagiotaropoulos, TI, Deco, G, Kapoor, V, & Logothetis, NK. (2012, Jun 7). Neuronal Discharges and Gamma Oscillations Explicitly Reflect Visual Consciousness in the Lateral Prefrontal Cortex. Neuron, Volume 74, Issue 5. doi: 10.1016/j.neuron.2012.04.013 

Neuronal Discharges and Gamma Oscillations Explicitly Reflect Visual Consciousness in the Lateral Prefrontal Cortex

Neuron, Volume 74, Issue 5, 924-935, 7 June 2012
Copyright © 2012 Elsevier Inc. All rights reserved.
10.1016/j.neuron.2012.04.013

Authors: Theofanis I. Panagiotaropoulos, Gustavo Deco, Vishal Kapoor, Nikos K. Logothetis

Highlights:
  • Single- and multiunit discharges reflect visual consciousness in the macaque LPFC
  • Gamma (>50 Hz) power reflects visual consciousness in the macaque LPFC
  • Subliminal spiking activity reflects weak traces of nonconscious processing

Summary

Neuronal discharges in the primate temporal lobe, but not in the striate and extrastriate cortex, reliably reflect stimulus awareness. However, it is not clear whether visual consciousness should be uniquely localized in the temporal association cortex. Here we used binocular flash suppression to investigate whether visual awareness is also explicitly reflected in feature-selective neural activity of the macaque lateral prefrontal cortex (LPFC), a cortical area reciprocally connected to the temporal lobe. We show that neuronal discharges in the majority of single units and recording sites in the LPFC follow the phenomenal perception of a preferred stimulus. Furthermore, visual awareness is reliably reflected in the power modulation of high-frequency (>50 Hz) local field potentials in sites where spiking activity is found to be perceptually modulated. Our results suggest that the activity of neuronal populations in at least two association cortical areas represents the content of conscious visual perception.
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