Motion Perception and Flash-lag effect
To catch a flying ball, you need to precisely predict the current moving position and react yourself at the right time and right location. This seems to be a mundane conclusion. However, if you consider neural transmission delay (e.g. about 70-100 ms from retinal to visual cortex), our visuomotor system has to compensate such delay in order to have precise actions. In our recent studies, we show that such compensation exists in our visual system.
Motion extrapolation in central fovea
Abstract of Shi & Nijhawan, 2012: Neural transmission latency would introduce a spatial lag when an object moves across the visual field, if the latency was not compensated. A visual predictive mechanism has been proposed, which overcomes such spatial lag by extrapolating the position of the moving object forward. However, a forward position shift is often absent if the object abruptly stops moving (motion-termination). A recent “correction-for-extrapolation” hypothesis suggests that the absence of forward shifts is caused by sensory signals representing ‘failed’ predictions. Thus far, this hypothesis has been tested only for extra-foveal retinal locations. We tested this hypothesis using two foveal scotomas: scotoma to dim light and scotoma to blue light. We found that the perceived position of a dim dot is extrapolated into the fovea during motion-termination. Next, we compared the perceived position shifts of a blue versus a green moving dot. As predicted the extrapolation at motion-termination was only found with the blue moving dot. The results provide new evidence for the correction-for-extrapolation hypothesis for the region with highest spatial acuity, the fovea.
It is well known that in the central fovea there is a 0.3 mm diameter rod-free area, where the low intensity objects fail to yield a visual percept (Hecht, 2002). Thus a continuous dim moving object moves across rod-free fovea, one would perceive a discontinuous movement. If the motion percept follows faithfully to the retinotopic map (no motion extrapolation for the neural delay), one should see a discontinuous movement is symmetrically located near fovea (i.e. the dim moving object vanishes at the boundary of rod-free fovea, and reappears at the boundary of fovea, as illustrated in the following figure).
Without motion extrapolation
However, the motion extrapolation hypothesis would predict that the dim object moves into the fovea (extrapolation) and reappears further away from the fovea (a Fröhlich effect). The following figure shows how you would observe with the dim moving object.
With motion extrapolation
To see such asymmetric extrapolation phenomenon, please download the following flash demo, and then open with full screen.
This scotopic illusion can only be seen in a dark room with a low luminance. So please reduce the brightness and contrast of your monitor, such that you barely see the moving object. In addition, you need several minutes of the dark adaptation for a better illusion.
Alternatively, you can use a Tokyo blue filter, through which you should be able to see the illusion in a normal light condition.
Anisotropic flash-lag and flash-mislocalization effect
Abstract of Shi & Nijhawan, 2008: Motion from periphery to central vision (foveopetal motion) causes a greater flash-lag effect than motion in the opposite direction (foveofugal motion). In order to examine the factors that contribute to the motion direction-based anisotropic flash-lag effect, we investigated the mislocalization of the flash caused by motion and the mislocalization of the moving object per se. We observed that for foveofugal motion, flashes were perceived shifted in the direction of motion but mislocalized in the opposite direction for foveopetal motion. Additionally the mislocalization of the moving object was larger in foveopetal motion than in foveofugal motion. Thus, both factors contribute to the anisotropic flash-lag effect. We interpret these findings in terms of greater behavioral significance of foveopetal motion in relation to foveofugal motion.
Shi, Z., & Nijhawan, R. (2012). Motion extrapolation in the central fovea. PloS one, 7(3), e33651. doi:10.1371/journal.pone.0033651
Shi, Z., & Nijhawan, R. (2008). Behavioral significance of motion direction causes anisotropic flash-lag and flash-mislocalization effects. Journal of Vision, 8(7), 24:1–14.