No matter how hard we fix our gaze, our eyes are in constant motion due to involuntary head and eye movements. The resulting image motion on our retinas can be quite substantial as is aptly demonstrated by the Hermann-Hering illusion . During normal viewing we are happily unaware of this perpetual retinal image dance, yet we remain exquisitely sensitive to small movements created by objects in the outside world. How does our visual system accomplish this feat?
A few years ago we showed that the segregation of object and background motion occurs already in the retina (Nature), the first stage of visual processing. Now, in a study published in Neuron , we show that the retinal circuit underlying this computation is even more sophisticated: the cells signaling object motion are most sensitive when an object begins to move. This makes a lot of sense: Prey and predators often reveal themselves by initiating movement and we, along with the rest of the animal kingdom, are well advised to pay attention to such stimuli.
Thus the neural circuits that sense object motion can distinguish three flavors of retinal image motion, ranked according to their ecological importance:
1. An object has just initiated a movement. Object motion sensitive (OMS) neurons in the retina are highly sensitive to this scenario and signal to the brain using high firing rates.
2. Object motion has been ongoing for several seconds or more. Such stimuli have lost their novelty, OMS cells respond in a lukewarm manner, firing much less vigorously than for newly initiated movements.
3. There is no motion in the visual scene, but its retinal image is jittered owing to eye movements. Here the OMS cells remain silent.
We tracked down where and how the adaptation to object motion occurs within the retinal circuit. All the evidence points to plasticity at one important neural link: the synapse between bipolar and ganglion cells. We now want to explore whether this same mechanism can give a unified explanation for a whole panoply of visual adaptations that occur in the retina (Nature).