Mislocalization of flashes during smooth pursuit hardly depends on the lighting conditions

Targets that are briefly flashed during smooth pursuit eye movements are mislocalized in the direction of motion (forward shift) and away from the fovea (spatial expansion). Hansen [Hansen, R. M. (1979). Spatial localization during pursuit eye movements. Vision Research 19(11), 1213-1221] reported that these errors are not present for fast motor responses in the dark, whereas Rotman et al. [Rotman, G., Brenner, E., Smeets, J. B. (2004). Quickly tapping targets that are flashed during smooth pursuit reveals perceptual mislocalizations. Experimental Brain Research 156(4), 409-414] reported that they are present for fast motor responses in the light. To evaluate whether the lighting conditions are the critical factor, we asked observers to point to the positions of flashed objects during smooth pursuit either in the dark or with the room lights on. In a first experiment, the flash, which could appear at 1 of 15 different positions, was always shown when the eye had reached a certain spatial position. We found a forward bias and spatial expansion that were independent of the target and ambient luminance. In a second experiment, the flash was always shown at the same retinal position, but the spatial position of the eye at the moment of flash presentation was varied. In this case we found differences between the luminance conditions, in terms of how the errors depended on the velocity and position on the trajectory. We also found specific conditions in which people did not mislocalize the target in the direction of pursuit at all. These findings may account for the above-mentioned discrepancy. We conclude that although the lighting conditions do influence the localization errors under some circumstances, it is certainly not so that such errors are absent whenever the experiment is conducted in the dark.