"Consequently, with the help of Arc, the overall orientation selectivity in the visual cortex is sharpened by visual experience," like a camera that learns to focus better over time, Wang said. What's more, he said, "we suspect that this molecular filtering mechanism may also be applicable to other information processing systems in the brain."
Although baby animals are born with a handful of neurons tuned to respond to edges of light at specific orientations, the ability to detect these orientations improves with experience. The more the animal is exposed to shapes, objects and light, the better it can perceive them.
Plasticity is the amazing ability of a neuron or a synapse to change in response to experience. Changes in synaptic strength require rapid protein synthesis, but at the molecular level, little is known about the factors contributing to experience-dependent changes in orientation selectivity in the visual cortex, Wang said.
To come up with a better way to investigate this, the MIT team developed a state-of-the-art imaging system in which transparent cranial windows were implanted over the primary visual cortex, allowing the researchers to monitor over time the expression of proteins in the brains of live mice.
The study exploited the power of two-photon microscopy (so-called because it uses two infrared photons to emit fluorescence in tissue), which allows imaging of living tissue up to 1 millimeter deep, enough for researchers to see proteins expressed within individual neurons within the brain.
They then created a mouse model in which a coding portion of the Arc gene was replaced with a jellyfish gene encoding a green fluorescent protein (GFP). Neural activities that normally activate the Arc gene then activated the GFP, leaving a fluorescent trace detectable by two-photon microscopy. This allowed the researchers to image neuronal activation patterns induced by visu
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Contact: Elizabeth Thomson
thomson@mit.edu
617-258-5402
Massachusetts Institute of Technology
28-Jul-2006