The purpose of blinks is to keep the eyes hydrated and to protect them. Blinks are rarely noticed by the subject as blink-induced alterations of visual input are blanked out without jeopardizing the perception of visual continuity. Although not perceived, the blink-induced disconnection from the visual environment leads to a loss of information. Therefore there is critical need to minimize it. Now, a study from researchers at the University of Tübingen identifies a new type of eye movement serving a distinct oculomotor demand, namely the resetting of eye torsion, likewise inevitably causing a loss of visual information. The team state that their findings suggest by integrating this eye movement into blinks, the downtimes of vision associated with each of the two behaviours are synchronized and the overall downtime minimized. The opensource study is published in the journal eLife.
Previous studies show that although it is brief, blinking creates an interruption in a human’s visual perception. A person spends up to a tenth of their waking hours blinking, with it serving an essential role in lubricating the eye and may even provide the brain with small, frequent mental breaks. Studies in humans have shown that the visual system is able to ensure an illusion of continuity by bridging the perceptual gaps with oculomotor reflexes. One of these reflexes is the torsional optokinetic nystagmus (tOKN) which consists of a slow phase in which the eyes pursue the rotation of the visual stimulus in order to reduce the slip of its retinal image, followed by a torsional fast phase which ensures that the eyes stay within the mechanical limits of the plant. Therefore, the lab also expected the involuntary fast phases of the tOKN might be synchronized with blinks in order to minimize the overall downtime of the visual system. The current study shows that rather than combining fast phases with blinks, the oculomotor system deploys a novel type of resetting movement.
The current study assessed the eye movements of 11 subjects using tiny wires attached to the cornea and with infrared video tracking, to show a new type of eye movement that is synchronised with blinking. Results show that the movement helps to reset the eye after it twists when viewing a rotating object, like avoiding tiny rotations of a camera to stabilise the image perceived. The group explain that people don’t notice the eye resetting in this way because it happens automatically when they blink.
Data findings show that as the participant’s eyes twisted to follow the dots, they frequently reset via tOKN to avoid moving beyond the mechanical limits of the eye muscles. The team observed that this resetting was imperfect, and the eyes gradually twisted until the muscles couldn’t twist any more. Results show that once they reached their maximum, the eyes reset so they were no longer twisted at all, which happened at the same time as blinking. The lab named this newly identified movement blink-associated resetting movement (BARM).
The team surmise that their findings demonstrate the existence of a distinct, hitherto unknown type of eye movement. It helps to avoid the accumulation of excessive eye torsion, building up in response to torsional optic flow by time and again resetting the torsional deviation. For the future, the researchers state that BARMs are a distinct type of eye movement that is a ubiquitous partner of eye blinks, new information which should greatly change the landscape of optometry and ophthalmology.
Source: University of Tübingen
