Johnston Lab
​Computations in Neural Circuits

Early computation of Orientation in the Retina

General Features of the retinal connectome determine the computation of motion anticipation

A Reinal Ganglion Cell filled with Alexa 488

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Light is converted into electrical signals by specialized cells in the retina called photoreceptors. This conversion process, termed phototransduction, is relatively slow, taking around a tenth of a second. Although this might not sound like a long time, it is enough for a tennis ball to have travelled 3-4 meters when served by a professional.​
 
Despite this, we do not experience moving objects being delayed, which indicates that moving objects are somehow processed faster. An example of how moving objects are processed differently to static objects can be seen in the flash lag illusion. This apparent faster processing of moving objects begins in the eye.
Retinal ganglion cells (pictured), which send signals from the retina to the rest of the brain, respond earlier than expected for moving objects, overcoming the slow process of phototransduction. This phenomenon has been termed “motion anticipation”. This project revealed how motion anticipation arises from the circuitry of the retina.

Using a combination of electrical recordings and computer simulations, Johnston and Lagnado showed that inhibitory signals play a key role in motion anticipation. In particular, they found that an excess of inhibitory inputs onto retinal ganglion cells enables motion anticipation for objects moving in any direction across the retina. 
The project described how the non-linear interaction of excitatory and inhibitory synapses, within ganglion cell dendrites, enables encoding of the actual position of a moving object instead of its delayed representation. The fact that retinal ganglion cells receive an excess of inhibitory inputs is a long observed feature of retinal wiring, and a role for this can now be understood in terms of providing the mechanism of motion anticipation.