Weak synapses may cause lines to blur

USC (US) — Neurons in the primary visual cortex respond selectively to lines and edges of visual images, allowing the brain to distinguish their orientation, according to a new study.

The research, published in the journal Neuron, focuses on the crucial role inhibitory synapses play in recognizing orientation and may have implications for treating decreased cognitive function in the aging brain.

Synapses are the junctions between neurons and other cells that allow information to travel from one cell to the next. They can exert either excitatory or inhibitory influences on the target cell depending on the neurotransmitters and receptors at the synapse.

“We are very interested in how the positive (excitatory) and negative (inhibitory) inputs interact,” says Huizhong W. Tao assistant professor in the department of cell and neurobiology at University of Southern California.

“We found something totally unexpected—inhibitory input is necessary for maintaining and sharpening orientation selectivity.” In other words, without inhibitory synapses, an image’s edges and contours are blurred.

Previous studies have shown that the arrangements of excitatory neuron circuits are sufficient for the brain to recognize shapes, but that conclusion is mostly based on output (nerve impulse) measurements only, says Tao, who looked at both output and input.

Tao and colleagues measured the nerve impulse as well as the excitatory and inhibitory currents that passed through individual cells in live mice as they were presented various visual stimuli.

They used a neuron model to determine how the inhibitory current affected the output response of the neuron and found that the ability to perceive orientation was most keen when both excitatory and inhibitory inputs were present and when the inhibitory input was strong.

“Inhibitory synapses weaken as people age,” Tao says. “This suggests that drugs that strengthen inhibitory activity may improve cognitive function among the elderly.”

The study was funded by the National Institutes of Health.

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