Abstract
When exploring a visual scene, humans make more saccades in the horizontal direction than any other direction. This horizontal saccade bias is well documented despite its unknown origin. While many have shown that the horizontal saccade bias rotates in response to natural scene tilt, it is unclear whether the direction distribution of microsaccades made while fixating a small target might rotate with background image tilt. Here we tested whether scene tilt influenced saccade direction distributions during periods of fixation. Study participants (n=9) viewed tilted scenes at -30, 0, and 30 degrees, and were asked to either fixate a central round dot (0.5 degrees) or free view the image. We recorded eye movements (binocularly) with a custom-built eye tracking system running at 250 Hz. The scenes were 20 x 20 degrees of visual angle and were viewed from 90 cm away. To determine if the biases in saccade direction changed with image tilt, we calculated polar histograms of saccade directions for both fixation and free viewing tasks. We then cross correlated the distributions for -30 and 30 degree image tilts to find the angular displacement that resulted in the maximum correlation. Saccade distributions during fixation did not rotate significantly between image tilts (2.1 ±2.01 degrees; t(8) = 1.05, p = 0.32). In contrast, saccade distributions during free viewing were significantly rotated by a mean angle of 14.6 ±5.41 degrees (t(8) = 2.69, p = 0.03). These results suggest that image tilt caused a rotation of the bias during free viewing but not during fixation, despite a common brain circuit that generates both saccades and microsaccades. It is possible that bias reorientation is controlled by upstream areas in the brain, though the precise mechanism for the bias and its reorientation is currently unknown.