Eagleman and Sejnowski report a series of experiments that go a long way to pinning down the nature of our conscious perception of movement. A number of illusions were used in experiments showing that they shared a common process: flash-lag (moving object aligned with flash is offset), flash-drag (flash is offset as result of nearby motion), feature flash-drag (a change in a moving object is mis-located) and Frohlich illusion (the starting position of a suddenly appearing moving object is offset). The question that the researchers were answering is whether it is the position or the time that is altered in these illusion.

By varying the classic setups, the researchers found they could have the consciously perceived location of an object to be at a position where the object could not have physically been. Thus the illusion could not be based on an actual location with a fiddled time. It was the position that was being fiddled.

Several other characteristics of motion bias were also shown in the experiments.

• In all the setups, there was a trigger, a particular event for the subject to use as the reference for ‘now’. The motion that was used to bias the position occurs after the trigger during about the next 80 ms and not before the trigger.

• There does not appear to be two types of perception, one for stationary and one for moving objects. “the configuration of motion in the visual field influences the localization of both moving and stationary stimuli”. There can therefore be a trade-off between flash lag and flash drag.

• Where features like colour change during the motion of an object. The binding of the feature is not changed but only the position of the object when the change is bound to it.

One aspect of the discussion is a problem for me. The authors appear to consider only one reason for this motion biasing. “The visual system attempts to correct for the processing delays in signals from eye to perception and accounts for these delays by shifting its localizations closer to where they would be if there were no neural delay.” They also assume, “localization computations might only be triggered on a need-to-know basis. If true, this suggests that it may be computationally expensive”.

I have for some time thought that prediction of the very near future was one of the functions (perhaps the main function) of consciousness. Far from the idea that prediction is only an occasional operation in ‘need to know’ situations, I think it may be continuously done with all motion all the time. As well as removing the experience of a lag in ‘now’, there for two other reasons for prediction. Comparison of predictive conscious experience with fresh sensory information is a possible method of monitoring the accuracy of perception and checking the validity of our understanding of the world. It is also a possible method for avoiding motor plans that conflict with each other and instead facilitate smoothly integrated motor programs.

Eagleman, D., & Sejnowski, T. (2007). Motion signals bias localization judgments: A unified explanation for the flash-lag, flash-drag, flash-jump, and Frohlich illusions Journal of Vision, 7 (4), 3-3 DOI: 10.1167/7.4.3