Upright Perception and Ocular Torsion Change Independently during Head Tilt

Abstract

We maintain a stable perception of the visual world despite continuous movements of our eyes, head and body. Perception of upright is a key aspect of such orientation constancy. Here we investigated whether changes in upright perception during sustained head tilt were related to simultaneous changes in torsional position of the eyes. We used a subjective visual vertical (SVV) task, modified to track changes in upright perception over time, and a custom video method to measure ocular torsion simultaneously. We tested 12 subjects in upright position, during prolonged (~15 min) lateral head tilts of 20 degrees, and also after the head returned to upright position. While the head was tilted, SVV drifted in the same direction as the head tilt (left tilt: −5.4 ± 1.4° and right tilt: +2.2 ± 2.1°). After the head returned to upright position, there was an SVV aftereffect with respect to the pre-tilt baseline, which was also in the same direction as the head tilt (left tilt: −3.9 ± 0.6° and right tilt: +2.55 ± 1.0°). Neither the SVV drift nor the SVV aftereffect were correlated with the changes in ocular torsion. Using the Bayesian spatial-perception model we show that the pattern of SVV drift and aftereffect in our results could be explained by a drift and an adaptation in sensory inputs that encode head orientation. The fact that ocular torsion (mainly driven by the otoliths) could not account for the perceptual changes suggests that neck proprioception could be the primary source of drift in upright perception during head tilt, and subsequently the aftereffect in upright position.

Perception of Upright: Multisensory Convergence and the Role of Temporo-Parietal Cortex

“…In addition, the torsional position of the eyes can change in the direction of the visual line rotation, and such “torsional entrainment” may introduce biases when SVV is measured using the line rotation ( 14 ). Considering all these sources of error, a forced-choice task with a random line orientation in each trial would be the least biased method for SVV measurement, as it would remove the effects of line movement on SVV responses ( 15 ).…”

“…Upright perception may drift during prolonged tilts of the whole body or prolonged tilts of the head on body ( 15 , 31 , 61 , 157 , 158 ). The drift pattern is usually variable across individuals ( 157 ), but often there is a gradual change in the direction of the tilt, followed by a post-tilt bias referred to as the aftereffect (Figure 8 ) ( 15 , 61 , 157 – 161 ). When this aftereffect was studied across a wide range of body orientations, there was a “local” effect (as opposed to a “global” effect), where the post tilt bias was mainly seen in the tilt orientations adjacent to the initial, adapting position ( 162 ).…”

Dynamic arm movements attenuate perceptual distortion of visual vertical induced during prolonged whole-body tilt

“…5) when the participants performed dynamic arm movements during prolonged tilt. As mentioned above, the SVV shifts during prolonged tilt are mainly due to sensory adaptation in the vestibular and body somatosensory systems [11,26].…”

“…Other studies suggest the possibility that somatosensory adaptation derived from trunk receptors may also contribute to SVV shift during prolonged tilt [11,16]. Taken together, head and body angle relative to gravity would be sensed to be smaller due to vestibular and somatosensory adaptation, leading to the shifts of the perceived direction of gravity towards the tilt side [26].…”

Updating of visual orientation in a gravity-based reference frame

“…terroll; see de Vrijer et al, 2009;Otero-Millan & Kheradmand, 2016) and further varies with the frameinduced modulation of internal gravity. Both the bias and modulation of stored line orientation are revealed when the gravity-based reference is used in the updating task, as seen in Figure 3.…”