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

Area of Science:

• Neuroscience
• Comparative physiology
• Oculomotor function

Background:

• The optokinetic response (OKR) is crucial for maintaining stable vision via compensatory eye movements.
• Vertebrates typically show an asymmetry in OKR, favoring temporal-to-nasal motion over nasal-to-temporal motion, linked to pretectal neuronal biases.
• Hummingbirds, unique in stabilizing visual motion with whole-body movements and lacking a pretectal bias, were hypothesized to have symmetrical OKR.

Purpose of the Study:

• To investigate the optokinetic response (OKR) in hummingbirds, particularly their eye movement patterns during visual motion.
• To determine if hummingbirds exhibit the predicted symmetrical OKR, given their unique visual stabilization strategies and lack of pretectal bias.
• To explore the potential role of compensatory eye movements in hummingbirds during flight.

Main Methods:

• Restrained hummingbirds were used to measure the optokinetic response (OKR).
• Visual stimuli, specifically drifting gratings at various speeds, were presented to evoke OKR.
• Eye movements, including independent eye motion and disjunctive responses, were analyzed in relation to visual motion direction and speed.

Main Results:

• Hummingbirds displayed an asymmetrical optokinetic response (OKR), contradicting the initial hypothesis.
• The direction of OKR asymmetry varied depending on the speed of the visual stimulus.
• Hummingbirds exhibited largely independent eye movements and disjunctive OKR, indicating weak eye-to-eye coupling.

Conclusions:

• Contrary to predictions, hummingbirds demonstrate an asymmetrical optokinetic response (OKR).
• The observed disjunctive eye movements and speed-dependent asymmetry suggest unique oculomotor control in hummingbirds.
• This unexpected behavior indicates a potential, previously unrecognized role for compensatory eye movements in avian flight stabilization.