Huizhong W. Tao, Ph.D., assistant professor in the Department of Cell and Neurobiology, Zilkha Neurogenetic Institute / by Christine Chan
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 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, Ph.D., 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 conclude 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 says. 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.”
Funded by the National Institutes of Health, the study is slated to
appear in the August 11, 2011, print edition of
Neuron.