Eye cells may use math to detect motion

NIH | APRIL 5, 2016

Our eyes constantly send bits of information about the world around us to our brains where the information is assembled into objects we recognize. Along the way, a series of neurons in the eye uses electrical and chemical signals to relay the information.

The human eye. Image credit: Paul Savage/Wikipedia (CC BY 2.0)

A new study suggests that the NMDA receptor, a protein normally associated with learning and memory, may help neurons in the eye and the brain relay that information.

Using retinas isolated from mice, the scientist studied neurons called directionally selective retinal ganglion cells (DSGCs), which are known to fire and send signals to the brain in response to objects moving in specific directions across the eye.

Electrical recordings showed that some of these cells fired when a bar of light passed across the retina from left to right, whereas others responded to light crossing in the opposite direction. These unique responses are controlled by incoming signals sent from neighboring cells at chemical communication points called synapses.

The activity of NMDA receptors at one set of synapses may regulate whether DSGCs sent direction-sensitive information to the brain.

NMDA receptors are proteins that generate electrical signals in response to the neurochemicals glutamate and glycine. When activated, they allow electrically charged ions to flow in and out of cells like water through an unlocked canal.

Magnesium blocks the flow until the neuron is strongly activated and its electrical state rises above a certain voltage. This regulation is thought to be critical for certain types of learning and memory, and in amplifying signals in neurons.

The variable magnesium block that ensured the cells consistently sent information to the brain in response to the passing bars of light despite the distracting incoming stream of signals generated by the background lights. The NMDA receptors did this by amplifying the cells’ responses to the bars in a process called multiplicative scaling.

Neurons in the eye and brain receive a constant stream of information. The results of this study support a growing body of evidence suggesting that NMDA receptors play in critical role in how neurons relay information.

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