PT - JOURNAL ARTICLE AU - Stephanie M. Reeves AU - Emily A. Cooper AU - Raul Rodriguez AU - Jorge Otero-Millan TI - Head orientation influences saccade directions during free viewing AID - 10.1523/ENEURO.0273-22.2022 DP - 2022 Nov 09 TA - eneuro PG - ENEURO.0273-22.2022 4099 - http://www.eneuro.org/content/early/2022/11/09/ENEURO.0273-22.2022.short 4100 - http://www.eneuro.org/content/early/2022/11/09/ENEURO.0273-22.2022.full AB - When looking around a visual scene, humans make saccadic eye movements to fixate objects of interest. While the extraocular muscles can execute saccades in any direction, not all saccade directions are equally likely: saccades in horizontal and vertical directions are most prevalent. Here, we asked if head orientation plays a role in determining saccade direction biases. Study participants (n = 14) viewed natural scenes and abstract fractals (radially symmetric patterns) through a virtual reality headset equipped with eye tracking. Participants’ heads were stabilized and tilted at -30°, 0°, or 30° while viewing the images, which could also be tilted by -30°, 0°, and 30° relative to the head. To determine if the biases in saccade direction changed with head tilt, we calculated polar histograms of saccade directions and cross-correlated pairs of histograms to find the angular displacement resulting in the maximum correlation. During free viewing of fractals, saccade biases largely followed the orientation of the head with an average displacement value of 24° when comparing head upright to head tilt in world-referenced coordinates (t(13)=17.63, p<0.001). There was a systematic offset of 2.6° in saccade directions, likely reflecting ocular counter roll (OCR) (t(13)=3.13, p=0.008). When participants viewed an Earth upright natural scene during head tilt, we found that the orientation of the head still influenced saccade directions (t(13)=3.7, p=0.001). These results suggest that non-visual information about head orientation, such as that acquired by vestibular sensors, likely plays a role in saccade generation.Significance StatementWe show that the statistics of saccade directions, from data collected during free viewing of fractal (radially symmetric) and natural scene images, are influenced by head orientation. During fractal viewing, saccade directions largely followed the orientation of the head with systematic offsets likely explained by ocular counter roll. During natural scene viewing of an Earth upright image, saccade directions were still influenced by head orientation. These results suggest that head and retinal orientation relative to gravity play a key role in saccade generation. Future work should consider the influence of head orientation when predicting saccade landing points or when using existing saccade generation models.