USC Discovery Sheds Light on Sight

Neurons in the primary visual cortex respond selectively to lines and edges of visual images, allowing the brain to distinguish their orientation.

How and why those neurons respond selectively is not clearly understood, but scientists at the Keck School of Medicine of USC have discovered that inhibitory synapses play a crucial role in recognizing orientation.

The discovery could have implications for treating decreased cognitive function in the aging brain, according to the study’s principal investigator, Huizhong W. Tao, assistant professor in the Department of Cell and Neurobiology at the Keck School-affiliated Zilkha Neurogenetic Institute.

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,” Tao said. “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 concluded 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, Tao said. Her study looks 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. They 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 said. “This suggests that drugs that strengthen inhibitory activity may improve cognitive function among the elderly.”

The National Institutes of Health funded the study.

Source: University of Florida