Bridge locus
Encyclopedia
In neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

 the bridge locus for a particular sensory percept is a hypothetical set of neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s whose activity is the basis of that sensory percept. The term was introduced by D.N. Teller and E.Y. Pugh, Jr. in 1983, and has been sparingly used. Activity in the bridge locus neurons is postulated to be necessary and sufficient
Necessary and sufficient conditions
In logic, the words necessity and sufficiency refer to the implicational relationships between statements. The assertion that one statement is a necessary and sufficient condition of another means that the former statement is true if and only if the latter is true.-Definitions:A necessary condition...

 for sensory perception: if the bridge locus neurons are not active, then the sensory perception does not occur, regardless of the actual sensory input. Conversely if the bridge locus neurons are active, then sensory perception occurs, regardless of the actual sensory input. It is the highest neural level of a sensory perception. So, for example, retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

l neurons are not considered a bridge locus for visual perception because stimulating visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....

 can give rise to visual percepts
Phosphene
A phosphene is a phenomenon characterized by the experience of seeing light without light actually entering the eye. The word phosphene comes from the Greek words phos and phainein...

.

Not all scholars believe in such a neural correlate of consciousness. Pessoa et al., for example, argue that there is no necessity for a bridge locus, basing their argument on the requirement of an isomorphism between neural states and conscious states. Thompson argues that there are good reasons to think that the notion of a bridge locus, which he calls a "localizationist approach", is misguided, questioning the premise that there has to be one particular neural stage whose activity forms the immediate substrate of perception. He argues, based upon work by Zeki & Shipp, DeYoe & Van Essen, and others, that brain regions are not independent stages or modules but have dense forward and backward projections that act reciprocally, and that visual processing is highly interactive and context-dependent. He also argues that cells in the visual cortex "are not mere 'feature detectors, and that neuroscience has revealed that the brain in fact employs distributed networks, rather than centralized representations. He equates the notion of a bridge locus to a Cartesian theatre and suggests that as a notion it should be abandoned.
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