When you move a camera quickly from one object to another, the abrupt shift causes a blur in the shot that can even make viewers nauseous. However, our eyes actually perform such rapid movements, known as saccades, two or three times per second. However, although the visual stimulus moves across the retina during a saccade, we do not perceive the shift.
According to a study published in the journal Nature Communications by Prof Dr Martin Rolfs from the Department of Psychology at Humboldt-Universit?t zu Berlin (HU) and colleagues at the Cluster of Excellence Science of Intelligence (TU Berlin), visual stimuli - such as those caused by the sight of a chipmunk dashing around or a tennis ball hit with full force - become invisible if the objects move at a speed, duration and distance corresponding to that of one of our saccades. In this case, the speed of the saccades apparently predicts the speed limit of our vision. This suggests that the properties of the human visual system are best understood in the context of the movements of our eyes.
Our movements shape our perception
"Which parts of the physical world we can perceive essentially depends on how good our sensors are," explains Martin Rolfs, the lead author of the study. "For example, we don't see infrared light because our eyes are not sensitive to it, and we don't see flicker on our screens because they flicker at higher frequencies than our eyes can resolve. In this paper, however, we show that the limits of vision are not only defined by these biophysical limitations, but also by the actions and movements that directly affect the sensory system. To show this, we used the fastest and most frequent movements of the body - the saccadic eye movements that people perform more than a hundred thousand times a day." Since the speed of these eye movements differs from person to person, people who make particularly fast eye movements can also see objects that move at higher speeds. This could mean that the best baseball players, action video gamers or wildlife photographers are those who have faster eye movements.
Saccades: A movement that we do not perceive
Similar to a camera movement in a film, saccades create patterns of movement on the retina. "But we never consciously perceive this movement," says Rolfs. "We have shown that stimuli that follow the same, very specific movement patterns as saccades - while people keep their gaze fixed in one place - also become invisible. This suggests that the kinematics of our actions, in this case saccades, fundamentally restrict a sensory system's access to the physical world around us." According to Rolfs, this can be interpreted as an intelligent property of the visual system, as it remains sensitive to fast movements, but only up to speeds that result specifically from saccades. "Put simply, the properties of a sensory system such as the human visual system are best understood in the context of the kinematics of the actions that control its input, in this case rapid eye movements," says Rolfs.
Finely tuned systems
"Our visual and motor systems are finely tuned to each other, but this was ignored for a long time," says Martin Rolfs. "One of the problems is that the people who deal with motor control are not the same people who deal with perception. They attend different conferences, publish in different journals - but they should talk to each other!" This study suggests that our visual system can recognise when a stimulus moves in a way that resembles our own eye movements and then filters out the conscious perception of that movement. This reveals a previously unknown mechanism that explains why we do not perceive the visual blur effect of eye movements as we would when using a camera.
Further information
Contact Prof. Dr Martin Rolfs
Prof Dr Martin Rolfs
Department of Psychology, Humboldt-Universit?t zu Berlin
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