The Koch Lab - Consciousness Research

Researchers at the Koch lab and Dr Francis Crick of the Salk Institute believe that the time is now ripe for a scientific attack on the problem of consciousness. Their approach is to leave the philosophical questions behind and instead to concentrate on looking for what they call "the neuronal correlate of consciousness", or "NCC". The NCC is thought to consist of particular neurons in the brain, and particular patterns of activity, which lead directly to consciousness. They assume that the different aspects of consciousness (pain, visual awareness, self-consciousness, etc) all employ the same basic mechanism. Thus they have chosen visual consciousness as the primary example target for study.

The working hypothesis:
Crick and Koch maintain that in order to be aware of an object or event, the brain has to construct a multilevel interpretation of the visual scene. The levels can correspond to lines, or eyes or faces. It may be that only a small group of neurons fire during the representation of some aspect of a visual scene. This group is likely to number 100-1000 neurons for the representation of an aspect such as a face. A representation of an object or an event will usually consist of representations of many of the relevant aspects of it, and these are likely to be distributed over different parts of the visual system. How these different representations are bound together is known as the binding problem. Much neural activity is needed for the brain to construct a representation. Most of this activity is probably unconscious. As a working hypothesis they assume that only some types of specific neurons will express the NCC. It is already known that the firing of many cortical cells does not correspond to what is currently being seen.

Bistable Percepts:
Bistable percepts are considered the most important experimental approach to finding the NCC. A bistable percept is when the visual input is constant, but the subject's perception can take one of two alternative forms (for example when looking at the famous Necker cube). Single neurons in the monkey's brain are studied when it is looking at something which produces a bistable percept. If the neuron fires when the monkey is consciously aware of a stimulus, but doesn't fire when the stimulus is presented and the monkey remains unaware of it, then this neuron can be considered to be part of the NCC.

The V1/Frontal Lobe Hypothesis:
The conscious visual representation is likely to be distributed over more than one area of the cerebral cortex and possibly over certain subcortical structures as well. Crick and Koch argue that it is not located in cortical area V1. One is not directly conscious of the features represented by neural activity in the primary visual cortex (V1). Activity there is necessary for visual consciousness, but none of the neurons correlates directly to what we see. Neurons expressing NCC must project directly to part of the brain that plan voluntary action (this is what seeing/consciousness is for). These plans are made in parts of the frontal cortex. Neuroanatomy of the macaque shows V1 does not project to frontal cortex. This is probably true for humans too. In experiments on monkeys, neuronal activity in V1 can clearly represent certain retinal stimulation, yet is not perceived. This is supported by recent fMRI studies on humans. If this theory is correct, it would narrow the search to areas of the brain farther removed from the sensory periphery. The recent findings that neurons in the inferior prefrontal cortex of the macaque respond selectively to faces, and receive direct input from regions around the superior temporal sulcus and the inferior temporal gyrus that are well known to contain face-selective neurons.

Future experiments:
More work using bistable percepts, and studies in cortical areas as to which neurons express NCC (ie are active during conscious awareness). What neuronal subtype are they, in which cortical layer are they, how do they fire, where do they project? Further study of the neuroanatomy of connectivity - easy to do in macaque, more difficult in humans. What is exact role of frontal cortex in visual perception? Advances in molecular biological markers will be used to selectively and reversibly inactivate specific neurons. Further investigation through direct stimulation of cortical neurons.

Links:
The Koch Lab: http://www.klab.caltech.edu/consciousness.html