Hummingbirds use compensatory eye movements to stabilize both rotational and translational visual motion

To maintain stable vision, behaving animals make compensatory eye
movements in response to image slip, a reflex known as the optokinetic
response (OKR). Although OKR has been studied in several avian species,
eye movements during flight are expected to be minimal. This is because
vertebrates with laterally placed eyes typically show weak OKR to
nasal-to-temporal motion (NT), which simulates typical forward locomotion,
compared to temporal-to-nasal motion (TN), which simulates atypical
backward locomotion. This OKR asymmetry is also reflected in the
pretectum, wherein neurons sensitive to global visual motion also exhibit
a TN bias. Hummingbirds, however, stabilize visual motion in all
directions through whole-body movements and are unique among vertebrates
in that they lack a pretectal bias. We therefore predicted that OKR in
hummingbirds would be symmetrical. We measured OKR in restrained
hummingbirds by presenting gratings drifting across a range of speeds. OKR
in hummingbirds was asymmetrical, though the direction of asymmetry varied
with stimulus speed. Hummingbirds moved their eyes largely independent of
one another. Consistent with weak eye-to-eye coupling, hummingbirds also
exhibited disjunctive OKR to visual motion simulating forward and backward
translation. This unexpected oculomotor behavior, previously unexplored in
birds, suggests a potential role for compensatory eye movements during
flight.