Saccadic oscillation
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A common finding when videoing bird's eyes (but only at high frame rates - 100-200fps) is the association of blinking with bursts of rhythmical rotatory eye movements. These may continue for more than a second. The blinks involve the nictitating membrane often combined with elevation of the lower lid. The term coined to describe these eye movements is 'saccadic oscillations' and they have been shown to increase perfusion of oxygen and nutrients to the retina by shaking the pecten oculi (a vascular structure which protrudes into the vitreous humour of the eye (Pettigrew et al. 1990). Birds lack the retinal vasculature of mammals.
A brief burst of rhythmic rotatory eye movements at the onset of a nictitating membrane blink in a marabou stork (Leptoptilos crumenifer) played at 1/8th speed.
Nictitating membrane blinks associated with saccadic oscillations and elevation of the lower lid tend to be more prolonged than simple nictitating membrane blinks.
Nictitating membrane blink with saccadic oscillations and elevation of the lower lid in a muscovy duck (Cairina moschata), played at 1/8th speed. Blood vessels can be seen in the membrane.
Simple nictitating membrane blink.
Blinks associated with saccadic oscillations tend to be more prolonged than simple blinks.
Two brief nictitating membrane blinks with the head still followed by a prolonged blink associated with rhythmical movements of the eyeball and slight narrowing of the palpebral fissure in a male black-necked stork
Saccadic oscillation can also occur between blinks:
Saccadic oscillations between blinks in a Nicobar pigeon (Caloenas nicobarica), played at 1/4 speed.
Reference
Pettigrew, J.D., Wallman, J. and Wildsoet, C.F. 1990. Saccadic oscillations facilitate ocular perfusion from the avian pectin - Nature. 343:362-3.