'Traffic cops' of the human eye

Dalhousie researchers have made a significant breakthrough in understanding how the retina detects motion. Their discovery was published in last week’s issue of Neuron, one of the world’s leading neuroscience journals.

The process of sensing motion has been known to begin in the retina, which is a thin screen of neural mesh that lies in the back of the eye. Dr. Gautam Awatramani’s laboratory at Dalhousie Medical School has determined how the retina does this.

“For nearly 50 years scientists have been trying to figure out how motion is computed by the retina,” says Dr. Awatramani, assistant professor of anatomy and neurobiology and a member of the Retina and Optic Nerve Research Lab.

“The current notion is that an elaborate neural circuitry within the inner retina is required to compute direction,” he explains. “We’ve found that in addition to these mechanisms, the dendrites (branches) of retinal neurons play an important role in processing motion information. This opens a new a chapter in the field of eye and vision research.”

The Dalhousie lab identified that neurons in the retina called directional-selective ganglion cells (DSGCs) have a special property to compute direction. These cells come in four types, and each one encodes one of four directions – left, right, up, and down. When viewed under a microscope, researchers found that the population of retinal cells that coded for anterior motion had an asymmetric shape, and their thin branch-like structures – called dendrites – were found to all point in the same direction.

Using their dendrites, DSGCs respond to moving images in a person’s environment, and they then signal this information to the brain.

“What we have discovered is that individual cells in the eye act like little traffic cops, and point in the direction of objects in motion; directional selective ganglion cells allow our brains to make sense of the moving world around us,” says Stuart Trenholm, a PhD candidate in the Awatramani lab.

“Our lab focuses on trying to understand how the neural circuits in both the healthy and diseased retina work,” says Dr. Awatramani. “Once we understand how the healthy retina works, we can begin to devise strategies to correct retinal dysfunction in injured and diseased eyes.”

Dr. Awatramani’s lab is funded by the Dalhousie Medical Research Foundation, the Natural Sciences and Engineering Research Council of Canada, the Foundation for Fighting Blindness, and the Canadian Institutes of Health Research.