Body position and direction of a moving object influence visual motion perception

Introduction: The perception of visual motion is essential for differentiating between moving and static objects. A recent study showed that it diminishes in normal subjects when an object moves at lower velocities[1]. Patients with bilateral vestibulopathy have higher motion coherence thresholds (MCT) than normal subjects, ranging between velocities of 0.15 to 40 deg/s. Their thresholds are also higher at low velocities[2,3]. This was hypothesized to indicate a compensatory mechanism for reducing oscillopsia caused by head movements [1,2,3].

In the current study healthy subjects were examined to determine if the direction of the moving object and gravity influence visual perception and thus motion coherence thresholds (MCT).

Methods: Nine healthy subjects (four males, ages 26–35 years, mean 28.8±2.8 years) were examined using a random dot pattern in which variable percentages of dots moved in a particular direction, i.e., upwards or downwards at random. Participants were asked to indicate in which of the two possible directions they perceived the coherent motion. Vertical planes were tested at speeds of 0.1 to 25 deg/s when the subjects were upright or lying on their sides.

Results: 1. When upright, subjects had a significantly higher MCT for dots moving upwards than for those moving downwards across all velocities (p=0.049). Conditions with a higher velocity had a lower MCT for all directions (p<0.0001). There was a significant interaction between direction and velocity (p<0.0001). Post-hoc Bonferroni test revealed a significant difference between the upward and the downward condition as regards MCTs at low velocities (0.1, 0.2, 0.4 deg/s; p<0.05); lower MCTs were found for the downward-moving dots.

2. When lying on their sides, subjects did not exhibit any significant effect of the direction on the MCT (p=0.813). High velocities corresponded to low MCTs (p=0.002). Post-hoc Bonferroni test did not reveal any significant difference between the two directions for each velocity.

Conclusion: This study demonstrates that the perception of slowly moving objects depends on the direction of the vertically moving object as well as on the position of the body. Our observation that the direction of vertically moving objects has an influence only when the subject is upright supports the view that gravity affects visual motion perception. Objects moving in the direction of gravity are easier to detect, perhaps because vestibular and visual inputs are then congruent.

References:

[1] Spiegel R, Kalla R, Muggleton N, Bueti D, Claassen J, Walsh V, Bronstein A. Adaptive mechanisms in visual motion processing and a possible link to evolution. Klin Neurophysiol 2010; 41: 42. DOI: 10.1055/s-0030–1250846.

[2] Grünbauer WM, Dieterich M, Brandt T. Bilateral vestibular failure impairs visual motion perception even with the head still. Neuroreport 1998; 9:1807–10.

[3] Shallo-Hoffmann J, Bronstein AM. Visual motion detection in patients with absent vestibular function. Vision Res 2003; 43:1589–94.