Seeing Movement In The Dark

Seeing is much harder at night: It has long been known that visual acuity is much poorer and that colors fade completely. New experimental findings by researchers at the Max Planck Institute for Biological Cybernetics and the University Eye Clinic in Tuebingen published in the April 8 issue of the journal Nature show that moving objects appear slower, when viewed through the rod photoreceptors of night vision than when viewed through the cone photoreceptors of day vision.

Perceived speed for stimuli seen by the rod- (night-) vision system is underestimated by about 25% compared to movement seen by the cone- (day-) vision system. Analogously, cone stimuli are seen to move about 30% faster than a rod stimulus of a given speed. Full size image available through contact

In their experiments, two moving stimuli were presented to the observers. The colors of the stimuli were carefully chosen so that one stimulus was only visible to the cone photoreceptors and the other only to the rod photoreceptors. Observers typically reported that the rod perceived stimulus appeared to move slower than the stimulus perceived through the cones, although the two actually moved at the same physical speed. To obtain a speed match between the rod and cone perceived stimuli, the physical speed of the rod stimulus had to be increased by about 25%. Since speed judgments by daylight (based on cone photoreceptors) are known to be quite veridical, this implies that, under low light conditions, observers underestimate the speed of moving objects.

When driving at night, the region illuminated by the headlights is bright enough to activate cone photoreceptors, while the remaining visual field is in the dark, and therefore seen by rod photoreceptors. Full size image available through contact

The situation closely resembles the situation of driving at night. The center of the visual field is illuminated by the bright headlights - and therefore seen by cones, whereas the remaining visual scene, including the sides of the road, are unlit and presumably processed by rods. This might lead to an underestimation of the speed of our movement through the environment, which in turn might elicit a compensatory and possibly fatal speeding-up. It is now planned to use the Virtual Reality setup at the Max Planck Institute to test this prediction under more realistic settings.

###

---

---

---