Visual acuity for moving objects in first- and second-order neurons of the fly compound eye

The early stages of visual systems contain a variety of components that limit both the spatial resolution and the temporal resolution of vision. When an animal sees a moving object, or moves relative to its environment, both spatial and temporal factors contribute to its ability to resolve the movement. In the present work we have combined currently available knowledge about the early stages of fly vision (optical system, photoreceptors, and large monopolar cells) to predict the resolution of the first two cell layers to moving point objects. These calculations included recent measurements of nonlinear light responses. Because background light level has a strong effect on the temporal behavior of these early visual layers, we examined the effects of light level on motion resolution. We also studied the effect of position within the eye, which is known to affect the static resolution of vision. Our results indicate that responses in large monopolar cells to moving point objects are maximal at angular velocities of 100-200 degrees/s. The resolution of point objects by both these early stages of the visual system is similar from stationary to an angular velocity of approximately 200 degrees/s. Above this, resolution deteriorates approximately linearly with velocity.