By Eric Craypo
A new study by optometry & vision science PhD student Stephanie Reeves and professor Jorge Otero-Millan has been published in the Journal of Neurological Sciences.
"A key part of understanding how we see is to understand the strategies we use to move our eyes. In this work, we show that the way we move our eyes changes depending on the size of the eye movement. Small saccades, the rapid eye movements that occur 2-3 times per second, are likely to be directed egocentrically (aligned with the orientation of the head) while large saccades are likely to be directed allocentrically (aligned with the orientation of the image). This work is exciting because it may help us better understand how saccades are generated and improve gaze prediction algorithms."
When exploring a visual scene, humans make more saccades in the horizontal direction than any other direction. While many have shown that the horizontal saccade bias rotates in response to scene tilt, it is unclear whether this effect depends on saccade amplitude. We addressed this question by examining the effect of image tilt on the saccade direction distributions recorded during freely viewing natural scenes. Participants (n = 20) viewed scenes tilted at −30°, 0°, and 30°. Saccade distributions during free viewing rotated by an angle of 12.1° ± 6.7° (t(19) = 8.04, p < 0.001) in the direction of the image tilt. When we partitioned the saccades according to their amplitude we found that small amplitude saccades occurred most in the horizontal direction while large amplitude saccades were more oriented to the scene tilt (p < 0.001). To further study the characteristics of small saccades and how they are affected by scene tilt, we looked at the effect of image tilt on small fixational saccades made while fixating a central target amidst a larger scene and found that fixational saccade distributions did not rotate with scene tilt (−0.3° ±1.7° degrees; t(19) = −0.8, p = 0.39). These results suggest a combined effect of two reference frames in saccade generation: one egocentric reference frame that dominates for small saccades, biases them horizontally, and may be common for different tasks, and another allocentric reference frame that biases larger saccades along the orientation of an image during free viewing.
Journal of Cognitive Neurological Sciences